LUMBAR SPINAL NERVES

Lumbar spinal nerves. Lumbar plexus: composition, topography, distribution of branches. sacral and coccygeal spinal nerves. sacral and coccygeal plexus.

Practical skills , anatomy of vessels and nerves of trunk and extremities.

Lesson # 35

Theme 1.     Lumbar spinal nerves. Lumbar plexus: composition, topography, distribution of branches

 

The lumbar plexus is formed from part of Th₁₂  and L₁ - L₄ ventral rami of the thoracic and lumbar spinal nerves. Lumbar plexus lies inside the psoas major muscle. The lumbar plexus gives off motor branches directly to the psoas major and psoas minor muscles, the quadratus lumborum muscle.

1.     The iliohypogastric nerve runs on the inner surface of the quadratus lumborum muscle and then between the transversus abdominus muscle and the internal oblique mus­cle of the abdomen and innervates these muscles. It gives off the lateral cutaneous branch which supplies the lateral re­gion of the hip and the cutane­ous branch for skin above the inguinal ligament.

2.     The ilioinguinal nerve runs along the inguinal ligament and takes part in the innervation of the broad abdominal muscles, the sen­sory innervation of the skin of the mons pubis, and the upper part of the scrotum or the labia majora in the female.

3.     The lateral femoral cutaneous nerve passes over the iliacus muscle to just below the anterior superior iliac spine. It then extends through the lateral part of the muscular lacuna, to the skin of lateral surface of the thigh.

4.     The femoral nerve passes on the lateral mar­gin of the psoas major muscle as far as the inguinal ligament, and through the muscular lacuna to the anterior surface of the thigh. Beneath the inguinal ligament the nerve trunk divides into a number of branches: a primarily sensory ventral group, anterior cutaneous branches for the skin of thigh, a motor branches to the anterior muscles of the thigh, and the saphenous nerve. The femoral nerve supplies the rectus femoris, the vastus medialis, lateralis and the vastus intermedius; the sartorius mus­cle. It gives off small sensory twigs to the capsule of the knee joint and vessels. The saphenous nerve runs into the adduc­tor canal. It is a purely sensory nerve and gives off an intrapatellar branch, to supply the skin below the patella. Then saphenous nerve supplies the skin on the an­terior and medial surfaces of the lower leg, the medial margin of the foot as far as the great toe.

5.     The obturator nerve supplies motor fibers to the adductor muscles (*) of the thigh. The nerve passes through the obturator canal where it extends to the thigh. It supplies a muscular branch to the obturator externus muscle and then divides into a superficial and a deep branches. Cutaneous branch to the skin of the medial surface of the thigh.

(* - adductor longus, adductor brevis mus­cles, pectineus and the gracilis muscles, ad­ductor magnus muscle).

6.     The genitofemoral nerve divides early into two branches, either in or on the psoas muscle. The genital branch passes over the inguinal ligament and innervates the cremaster muscle or the ligamentum teres and supplies the sensory innervation of the skin of the scrotum or the labia majora. The femoral branch passes below the in­guinal ligament and reaches the saphenous hiatus. It supplies the skin of the thigh lateral to the area of the genital branch.

The anterior divisions of the lumbar, sacral, and coccygeal nerves form the lumbosacral plexus, the first lumbar nerve being frequently joined by a branch from the twelfth thoracic. For descriptive purposes this plexus is usually divided into three parts—the lumbar, sacral, and pudendal plexuses.

 

The Lumbar Nerves (Nn. Lumbales)

  The anterior divisions of the lumbar nerves (rami anteriores) increase in size from above downward. They are joined, near their origins, by gray rami communicantes from the lumbar ganglia of the sympathetic trunk. These rami consist of long, slender branches which accompany the lumbar arteries around the sides of the vertebral bodies, beneath the Psoas major. Their arrangement is somewhat irregular: one ganglion may give rami to two lumbar nerves, or one lumbar nerve may receive rami from two ganglia. The first and second, and sometimes the third and fourth lumbar nerves are each connected with the lumbar part of the sympathetic trunk by a white ramus communicans.

  The nerves pass obliquely outward behind the Psoas major, or between its fasciculi, distributing filaments to it and the Quadratus lumborum. The first three and the greater part of the fourth are connected together in this situation by anastomotic loops, and form the lumbar plexus. The smaller part of the fourth joins with the fifth to form the lumbosacral trunk, which assists in the formation of the sacral plexus. The fourth nerve is named the nervus furcalis, from the fact that it is subdivided between the two plexuses.  

The Lumbar Plexus (plexus lumbalis).—The lumbar plexus is formed by the loops of communication between the anterior divisions of the first three and the greater part of the fourth lumbar nerves; the first lumbar often receives a branch from the last thoracic nerve. It is situated in the posterior part of the Psoas major, in front of the transverse processes of the lumbar vertebræ.

Plan of lumbar plexus.

 

  The mode in which the plexus is arranged varies in different subjects. It differs from the brachial plexus in not forming an intricate interlacement, but the several nerves of distribution arise from one or more of the spinal nerves, in the following manner: the first lumbar nerve, frequently supplemented by a twig from the last thoracic, splits into an upper and lower branch; the upper and larger branch divides into the iliohypogastric and ilioinguinal nerves; the lower and smaller branch unites with a branch of the second lumbar to form the genitofemoral nerve. The remainder of the second nerve, and the third and fourth nerves, divide into ventral and dorsal divisions. The ventral division of the second unites with the ventral divisions of the third and fourth nerves to form the obturator nerve. The dorsal divisions of the second and third nerves divide into two branches, a smaller branch from each uniting to form the lateral femoral cutaneous nerve, and a larger branch from each joining with the dorsal division of the fourth nerve to form the femoral nerve. The accessory obturator, when it exists, is formed by the union of two small branches given off from the third and fourth nerves.

The lumbar plexus and its branches.

 

  The branches of the lumbar plexus may therefore be arranged as follows:

Iliohypogastric       1 L.

Ilioinguinal             1 L.

Genitofemoral        1, 2 L.

 

Dorsal divisions.

Lateral femoral cutaneous    2, 3 L.

Femoral                              2, 3, 4 L.

 

Ventral divisions.

Obturator                            2, 3, 4 L.

Accessory obturator            3, 4 L.

 

  The Iliohypogastric Nerve (n. iliohypogastricus) arises from the first lumbar nerve. It emerges from the upper part of the lateral border of the Psoas major, and crosses obliquely in front of the Quadratus lumborum to the iliac crest. It then perforates the posterior part of the Transversus abdominis, near the crest of the ilium, and divides between that muscle and the Obliquus internus abdominis into a lateral and an anterior cutaneous branch.

  The lateral cutaneous branch (ramus cutaneus lateralis; iliac branch) pierces the Obliqui internus and externus immediately above the iliac crest, and is distributed to the skin of the gluteal region, behind the lateral cutaneous branch of the last thoracic nerve the size of this branch bears an inverse proportion to that of the lateral cutaneous branch of the last thoracic nerve.

Deep and superficial dissection of the lumbar plexus.

 

  The anterior cutaneous branch (ramus cutaneus anterior; hypogastric branch) continues onward between the Obliquus internus and Transversus. It then pierces the Obliquus internus, becomes cutaneous by perforating the aponeurosis of the Obliquus externus about 2.5 cm. above the subcutaneous inguinal ring, and is distributed to the skin of the hypogastric region.

