General doctrine about joints.

Ontogenesis of articulations




The bones of the skeleton are joined to one another at different parts of their surfaces, and such connections are termed Joints or Articulations. Where the joints are immovable, as in the articulations between practically all the bones of the skull, the adjacent margins of the bones are almost in contact, being separated merely by a thin layer of fibrous membrane, named the sutural ligament. In certain regions at the base of the skull this fibrous membrane is replaced by a layer of cartilage. Where slight movement combined with great strength is required, the osseous surfaces are united by tough and elastic fibrocartilages, as in the joints between the vertebral bodies, and in the interpubic articulation. In the freely movable joints the surfaces are completely separated; the bones forming the articulation are expanded for greater convenience of mutual connection, covered by cartilage and enveloped by capsules of fibrous tissue. The cells lining the interior of the fibrous capsule form an imperfect membranethe synovial membranewhich secretes a lubricating fluid. The joints are strengthened by strong fibrous bands called ligaments, which extend between the bones forming the joint.  

Bone constitutes the fundamental element of all the joints. In the long bones, the extremities are the parts which form the articulations; they are generally somewhat enlarged; and consist of spongy cancellous tissue with a thin coating of compact substance. In the flat bones, the articulations usually take place at the edges; and in the short bones at various parts of their surfaces. The layer of compact bone which forms the joint surface, and to which the articular cartilage is attached, is called the articular lamella. It differs from ordinary bone tissue in that it contains no Haversian canals, and its lacunæ are larger and have no canaliculi. The vessels of the cancellous tissue, as they approach the articular lamella, turn back in loops, and do not perforate it; this layer is consequently denser and firmer than ordinary bone, and is evidently designed to form an unyielding support for the articular cartilage.  

Cartilage is a non-vascular structure which is found in various parts of the bodyin adult life chiefly in the joints, in the parietes of the thorax, and in various tubes, such as the trachea and bronchi, nose, and ears, which require to be kept permanently open. In the fetus, at an early period, the greater part of the skeleton is cartilaginous; as this cartilage is afterward replaced by bone, it is called temporary, in contradistinction to that which remains unossified during the whole of life, and is called permanent.  Cartilage is divided, according to its minute structure, into hyaline cartilage, white fibrocartilage, and yellow or elastic fibrocartilage.   

Intervertebral Disks

Each intervertebral disk consists of an outer tense annulus fibrous and a soft jelly-like nucleus, the nucleus pulposus which contains remnants of the notochord ("chorda dorsalis"). The annulus fibrosus consists of concentrically arranged collagen fibers and fibrocartilage held under tension by the nucleus pulposus. The intervertebral disks lie between the bodies of the individual vertebrae. In a sagittal section they appear conical. In the cervical and lumbar region they are higher in front and lower behind. The reverse is true in the thoracic region, where disks are lower in front and higher behind. Basically, the thickness of the intervertebral disks increases from the cranial to the caudal region.

The surfaces of the intervertebral disks are covered by hyaline cartilage (remnants of the epiphyses of the vertebral bodies), and are united synchrondrotically to the vertebrae. In addition, the intervertebral disks are also held in position by the longitudinal ligaments (3). The posterior longitudinal ligament is united with the disks over a broad surface, while the anterior longitudinal ligament is only loosely attached to them.

The intervertebral disks and the longitudinal ligaments form a functional entity and together are known as the intervertebral symphysis. Clinical Tips: with increasing age, a reduction in internal nucleus pulposus. This causes lessening of tension in the anulus fibrosus so pressure may result in shrinkage of the it becomes torn more easily. Basically, each tear begins in the region of the nucleus pulposus (Schluter). Radially running tears (caused by excessive loads even in the young) should be distinguished from concentric tears. The latter are associated with degenerative processes. Finally, parts of the intervertebral disk may be displaced. Displacement with invasion of the adjacent vertebral body is known as a "Schmorl's nodule". It is clearly visible in radiographs. Pulposus herniation occurs if the jelly-like nucleus is pushed dorsally and laterally into the vertebral canal after damage to the anulus fibrosus. This may endanger the spinal cord, or individual spinal roots or spinal nerves. Herniation of the nucleus pulposus is commonest between the 3rd and 4th and the 4th and 5th lumbar vertebrae. In addition, it often affects the lowest two cervical intervertebral disks between the 5th and 6th and 6th and 7th vertebrae. Prolapse of a disk (i. e. of the nucleus) develops from a complete rupture of the annulus fibrosus. Reduction in the tension of the anulus fibrosus may lead to a loss of elasticity, followed by invasion of osteoblasts and ossification of parts of the disk. Function: the interveriebral disks act as shock sorbers. The nucleus pulposus distributes the pressure. Loading compresses then and when it is released, they regain their original shape after some time. In movements within the vertebral column the intervertebral disks, as elastic elements, are compressed or stretched unilateral.