  The iliohypogastric nerve communicates with the last thoracic and ilioinguinal nerves.

  The Ilioinguinal Nerve (n. ilioinguinalis), smaller than the preceding, arises with it from the first lumbar nerve. It emerges from the lateral border of the Psoas major just below the iliohypogastric, and, passing obliquely across the Quadratus lumborum and Iliacus, perforates the Transversus abdominis, near the anterior part of the iliac crest, and communicates with the iliohypogastric nerve between the Transversus and the Obliquus internus. The nerve then pierces the Obliquus internus, distributing filaments to it, and, accompanying the spermatic cord through the subcutaneous inguinal ring, is distributed to the skin of the upper and medial part of the thigh, to the skin over the root of the penis and upper part of the scrotum in the male, and to the skin covering the mons pubis and labium majus in the female. The size of this nerve is in inverse proportion to that of the iliohypogastric. Occasionally it is very small, and ends by joining the iliohypogastric; in such cases, a branch from the iliohypogastric takes the place of the ilioinguinal, or the latter nerve may be altogether absent.

Cutaneous nerves of right lower extremity. Front view.

 

 

Diagram of segmental distribution of the cutaneous nerves of the right lower extremity. Front view.

 

  The Genitofemoral Nerve (n. genitofemoralis; genitocrural nerve) arises from the first and second lumbar nerves. It passes obliquely through the substance of the Psoas major, and emerges from its medial border, close to the vertebral column, opposite the fibrocartilage between the third and fourth lumbar vertebræ; it then descends on the surface of the Psoas major, under cover of the peritoneum, and divides into the external spermatic and lumboinguinal nerves. Occasionally these two nerves emerge separately through the substance of the Psoas.

  The external spermatic nerve (n. spermaticus externus; genital branch of genitofemoral) passes outward on the Psoas major, and pierces the fascia transversalis, or passes through the abdominal inguinal ring; it then descends behind the spermatic cord to the scrotum, supplies the Cremaster, and gives a few filaments to the skin of the scrotum. In the female, it accompanies the round ligament of the uterus, and is lost upon it.

  The lumboinguinal nerve (n. lumboinguinalis; femoral or crural branch of genitofemoral) descends on the external iliac artery, sending a few filaments around it, and, passing beneath the inguinal ligament, enters the sheath of the femoral vessels, lying superficial and lateral to the femoral artery. It pierces the anterior layer of the sheath of the vessels and the fascia lata, and supplies the skin of the anterior surface of the upper part of the thigh.  On the front of the thigh it communicates with the anterior cutaneous branches of the femoral nerve. A few filaments from the lumboinguinal nerve may be traced to the femoral artery.

  The Lateral Femoral Cutaneous Nerve (n. cutaneus femoralis lateralis; external cutaneous nerve) arises from the dorsal divisions of the second and third lumbar nerves. It emerges from the lateral border of the Psoas major about its middle, and crosses the Iliacus obliquely, toward the anterior superior iliac spine. It then passes under the inguinal ligament and over the Sartorius muscle into the thigh, where it divides into two branches, and anterior and a posterior/

  The anterior branch becomes superficial about 10 cm. below the inguinal ligament, and divides into branches which are distributed to the skin of the anterior and lateral parts of the thigh, as far as the knee. The terminal filaments of this nerve frequently communicate with the anterior cutaneous branches of the femoral nerve, and with the infrapatellar branch of the saphenous nerve, forming with them the patellar plexus.

  The posterior branch pierces the fascia lata, and subdivides into filaments which pass backward across the lateral and posterior surfaces of the thigh, supplying the skin from the level of the greater trochanter to the middle of the thigh.

  The Obturator Nerve (n. obturatorius) arises from the ventral divisions of the second, third, and fourth lumbar nerves; the branch from the third is the largest, while that from the second is often very small. It descends through the fibers of the Psoas major, and emerges from its medial border near the brim of the pelvis; it then passes behind the common iliac vessels, and on the lateral side of the hypogastric vessels and ureter, which separate it from the ureter, and runs along the lateral wall of the lesser pelvis, above and in front of the obturator vessels, to the upper part of the obturator foramen. Here it enters the thigh, and divides into an anterior and a posterior branch, which are separated at first by some of the fibers of the Obturator externus, and lower down by the Adductor brevis.

  The anterior branch (ramus anterior) leaves the pelvis in front of the Obturator externus and descends in front of the Adductor brevis, and behind the Pectineus and Adductor longus; at the lower border of the latter muscle it communicates with the anterior cutaneous and saphenous branches of the femoral nerve, forming a kind of plexus. It then descends upon the femoral artery, to which it is finally distributed. Near the obturator foramen the nerve gives off an articular branch to the hipjoint. Behind the Pectineus, it distributes branches to the Adductor longus and Gracilis, and usually to the Adductor brevis, and in rare cases to the Pectineus; it receives a communicating branch from the accessory obturator nerve when that nerve is present.

Nerves of the right lower extremity. Front view.

 

  Occasionally the communicating branch to the anterior cutaneous and saphenous branches of the femoral is continued down, as a cutaneous branch, to the thigh and leg. When this is so, it emerges from beneath the lower border of the Adductor longus, descends along the posterior margin of the Sartorius to the medial side of the knee, where it pierces the deep fascia, communicates with the saphenous nerve, and is distributed to the skin of the tibial side of the leg as low down as its middle.

  The posterior branch (ramus posterior) pierces the anterior part of the Obturator externus, and supplies this muscle; it then passes behind the Adductor brevis on the front of the Adductor magnus, where it divides into numerous muscular branches which are distributed to the Adductor magnus and the Adductor brevis when the latter does not receive a branch from the anterior division of the nerve. It usually gives off an articular filament to the knee-joint.

  The articular branch for the knee-joint is sometimes absent; it either perforates the lower part of the Adductor magnus, or passes through the opening which transmits the femoral artery, and enters the popliteal fossa; it then descends upon the popliteal artery, as far as the back part of the knee-joint, where it perforates the oblique popliteal ligament, and is distributed to the synovial membrane. It gives filaments to the popliteal artery.

  The Accessory Obturator Nerve (n. obturatorius accessorius) is present in about 29 per cent. of cases. It is of small size, and arises from the ventral divisions of the third and fourth lumbar nerves. It descends along the medial border of the Psoas major, crosses the superior ramus of the pubis, and passes under the Pectineus, where it divides into numerous branches. One of these supplies the Pectineus, penetrating its deep surface, another is distributed to the hip-joint; while a third communicates with the anterior branch of the obturator nerve. Occasionally the accessory obturator nerve is very small and is lost in the capsule of the hip-joint. When it is absent, the hip-joint receives two branches from the obturator nerve.

  The Femoral Nerve (n. femoralis; anterior crural nerve) the largest branch of the lumbar plexus, arises from the dorsal divisions of the second, third, and fourth lumbar nerves. It descends through the fibers of the Psoas major, emerging from the muscle at the lower part of its lateral border, and passes down between it and the Iliacus, behind the iliac fascia; it then runs beneath the inguinal ligament, into the thigh, and splits into an anterior and a posterior division. Under the inguinal ligament, it is separated from the femoral artery by a portion of the Psoas major.

  Within the abdomen the femoral nerve gives off small branches to the Iliacus, and a branch which is distributed upon the upper part of the femoral artery; the latter branch may arise in the thigh.

  In the thigh the anterior division of the femoral nerve gives off anterior cutaneous and muscular branches. The anterior cutaneous branches comprise the intermediate and medial cutaneous nerves.

  The intermediate cutaneous nerve (ramus cutaneus anterior; middle cutaneous nerve) pierces the fascia lata (and generally the Sartorius) about 7.5 cm. below the inguinal ligament, and divides into two branches which descend in immediate proximity along the forepart of the thigh, to supply the skin as low as the front of the knee. Here they communicate with the medial cutaneous nerve and the infrapatellar branch of the saphenous, to form the patellar plexus. In the upper part of the thigh the lateral branch of the intermediate cutaneous communicates with the lumboinguinal branch of the genitofemoral nerve.

  The medial cutaneous nerve (ramus cutaneus anterior; internal cutaneous nerve) passes obliquely across the upper part of the sheath of the femoral artery, and divides in front, or at the medial side of that vessel, into two branches, an anterior and a posterior. The anterior branch runs downward on the Sartorius, perforates the fascia lata at the lower third of the thigh, and divides into two branches: one supplies the integument as low down as the medial side of the knee; the other crosses to the lateral side of the patella, communicating in its course with the infrapatellar branch of the saphenous nerve. The posterior branch descends along the medial border of the Sartorius muscle to the knee, where it pierces the fascia lata, communicates with the saphenous nerve, and gives off several cutaneous branches. It then passes down to supply the integument of the medial side of the leg. Beneath the fascia lata, at the lower border of the Adductor longus, it joins to form a plexiform net-work (subsartorial plexus) with branches of the saphenous and obturator nerves. When the communicating branch from the obturator nerve is large and continued to the integument of the leg, the posterior branch of the medial cutaneous is small, and terminates in the plexus, occasionally giving off a few cutaneous filaments. The medial cutaneous nerve, before dividing, gives off a few filaments, which pierce the fascia lata, to supply the integument of the medial side of the thigh, accompanying the long saphenous vein. One of these filaments passes through the saphenous opening; a second becomes subcutaneous about the middle of the thigh; a third pierces the fascia at its lower third.

 

MUSCULAR BRANCHES (rami musculares).—The nerve to the Pectineus arises immediately below the inguinal ligament, and passes behind the femoral sheath to enter the anterior surface of the muscle; it is often duplicated. The nerve to the Sartorius arises in common with the intermediate cutaneous.

  The posterior division of the femoral nerve gives off the saphenous nerve, and muscular and articular branches.

  The Saphenous Nerve (n. saphenus; long or internal saphenous nerve) is the largest cutaneous branch of the femoral nerve. It approaches the femoral artery where this vessel passes beneath the Sartorius, and lies in front of it, behind the aponeurotic covering of the adductor canal, as far as the opening in the lower part of the Adductor magnus. Here it quits the artery, and emerges from behind the lower edge of the aponeurotic covering of the canal; it descends vertically along the medial side of the knee behind the Sartorius, pierces the fascia lata, between the tendons of the Sartorius and Gracilis, and becomes subcutaneous. The nerve then passes along the tibial side of the leg, accompanied by the great saphenous vein, descends behind the medial border of the tibia, and, at the lower third of the leg, divides into two branches: one continues its course along the margin of the tibia, and ends at the ankle; the other passes in front of the ankle, and is distributed to the skin on the medial side of the foot, as far as the ball of the great toe, communicating with the medial branch of the superficial peroneal nerve.

 

BRANCHES.

  —The saphenous nerve, about the middle of the thigh, gives off a branch which joins the subsartorial plexus.

  At the medial side of the knee it gives off a large infrapatellar branch, which pierces the Sartorius and fascia lata, and is distributed to the skin in front of the patella. This nerve communicates above the knee with the anterior cutaneous branches of the femoral nerve; below the knee, with other branches of the saphenous; and, on the lateral side of the joint, with branches of the lateral femoral cutaneous nerve, forming a plexiform net-work, the plexus patellæ. The infrapatellar branch is occasionally small, and ends by joining the anterior cutaneous branches of the femoral, which supply its place in front of the knee.

  Below the knee, the branches of the saphenous nerve are distributed to the skin of the front and medial side of the leg, communicating with the cutaneous branches of the femoral, or with filaments from the obturator nerve.

  The muscular branches supply the four parts of the Quadriceps femoris. The branch to the Rectus femoris enters the upper part of the deep surface of the muscle, and supplies a filament to the hip-joint. The branch to the Vastus lateralis, of large size, accompanies the descending branch of the lateral femoral circumflex artery to the lower part of the muscle. It gives off an articular filament to the knee-joint. The branch to the Vastus medialis descends lateral to the femoral vessels in company with the saphenous nerve. It enters the muscle about its middle, and gives off a filament, which can usually be traced downward, on the surface of the muscle, to the knee-joint. The branches to the Vastus intermedius, two or three in number, enter the anterior surface of the muscle about the middle of the thigh; a filament from one of these descends through the muscle to the Articularis genu and the knee-joint. The articular branch to the hip-joint is derived from the nerve to the Rectus femoris.

  The articular branches to the knee-joint are three in number. One, a long slender filament, is derived from the nerve to the Vastus lateralis; it penetrates the capsule of the joint on its anterior aspect. Another, derived from the nerve to the Vastus medialis, can usually be traced downward on the surface of this muscle to near the joint; it then penetrates the muscular fibers, and accompanies the articular branch of the highest genicular artery, pierces the medial side of the articular capsule, and supplies the synovial membrane. The third branch is derived from the nerve to the Vastus intermedius.

 

 

Theme.  Sacral and coccygeal spinal nerves. Sacral and coccygeal plexus

The sacral plexus is formed from L_IV-L_V, S_I-S_IV ventral rami of the lumbar and sacral spinal nerves. Sacral plexus lies on the piriform muscle and gives off short and long branches.

short branches:

·        small muscular branches pass directly to muscles in the pelvic region: internal obturator, superior and inferior gemelli piriform quadratus femoris muscles

·        superior gluteal nerve passes through the suprapiriform foramen to supply the motor innervation of the medius and minimus gluteus muscle and tensor fasciae latae

·        inferior gluteal nerve leaves the pelvis through the infrapiriform foramen and supplies several branches to the gluteus maximus muscle

·        pudendal nerve leaves the pelvis through the infrapiriform foramen and passes dorsally around the ischial spine to enter the ischiorectal fossa through the lesser sciatic foramen. It gives off here inferior rectal nerves to external anal sphincter, perineal nerves to ischiocavernosus, bulbospondiosus, transversi perinei superficial and profundus muscles and skin. Posterior scrotal (labial) nerves and dorsal penis (clitoridis) nerve supplies external genitals and sphincter urethrae.

long branches:

1.              Posterior femoral cutaneous nerve leaves the pelvis through the infrapiriform foramen and passes below the gluteus maximus muscle to the skin in posterior surface of the thigh. It is a purely sensory nerve and gives off branches to the lower margin of the buttocks, the inferior cluneal nerves, the perineal branches.