Hyaline Cartilage.Hyaline cartilage consists of a gristly mass of a firm consistence, but of considerable elasticity and pearly bluish color. Except where it coats the articular ends of bones, it is covered externally by a fibrous membrane, the perichondrium, from the vessels of which it imbibes its nutritive fluids, being itself destitute of bloodvessels. It contains no nerves. Its intimate structure is very simple. If a thin slice be examined under the microscope, it will be found to consist of cells of a rounded or bluntly angular form, lying in groups of two or more in a granular or almost homogeneous matrix. The cells, when arranged in groups of two or more, have generally straight outlines where they are in contact with each other, and in the rest of their circumference are rounded. They consist of clear translucent protoplasm in which fine interlacing filaments and minute granules are sometimes present; imbedded in this are one or two round nuclei, having the usual intranuclear network. The cells are contained in cavities in the matrix, called cartilage lacunæ; around these the matrix is arranged in concentric lines, as if it had been formed in sucessive portions around the cartilage cells. This constitutes the so-called capsule of the space. Each lacuna is generally occupied by a single cell, but during the division of the cells it may contain two, four, or eight cells. 

The matrix is transparent and apparently without structure, or else presents a dimly granular appearance, like ground glass. Some observers have shown that the matrix of hyaline cartilage, and especially of the articular variety, after prolonged maceration, can be broken up into fine fibrils. These fibrils are probably of the same nature, chemically, as the white fibers of connective tissue. It is believed by some histologists that the matrix is permeated by a number of fine channels, which connect the lacunæ with each other, and that these canals communicate with the lymphatics of the perichondrium, and thus the structure is permeated by a current of nutrient fluid.   6  Articular cartilage, costal cartilage, and temporary cartilage are all of the hyaline variety. They present differences in the size, shape, and arrangement of their cells.   

In Articular Cartilage, which shows no tendency to ossification, the matrix is finely granular; the cells and nuclei are small, and are disposed parallel to the surface in the superficial part, while nearer to the bone they are arranged in vertical rows. Articular cartilages have a tendency to split in a vertical direction; in disease this tendency becomes very manifest. The free surface of articular cartilage, where it is exposed to friction, is not covered by perichondrium, although a layer of connective tissue continuous with that of the synovial membrane can be traced in the adult over a small part of its circumference, and here the cartilage cells are more or less branched and pass insensibly into the branched connective tissue corpuscles of the synovial membrane. Articular cartilage forms a thin incrustation upon the joint surfaces of the bones, and its elasticity enables it to break the force of concussions, while its smoothness affords ease and freedom of movement. It varies in thickness according to the shape of the articular surface on which it lies; where this is convex the cartilage is thickest at the center, the reverse being the case on concave articular surfaces. It appears to derive its nutriment partly from the vessels of the neighboring synovial membrane and partly from those of the bone upon which it is implanted. Toynbee has shown that the minute vessels of the cancellous tissue as they approach the articular lamella dilate and form arches, and then return into the substance of the bone.   8  In Costal Cartilage the cells and nuclei are large, and the matrix has a tendency to fibrous striation, especially in old age. In the thickest parts of the costal cartilages a few large vascular channels may be detected. This appears, at first sight, to be an exception to the statement that cartilage is a non-vascular tissue, but is not so really, for the vessels give no branches to the cartilage substance itself, and the channels may rather be looked upon as involutions of the perichondrium. The xiphoid process and the cartilages of the nose, larynx, and trachea (except the epiglottis and corniculate cartilages of the larynx, which are composed of elastic fibrocartilage) resemble the costal cartilages in microscopic characteristics. The arytenoid cartilage of the larynx shows a transition from hyaline cartilage at its base to elastic cartilage at the apex.   