2.              Sciatic nerve (the largest in human body) nerve leaves the pelvis through the infrapiriform foramen and passes to­ward the knee beneath the gluteus maximus muscle and the biceps mus­cle. It supplies the long head of the biceps femoris muscle, semitendinosus and semimembranosus. Then it divides into the tibial nerve and common peroneal nerve above the knee joint.

Tibial nerve runs vertically through the middle of the popliteal fossa deep to the gastrocnemius muscle in cruropopliteal canal. The medial sural cutaneous nerve  sepa­rates in the popliteal fossa and runs downward between the two heads of the gastrocnemius muscle (supplies the skin of medial leg region) to combine with the peroneal communicating branch to form the sural nerve. This passes lateral to the Achilles tendon and behind and around the lateral malleolus to reach the lateral margin of the foot. It gives off lateral calcaneal branches to the skin of the lateral side of the heel, and the lateral dorsal cutaneous nerve to the lateral mar­gin of the foot. Motor branches of the tibial nerve arise in the popliteal fossa for: the gastrocnemius muscle, so­leus muscle and plantaris and popliteal muscles the tibialis posterior muscle, flexor digitorum longus muscles and the flexor hallucis longus also the interosseous membrane, the periosteum of the tibia, the ankle joint and tibiofibular joint. Below the medial malleolus tibial nerve divides into two terminal branches, the medial and lateral plantar nerves.

a)          The medial plantar nerve runs in the medial plantar sulcus and innervates the abductor hallucis mus­cle, the flexor digitorum brevis muscle and the flexor hallucis brevis muscles. It divides into the three common plantar digital nerves which supply lumbrical muscles 1 and 2 and divide further into the deep digital plantar nerves that sup­ply the skin of the spaces between the toes from the great to the fourth toe.

b)         The second terminal branch, the lateral plantar nerve runs in the lateral plantar sulcus, supplies the skin of the region of the small toe and to the interos­seous muscles, the adductor hal­lucis muscle and the lateral 3 and 4 lumbrical muscles, flexor digiti minimi brevis muscle.

·        Common peroneal nerve passes along the biceps muscle at the lateral margin of the popliteal fossa to the head of the fibula. It winds around the neck of the fibula toward the anterior surface of the lower leg and enters the peroneus longus muscle. Within the muscle it divides (common peroneal nerve) into the superficial and deep pero­neal nerves. The lateral sural cutaneous nerve sup­plies the skin of the lateral surface of the lower leg, and the communicating medial sural cutaneous nerve that together with the forms the sural nerve.

c)          The superficial peroneal nerve passes between the peroneus longus muscle and the fibula to the dorsum of the foot. It gives off muscular branches to the peroneus longus and brevis muscles. The remainder of the nerve is purely sensory; it divides into two terminal branches, the medial dorsal cutaneous nerve and the intermedi­ate dorsal cutaneous nerve which supply the skin of the dorsum of the foot, with the exception of the space between the great and second toes.

d)         The deep peroneal nerve, which is predominantly motor, turns anteriorly to the anterior mus­cles of the leg and passes to the dorsum of the foot. The deep peroneal nerve gives off several motor branches to the extensor muscles of the lower leg and the foot, to the tibialis anterior muscle, the extensors of the digits (lon­gus and brevis) and the extensors of the great toe (longus and brevis). The terminal branch is sensory and supplies the adjacent surfaces of the interspace between the great and second toes.

 

e)         

 

Coccygeal plexus

The coccygeal plexus is formed from ventral rami of the sacral and coccygeal spinal nerves (S_V , Co_I ). It is located on coccygeal muscle. Anococccygeal nerves start there and innervate the skin in adjacent area.

The anterior divisions of the sacral and coccygeal nerves (rami anteriores) form the sacral and pudendal plexuses. The anterior divisions of the upper four sacral nerves enter the pelvis through the anterior sacral foramina, that of the fifth between the sacrum and coccyx, while that of the coccygeal nerve curves forward below the rudimentary transverse process of the first piece of the coccyx. The first and second sacral nerves are large; the third, fourth, and fifth diminish progressively from above downward. Each receives a gray ramus communicans from the corresponding ganglion of the sympathetic trunk, while from the third and frequently from the second and the fourth sacral nerves, a white ramus communicans is given to the pelvic plexuses of the sympathetic.

 

The Sacral Plexus (plexus sacralis)

—The sacral plexus is formed by the lumbosacral trunk, the anterior division of the first, and portions of the anterior divisions of the second and third sacral nerves.

  The lumbosacral trunk comprises the whole of the anterior division of the fifth and a part of that of the fourth lumbar nerve; it appears at the medial margin of the Psoas major and runs downward over the pelvic brim to join the first sacral nerve. The anterior division of the third sacral nerve divides into an upper and a lower branch, the former entering the sacral and the latter the pudendal plexus.

  The nerves forming the sacral plexus converge toward the lower part of the greater sciatic foramen, and unite to form a flattened band, from the anterior and posterior surfaces of which several branches arise. The band itself is continued as the sciatic nerve. which splits on the back of the thigh into the tibial and common peroneal nerves; these two nerves sometimes arise separately from the plexus, and in all cases their independence can be shown by dissection.

 

Relation.—The sacral plexus lies on the back of the pelvis between the Piriformis and the pelvic fascia in front of it are the hypogastric vessels, the ureter and the sigmoid colon. The superior gluteal vessels run between the lumbosacral trunk and the first sacral nerve, and the inferior gluteal vessels between the second and third sacral nerves.

  All the nerves entering the plexus, with the exception of the third sacral, split into ventral and dorsal divisions, and the nerves arising from these are as follows:

Ventral divisions.

Dorsal divisions.

 

 

 

  The Nerve to the Quadratus Femoris and Gemellus Inferior arises from the ventral divisions of the fourth and fifth lumbar and first sacral nerves: it leaves the pelvis through the greater sciatic foramen, below the Piriformis, and runs down in front of the sciatic nerve, the Gemelli, and the tendon of the Obturator internus, and enters the anterior surfaces of the muscles; it gives an articular branch to the hip-joint.

Plan of sacral and pudendal plexuses.

 

  The Nerve to the Obturator Internus and Gemellus Superior arises from the ventral divisions of the fifth lumbar and first and second sacral nerves. It leaves the pelvis through the greater sciatic foramen below the Piriformis, and gives off the branch to the Gemellus superior, which enters the upper part of the posterior surface of the muscle. It then crosses the ischial spine, reënters the pelvis through the lesser sciatic foramen, and pierces the pelvic surface of the Obturator internus.

  The Nerve to the Piriformis arises from the dorsal division of the second sacral nerve, or the dorsal divisions of the first and second sacral nerves, and enters the anterior surface of the muscle; this nerve may be double.

Dissection of side wall of pelvis showing sacral and pudendal plexuses.

 

  The Superior Gluteal Nerve (n. glutæus superior) arises from the dorsal divisions of the fourth and fifth lumbar and first sacral nerves: it leaves the pelvis through the greater sciatic foramen above the Piriformis, accompanied by the superior gluteal vessels, and divides into a superior and an inferior branch. The superior branch accompanies the upper branch of the deep division of the superior gluteal artery and ends in the Glutæus minimus. The inferior branch runs with the lower branch of the deep division of the superior gluteal artery across the Glutæus minimus; it gives filaments to the Glutæi medius and minimus, and ends in the Tensor fasciæ latæ.

  The Inferior Gluteal Nerve (n. glutæus inferior) arises from the dorsal divisions of the fifth lumbar and first and second sacral nerves: it leaves the pelvis through the greater sciatic foramen, below the Piriformis, and divides into branches which enter the deep surface of the Glutæus maximus.

  The Posterior Femoral Cutaneous Nerve (n. cutaneus femoralis posterior; small sciatic nerve) is distributed to the skin of the perineum and posterior surface of the thigh and leg. It arises partly from the dorsal divisions of the first and second, and from the ventral divisions of the second and third sacral nerves, and issues from the pelvis through the greater sciatic foramen below the Piriformis. It then descends beneath the Glutæus maximus with the inferior gluteal artery, and runs down the back of the thigh beneath the fascia lata, and over the long head of the Biceps femoris to the back of the knee; here it pierces the deep fascia and accompanies the small saphenous vein to about the middle of the back of the leg, its terminal twigs communicating with the sural nerve.

  Its branches are all cutaneous, and are distributed to the gluteal region, the perineum, and the back of the thigh and leg.

  The gluteal branches (nn. clunium inferiores), three or four in number, turn upward around the lower border of the Glutæus maximus, and supply the skin covering the lower and lateral part of that muscle.

  The perineal branches (rami perineales) are distributed to the skin at the upper and medial side of the thigh. One long perineal branch, inferior pudendal (long scrotal nerve), curves forward below and in front of the ischial tuberosity, pierces the fascia lata, and runs forward beneath the superficial fascia of the perineum to the skin of the scrotum in the male, and of the labium majus in the female. It communicates with the inferior hemorrhoidal and posterior scrotal nerves.

  The branches to the back of the thigh and leg consist of numerous filaments derived from both sides of the nerve, and distributed to the skin covering the back and medial side of the thigh, the popliteal fossa, and the upper part of the back of the leg/

  The Sciatic (n. ischiadicus; great sciatic nerve) supplies nearly the whole of the skin of the leg, the muscles of the back of the thigh, and those of the leg and foot. It is the largest nerve in the body, measuring 2 cm. in breadth, and is the continuation of the flattened band of the sacral plexus. It passes out of the pelvis through the greater sciatic foramen, below the Piriformis muscle. It descends between the greater trochanter of the femur and the tuberosity of the ischium, and along the back of the thigh to about its lower third, where it divides into two large branches, the tibial and common peroneal nerves. This division may take place at any point between the sacral plexus and the lower third of the thigh. When it occurs at the plexus, the common peroneal nerve usually pierces the Piriformis.

  In the upper part of its course the nerve rests upon the posterior surface of the ischium, the nerve to the Quadratus femoris, the Obturator internus and Gemelli, and the Quadratus femoris; it is accompanied by the posterior femoral cutaneous nerve and the inferior gluteal artery, and is covered by the Glutæus maximus. Lower down, it lies upon the Adductor magnus, and is crossed obliquely by the long head of the Biceps femoris.

  The nerve gives off articular and muscular branches.

  The articular branches (rami articulares) arise from the upper part of the nerve and supply the hip-joint, perforating the posterior part of its capsule; they are sometimes derived from the sacral plexus.

  The muscular branches (rami musculares) are distributed to the Biceps femoris, Semitendinosus, Semimembranosus, and Adductor magnus. The nerve to the short head of the Biceps femoris comes from the common peroneal part of the sciatic, while the other muscular branches arise from the tibial portion, as may be seen in those cases where there is a high division of the sciatic nerve.

  The Tibial Nerve (n. tibialis; internal popliteal nerve) the larger of the two terminal branches of the sciatic, arises from the anterior branches of the fourth and fifth lumbar and first, second, and third sacral nerves. It descends along the back of the thigh and through the middle of the popliteal fossa, to the lower part of the Popliteus muscle, where it passes with the popliteal artery beneath the arch of the Soleus. It then runs along the back of the leg with the posterior tibial vessels to the interval between the medial malleolus and the heel, where it divides beneath the laciniate ligament into the medial and lateral plantar nerves. In the thigh it is overlapped by the hamstring muscles above, and then becomes more superficial, and lies lateral to, and some distance from, the popliteal vessels;opposite the knee-joint, it is in close relation with these vessels, and crosses to the medial side of the artery. In the leg it is covered in the upper part of its course by the muscles of the calf; lower down by the skin, the superficial and deep fasciæ. It is placed on the deep muscles, and lies at first to the medial side of the posterior tibial artery, but soon crosses that vessel and descends on its lateral side as far as the ankle. In the lower third of the leg it runs parallel with the medial margin of the tendo calcaneus.

Cutaneous nerves of right lower extremity. Posterior view.

 

Diagram of the segmental distribution of the cutaneous nerves of the right lower extremity. Posterior view.

 

  The branches of this nerve are: articular, muscular, medial sural cutaneous, medial calcaneal, medial and lateral plantar.

  Articular branches (rami articulares), usually three in number, supply the knee-joint; two of these accompany the superior and inferior medial genicular arteries; and a third, the middle genicular artery. Just above the bifurcation of the nerve an articular branch is given off to the ankle-joint.

  Muscular branches (rami musculares), four or five in number, arise from the nerve as it lies between the two heads of the Gastrocnemius muscle; they supply that muscle, and the Plantaris, Soleus, and Popliteus. The branch for the Popliteus turns around the lower border and is distributed to the deep surface of the muscle. Lower down, muscular branches arise separately or by a common trunk and supply the Soleus, Tibialis posterior, Flexor digitorum longus, and Flexor hallucis longus; the branch to the last muscle accompanies the peroneal artery; that to the Soleus enters the deep surface of the muscle.

  The medial sural cutaneous nerve (n. cutaneus suræ medialis; n. communicans tibialis) descends between the two heads of the Gastrocnemius, and, about the middle of the back of the leg, pierces the deep fascia, and unites with the anastomotic ramus of the common peroneal to form the sural nerve.

 

Nerves of the right lower extremity Posterior view.

   The sural nerve (n. suralis; short saphenous nerve), formed by the junction of the medial sural cutaneous with the peroneal anastomotic branch, passes downward near the lateral margin of the tendo calcaneus, lying close to the small saphenous vein, to the interval between the lateral malleolus and the calcaneus. It runs forward below the lateral malleolus, and is continued as the lateral dorsal cutaneous nerve along the lateral side of the foot and little toe, communicating on the dorsum of the foot with the intermediate dorsal cutaneous nerve, a branch of the superficial peroneal. In the leg, its branches communicate with those of the posterior femoral cutaneous.

  The medial calcaneal branches (rami calcanei mediales; internal calcaneal branches) perforate the laciniate ligament, and supply the skin of the heel and medial side of the sole of the foot.

  The medial plantar nerve (n. plantaris medialis; internal plantar nerve) the larger of the two terminal divisions of the tibial nerve, accompanies the medial plantar artery. From its origin under the laciniate ligament it passes under cover of the Abductor hallucis, and, appearing between this muscle and the Flexor digitorum brevis, gives off a proper digital plantar nerve and finally divides opposite the bases of the metatarsal bones into three common digital plantar nerves.

 

BRANCHES.—The branches of the medial plantar nerve are: (1) cutaneous, (2) muscular, (3) articular, (4) a proper digital nerve to the medial side of the great toe, and (5) three common digital nerves.

  The cutaneous branches pierce the plantar aponeurosis between the Abductor hallucis and the Flexor digitorum brevis and are distributed to the skin of the sole of the foot.

  The muscular branches supply the Abductor hallucis, the Flexor digitorum brevis, the Flexor hallucis brevis, and the first Lumbricalis; those for the Abductor hallucis and Flexor digitorum brevis arise from the trunk of the nerve near its origin and enter the deep surfaces of the muscles; the branch of the Flexor hallucis brevis springs from the proper digital nerve to the medial side of the great toe, and that for the first Lumbricalis from the first common digital nerve.

  The articular branches supply the articulations of the tarsus and metatarsus.

  The proper digital nerve of the great toe (nn. digitales plantares proprii; plantar digital branches) supplies the Flexor hallucis brevis and the skin on the medial side of the great toe.

  The three common digital nerves (nn. digitales plantares communes) pass between the divisions of the plantar aponeurosis, and each splits into two proper digital nerves—those of the first common digital nerve supply the adjacent sides of the great and second toes; those of the second, the adjacent sides of the second and third toes; and those of the third, the adjacent sides of the third and fourth toes. The third common digital nerve receives a communicating branch from the lateral plantar nerve; the first gives a twig to the first Lumbricalis. Each proper digital nerve gives off cutaneous and articular filaments; and opposite the last phalanx sends upward a dorsal branch, which supplies the structures around the nail, the continuation of the nerve being distributed to the ball of the toe. It will be observed that these digital nerves are similar in their distribution to those of the median nerve in the hand.

  The Lateral Plantar Nerve (n. plantaris lateralis; external plantar nerve) supplies the skin of the fifth toe and lateral half of the fourth, as well as most of the deep muscles, its distribution being similar to that of the ulnar nerve in the hand. It passes obliquely forward with the lateral plantar artery to the lateral side of the foot, lying between the Flexor digitorum brevis and Quadratus plantæ and, in the interval between the former muscle and the Abductor digiti quinti, divides into a superficial and a deep branch. Before its division, it supplies the Quadratus plantæ and Abductor digiti quinti.

  The superficial branch (ramus superficialis) splits into a proper and a common digital nerve; the proper digital nerve supplies the lateral side of the little toe, the Flexor digiti quinti brevis, and the two Interossei of the fourth intermetatarsal space; the common digital nerve communicates with the third common digital branch of the medial plantar nerve and divides into two proper digital nerves which supply the adjoining sides of the fourth and fifth toes.

The plantar nerves.

 

Diagram of the segmental distribution of the cutaneous nerves of the sole of the foot.

 

  The deep branch (ramus profundus; muscular branch) accompanies the lateral plantar artery on the deep surface of the tendons of the Flexor muscles and the Adductor hallucis, and supplies all the Interossei (except those in the fourth metatarsal space), the second, third, and fourth Lumbricales, and the Adductor hallucis.

  The Common Peroneal Nerve (n. peronæus communis; external popliteal nerve; peroneal nerve) about one-half the size of the tibial, is derived from the dorsal branches of the fourth and fifth lumbar and the first and second sacral nerves. It descends obliquely along the lateral side of the popliteal fossa to the head of the fibula, close to the medial margin of the Biceps femoris muscle. It lies between the tendon of the Biceps femoris and lateral head of the Gastrocnemius muscle, winds around the neck of the fibula, between the Peronæus longus and the bone, and divides beneath the muscle into the superficial and deep peroneal nerves. Previous to its division it gives off articular and lateral sural cutaneous nerves.

  The articular branches (rami articulares) are three in number; two of these accompany the superior and inferior lateral genicular arteries to the knee; the upper one occasionally arises from the trunk of the sciatic nerve. The third (recurrent) articular nerve is given off at the point of division of the common peroneal nerve; it ascends with the anterior recurrent tibial artery through the Tibialis anterior to the front of the knee.

  The lateral sural cutaneous nerve (n. cutaneus suræ lateralis; lateral cutaneous branch) supplies the skin on the posterior and lateral surfaces of the leg; one branch, the peroneal anastomotic (n. communicans fibularis), arises near the head of the fibula, crosses the lateral head of the Gastrocnemius to the middle of the leg, and joins with the medial sural cutaneous to form the sural nerve. The peroneal anastomotic is occasionally continued down as a separate branch as far as the heel.

  The Deep Peroneal Nerve (n. peronæus profundus; anterior tibial nerve) begins at the bifurcation of the common peroneal nerve, between the fibula and upper part of the Peronæus longus, passes obliquely forward beneath the Extensor digitorum longus to the front of the interosseous membrane, and comes into relation with the anterior tibial artery above the middle of the leg; it then descends with the artery to the front of the ankle-joint, where it divides into a lateral and a medial terminal branch. It lies at first on the lateral side of the anterior tibial artery, then in front of it, and again on its lateral side at the ankle-joint.

  In the leg, the deep peroneal nerve supplies muscular branches to the Tibialis anterior, Extensor digitorum longus, Peronæus tertius, and Extensor hallucis prop ius, and an articular branch to the ankle-joint.

  The lateral terminal branch (external or tarsal branch) passes across the tarsus, beneath the Extensor digitorum brevis, and, having become enlarged like the dorsal interosseous nerve at the wrist, supplies the Extensor digitorumbrevis. From the enlargement three minute interosseous branches are given off, which supply the tarsal joints and the metatarsophalangeal joints of the second, third, and fourth toes. The first of these sends a filament to the second Interosseus dorsalis muscle.

Deep nerves of the front of the leg.

 

  The medial terminal branch (internal branch) accompanies the dorsalis pedis artery along the dorsum of the foot, and, at the first interosseous space, divides into two dorsal digital nerves (nn. digitales dorsales hallucis lateralis et digiti secundi medialis) which supply the adjacent sides of the great and second toes, communicating with the medial dorsal cutaneous branch of the superficial peroneal nerve. Before it divides it gives off to the first space an interosseous branch which supplies the metatarsophalangeal joint of the great toe and sends a filament to the first Interosseous dorsalis muscle.

  The Superficial Peroneal Nerve (n. peronæus superficialis; musculocutaneous nerve) supplies the Peronei longus and brevis and the skin over the greater part of the dorsum of the foot. It passes forward between the Peronæi and the Extensor digitorum longus, pierces the deep fascia at the lower third of the leg, and divides into a medial and an intermediate dorsal cutaneous nerve. In its course between the muscles, the nerve gives off muscular branches to the Peronæi longus and brevis, and cutaneous filaments to the integument of the lower part of the leg.

Nerves of the dorsum of the foot.

 

  The medial dorsal cutaneous nerve (n. cutaneus dorsalis medialis; internal dorsal cutaneous branch) passes in front of the ankle-joint, and divides into two dorsal digital branches, one of which supplies the medial side of the great toe, the other, the adjacent side of the second and third toes. It also supplies the integument of the medial side of the foot and ankle, and communicates with the saphenous nerve, and with the deep peroneal nerve.

  The intermediate dorsal cutaneous nerve (n. cutaneus dorsalis intermedius; external dorsal cutaneous branch), the smaller, passes along the lateral part of the dorsum of the foot, and divides into dorsal digital branches, which supply the contiguous sides of the third and fourth, and of the fourth and fifth toes. It also supplies the skin of the lateral side of the foot and ankle, and communicates with the sural nerve. The branches of the superficial peroneal nerve supply the skin of the dorsal surfaces of all the toes excepting the lateral side of the little toe, and the adjoining sides of the great and second toes, the former being supplied by the lateral dorsal cutaneous nerve from the sural nerve, and the latter by the medial branch of the deep peroneal nerve. Frequently some of the lateral branches of the superficial peroneal are absent, and their places are then taken by branches of the sural nerve.

 

The Pudendal Plexus (plexus pudendus) —The pudendal plexus is not sharply marked off from the sacral plexus, and as a consequence some of the branches which spring from it may arise in conjunction with those of the sacral plexus. It lies on the posterior wall of the pelvis, and is usually formed by branches from the anterior divisions of the second and third sacral nerves, the whole of the anterior divisions of the fourth and fifth sacral nerves, and the coccygeal nerve.

  It gives off the following branches:

 

 

 

Sacral plexus of the right side.

 

  The Perforating Cutaneous Nerve (n. clunium inferior medialis) usually arises from the posterior surface of the second and third sacral nerves. It pierces the lower part of the sacrotuberous ligament, and winding around the inferior border of the Glutæus maximus supplies the skin covering the medial and lower parts of that muscle.

  The perforating cutaneous nerve may arise from the pudendal or it may be absent; in the latter case its place may be taken by a branch from the posterior femoral cutaneous nerve or by a branch from the third and fourth, or fourth and fifth, sacral nerves.

  The Pudendal Nerve (n. pudendus; internal pudic nerve) derives its fibers from the ventral branches of the second, third, and fourth sacral nerves. It passes between the Piriformis and Coccygeus muscles and leaves the pelvis through the lower part of the greater sciatic foramen. It then crosses the spine of the ischium, and reënters the pelvis through the lesser sciatic foramen. It accompanies the internal pudendal vessels upward and forward along the lateral wall of the ischiorectal fossa, being contained in a sheath of the obturator fascia termed Alcock’s canal, and divides into two terminal branches, viz., the perineal nerve, and the dorsal nerve of the penis or clitoris. Before its division it gives off the inferior hemorrhoidal nerve.

  The inferior hemorrhoidal nerve (n. hæmorrhoidalis inferior) occasionally arises directly from the sacral plexus; it crosses the ischiorectal fossa, with the inferior hemorrhoidal vessels, toward the anal canal and the lower end of the rectum, and is distributed to the Sphincter ani externus and to the integument around the anus. Branches of this nerve communicate with the perineal branch of the posterior femoral cutaneous and with the posterior scrotal nerves at the forepart of the perineum.

  The perineal nerve (n. perinei), the inferior and larger of the two terminal branches of the pudendal, is situated below the internal pudendal artery. It accompanies the perineal artery and divides into posterior scrotal (or labial) and muscular branches.

  The posterior scrotal (or labial) branches (nn. scrotales (or labiales) posteriores; superficial peroneal nerves) are two in number, medial and lateral. They pierce the fascia of the urogenital diaphragm, and run forward along the lateral part of the urethral triangle in company with the posterior scrotal branches of the perineal artery; they are distributed to the skin of the scrotum and communicate with the perineal branch of the posterior femoral cutaneous nerve. These nerves supply the labium majus in the female.

  The muscular branches are distributed to the Transversus perinæi superficialis, Bulbocavernous, Ischiocavernosus, and Constrictor urethræ. A branch, the nerve to the bulb, given off from the nerve to the Bulbocavernosus, pierces this muscle, and supplies the corpus cavernosum urethræ, ending in the mucous membrane of the urethra.

  The dorsal nerve of the penis (n. dorsalis penis) is the deepest division of the pudendal nerve; it accompanies the internal pudendal artery along the ramus of the ischium; it then runs forward along the margin of the inferior ramus of the pubis, between the superior and inferior layers of the fascia of the urogenital diaphragm. Piercing the inferior layer it gives a branch to the corpus cavernosum penis, and passes forward, in company with the dorsal artery of the penis, between the layers of the suspensory ligament, on to the dorsum of the penis, and ends on the glans penis. In the female this nerve is very small, and supplies the clitoris (n. dorsalis clitoridis).

  The Visceral Branches arise from the third and fourth, and sometimes from the second, sacral nerves, and are distributed to the bladder and rectum and, in the female, to the vagina; they communicate with the pelvic plexuses of the sympathetic.

  The Muscular Branches are derived from the fourth sacral, and supply the Levator ani, Coccygeus, and Sphincter ani externus. The branches to the Levator ani and Coccygeus enter their pelvic surfaces; that to the Sphincter ani externus (perineal branch) reaches the ischiorectal fossa by piercing the Coccygeus or by passing between it and the Levator ani. Cutaneous filaments from this branch supply the skin between the anus and the coccyx.

      Anococcygeal Nerves (nn. anococcygei).—The fifth sacral nerve receives a communicating filament from the fourth, and unites with the coccygeal nerve to form the coccygeal plexus. From this plexus the anococcygeal nerves take origin; they consist of a few fine filaments which pierce the sacrotuberous ligament to supply the skin in the region of the coccyx.

Phylogenesis of the nervous system¹ briefly amounts to the following. The single-celled protozoa (the amoeba) have no nervous system and their connection with the environment is accomplished by means of fluids present both in and outside of the organism. This is the humoral, preneural form of control.

Later, when the nervous system originates, another form of control, i.e. neural control, appears. With the gradual development of the nervous system the humoral control becomes more and more subordinate to neural control so that a single neurohumoral control forms in which the nervous system plays the leading role. In the process of phylogenesis the nervous system passes through a series of principal stages (Fig. 89).

Stage I, a network nervous system. In this stage (the coelenterata) the nervous system, e.g. that of the hydra, is formed of nerve cells whose numer­ous processes are interconnected in different directions to form a network

 

diffusely piercing the whole body of the animal. In stimulation of any point of the body, the stimulus spreads along the entire nervous network and the animal responds by movement of the whole body. The network structure of the intramural nervous system in man is a reflection of this stage (E.K. Sepp).

Stage II, a ganglionic nervous system. In this stage (higher worms) the nerve cells come together to form aggregations or groups; the aggregations of cell bodies form the nerve ganglia, the centres, whereas aggregations of the processes form trunks, the nerves. The number of processes in each cell reduces and they acquire a definite direction. According to the segmental structure of the animal’s body, e.g. in the segmented worm, each segment has segmental nerve ganglia and nerve trunks. The trunks join the ganglia in two directions: the transverse trunks connect the ganglia of the given segment while the longitudinal trunks join the ganglia of different segments. As a result, nerve impulses occurring in any point of the body spread not over the whole body but only along the transverse trunks within the given segment. The longitu-

dinal trunks unite the nerve segments into a single whole. Sensory organs develop in the animal’s head end, which comes in contact with different objects in the environment when the animal moves forward due to which the head ganglia are more developed than the other ganglia and are a proto­type of the future brain. The persistence in man of primitive features (scat­tered ganglia and microganglia on the periphery) in the structure of the vegetative nervous system is a reflection of this stage.

Type III, a tubular nervous system. The motor apparatus played an especially significant role in the initial developmental stage of animals. The principal condition for the animal’s existence, nutrition, depends on the perfection of this apparatus (movement in search of food, its grasping and' absorption).

The peristaltic mode of movement developed in the lower multicellular animals, which was associated with the smooth musculature and. its local nerve apparatus. In a higher developmental stage peristaltic movement was replaced by skeletal motion, i.e. movements by means of a system of rigid levers occurring over the muscles (arthropods) and inside the muscles (ver­tebrates). A consequence of this was the formation of the striated musculature and the central nervous system which coordinated the movement of the separate levers of the motor skeleton.

Such a central nervous system occurred in the chordates (the lancelet) in the form of a metameric neural tube giving off segmental nerves to all body segments, including the motor apparatus: this is the truncal cord. In vertebrates and man it becomes the spinal cord. The appearance of the truncal cord is therefore linked with the perfection of the animal’s motor equipment in the first place. In addition, receptors (olfactory and light) are already present in the lancelet. Further development of the nervous system and the origin of the brain are predominantly determined by the perfection of the receptor equipment.

Since most sensory organs arise in the end of the animal’s body facing in the direction of the movement, i.e. forward, to perceive the external impulses arriving through them the anterior end of the truncal cord develops and the brain forms, which coincides with the differentiation of the anterior part of the body into the head (cephalization, Gk kephale head).

In his manual on nervous diseases E. K. Sepp gives a diagram of brain phylogenesis, which is simplified but convenient for study. We represent it here. According to this diagram, in the first developmental stage, the brain consists of three parts: the hindbrain, the midbrain, and the forebrain. Of these parts, the hindbrain, or the rhombencephalon, develops predominantly (in lower fishes) under the effect of the acoustic and static receptors (receptors of the eighth pair of cranial nerves) which are of principal importance for the orientation in water.

During further evolution the hindbrain differentiates into the medulla oblongata which is a transitional part from the spinal cord to the brain and is also called the myelencephalon (Gk myelos marrow, egkephalos brain), and the hindbrain proper, the metencephalon from which the cerebellum and pons, develop.

In the process of adaptation of the organism to the environment by changes in metabolism, centres for the control of vitally important processes of vegetative life develop in the hindbrain which is the best developed part of the central nervous system in this stage; these centres are linked in par­ticular with the branchial apparatus (respiration, blood circulation, diges­tion, etc.). That is why the nuclei of the branchial nerves originate in the medulla oblongata (a group of the tenth pair, the vagus nerve). These vitally important centres of respiration and circulation remain in the human medulla oblongata. This explains why death can occur after an injury to the medulla. In stage II (the stage of fish development) the midbrain, the mesencephalon, develops in particular under the effect of the visual receptor. In stage III the olfactory receptor develops intensely due to the final change from life in water to life in air. This receptor perceives the content of chemical substances in the atmosphere whose odour signals prey, danger, and other vitally impor­tant phenomena of the environment.

The forebrain, the prosencephalon, develops under the action of the olfactory receptor and is at first concerned purely with the sense of smell. Later it grows and differentiates into the diencephalon and the telencephalon. According to Sepp, both these parts develop simultaneously.

Centres of all types of sensitivity appear in the telencephalon as it is a higher part of the central nervous system. Centres located below it do not disappear, however, but remain in subordination to those occurring at a higher level. Consequently, with each new stage of brain development new centres arise to which the old centres become subordinate. Hence, the function­al centres migrate, as it were, to the cephalic end with simultaneous subordin­ation of the phylogenetically old rudiments to them. As a result the auditory centres which first emerged in the hindbrain are also present in the mid­brain and forebrain; the centres of vision which first originated in the mid­brain are also found in the forebrain, whereas the olfactory centres are situated only in the forebrain. The olfactory receptor induces the development of a small part of the forebrain called, therefore, the rhinencephalon (Gk rhis nose, egkephalos brain); it is covered by a cortex of grey matter (palaeocortex).

Perfection of the receptors leads to progressive development of the forebrain which gradually becomes an organ controlling the animal’s behav­iour. Two forms of the animal’s behaviour are distinguished: instinctive, based on species reactions (unconditioned reflexes), and individual behaviour, based on the individual’s experience (conditioned reflexes). These two types of behaviour show that two groups of grey matter centres develop in the hindbrain: subcortical centres possessing the structure of nuclei (nuclear centres) and the grey matter cortex having the structure of a continuous screen (screen centres). The subcortex develops first and the cortex after it. The cortex emerged when the animals changed from life in water to life on the ground and is clearly detected in amphibians and reptiles. Further evolution of the nervous system is marked by the growing subordination of the activity of all lower located centres to the cerebral cortes; gradual corticalization of functions occurs. Pavlov claimed that the more perfect the nervous system of an animal, the more it is centralized, its higher part supervising to an ever­growing extent the activity of the organism, though this is never strikingly or openly demonstrated.

A new cortex lying on the surface of the hemispheres and acquiring a six-layer structure in the process of phylogenesis is a necessary formation for the accomplishment of higher nervous activity. Due to the intensive development of the new cortex the endbrain in higher vertebrates surpasses all the other parts of the brain and covers them like a mantle (the pallium). The developing new brain (neencephalon) pushes the old (olfactory) brain deeper which folds, as it were, to form the cornu Ammonii, or pes hippocampi, which remains the olfactory centre. As a result the pallium, i.e. the new brain sharply predominates over the other parts of the brain, the old brain (palaeo- encephalon).

Thus, the brain develops under the influence of the development of the receptors. This explains the fact that the higher part of the brain, the grey matter cortex, is, according to Pavlov, an aggregate of the cortical ends of the analysers, i.e. a continuous perceiving (receptor) surface. Further develop­ment of the human brain is subordinate to other laws associated with man’s social nature. In addition to the natural body organs (also possessed by animals) man uses instruments of labour. These instruments have become artificial organs and have supplemented the natural body organs to comprise the technical equipment of man.

With the help of this equipment man can not only adapt himself to nature, as is the case in animals, but can adapt nature to his needs. As it is pointed out in Volume I, labour proved to be the decisive factor in the for­mation of man, whereas in the process of social labour speech developed as a necessary means for establishing contact between people. “First labour, after it, and then with it, articulate speech—these were the two most essential stimuli under the influence of which the brain of the apes gradually changed into that of man, which for all its similarity to the former is far larger and more perfect.”¹ This perfection is a consequence of the maximum develop­ment of the telencephalon, its cortex in particular (the new cortex, neo­cortex).

In addition to the analysers that perceive different stimuli of the external environment and compose the material substrate of concrete-visual thinking inherent in animals (the first signalling system of reality, according to Pavlov), man developed the property of abstract thinking by means of the word, first heard (uttered speech) and later seen (written speech). This made up the second signalling system which, according to Pavlov, is an extraordinary addition to the mechanisms of nervous activity. The superficial layers of the new cortex became the material substrate of the second signalling system. That is why the telencephalon attains highest development in man. The whole evolution of *he nervous system, therefore, amounts to progressive development of the telencephalon, which becomes very large in higher ver­tebrates and especially in man due to sophistication of the nervous activity.