The hyaline cartilages, especially in adult and advanced life, are prone to calcifythat is to say, to have their matrix permeated by calcium salts without any appearance of true bone. The process of calcification occurs frequently, in such cartilages as those of the trachea and in the costal cartilages, where it may be succeeded by conversion into true bone.   

White fibrocartilage consists of a mixture of white fibrous tissue and cartilaginous tissue in various proportions; to the former of these constituents it owes its flexibility and toughness, and to the latter its elasticity. When examined under the microscope it is found to be made up of fibrous connective tissue arranged in bundles, with cartilage cells between the bundles; the cells to a certain extent resemble tendon cells, but may be distinguished from them by being surrounded by a concentrically striated area of cartilage matrix and by being less flattened. The white fibrocartilages admit of arrangement into four groupsinterarticular, connecting, circumferential, and stratiform.  

The Interarticular Fibrocartilages (menisci) are flattened fibrocartilaginous plates, of a round, oval, triangular, or sickle-like form, interposed between the articular cartilages of certain joints. They are free on both surfaces, usually thinner toward the center than at the circumference, and held in position by the attachment of their margins and extremities to the surrounding ligaments. The synovial membranes of the joints are prolonged over them. They are found in the temporomandibular, sternoclavicular, acromioclavicular, wrist, and knee jointsi. e., in those joints which are most exposed to violent concussion and subject to frequent movement. Their uses are to obliterate the intervals between opposed surfaces in their various motions; to increase the depths of the articular surfaces and give ease to the gliding movements; to moderate the effects of great pressure and deaden the intensity of the shocks to which the parts may be subjected. These interarticular fibrocartilages serve an important purpose in increasing the varieties of movement in a joint. Thus in the knee joint there are two kinds of motion, viz., angular movement and rotation, although it is a hinge joint, in which, as a rule, only one variety of motion is permitted; the former movement takes place between the condyles of the femur and the interarticular cartilages, the latter between the cartilages and the head of the tibia. So, also, in the temporomandibular joint, the movements of opening and shutting the mouth take place between the fibrocartilage and the mandible, the grinding movement between the mandibular fossa and the fibrocartilage, the latter moving with the mandible.  

The Connecting Fibrocartilages are interposed between the bony surfaces of those joints which admit of only slight mobility, as between the bodies of the vertebræ. They form disks which are closely adherent to the opposed surfaces. Each disk is composed of concentric rings of fibrous tissue, with cartilaginous laminæ interposed, the former tissue predominating toward the circumference, the latter toward the center. The Circumferential Fibrocartilages consist of rims of fibrocartilage, which surround the margins of some of the articular cavities, e. g., the glenoidal labrum of the hip, and of the shoulder; they serve to deepen the articular cavities and to protect their edges.  The Stratiform Fibrocartilages are those which form a thin coating to osseous grooves through which the tendons of certain muscles glide. Small masses of fibrocartilage are also developed in the tendons of some muscles, where they glide over bones, as in the tendons of the Peronæus longus and Tibialis posterior.   

The distinguishing feature of cartilage chemically is that it yields on boiling a substance called chondrin, very similar to gelatin, but differing from it in several of its reactions. It is now believed that chondrin is not a simple body, but a mixture of gelatin with mucinoid substances, chief among which, perhaps, is a compound termed chondro-mucoid.   

The Intervertebral Fibrocartilages (fibrocartilagines intervertebrales; intervertebral disks) The intervertebral fibrocartilages are interposed between the adjacent surfaces of the bodies of the vertebræ, from the axis to the sacrum, and form the chief bonds of connection between the vertebræ. They vary in shape, size, and thickness, in different parts of the vertebral column. In shape and size they correspond with the surfaces of the bodies between which they are placed, except in the cervical region, where they are slightly smaller from side to side than the corresponding bodies. In thickness they vary not only in the different regions of the column, but in different parts of the same fibrocartilage; they are thicker in front than behind in the cervical and lumbar regions, and thus contribute to the anterior convexities of these parts of the column; while they are of nearly uniform thickness in the thoracic region, the anterior concavity of this part of the column being almost entirely owing to the shape of the vertebral bodies. The intervertebral fibrocartilages constitute about one-fourth of the length of the vertebral column, exclusive of the first two vertebræ; but this amount is not equally distributed between the various bones, the cervical and lumbar portions having, in proportion to their length, a much greater amount than the thoracic region, with the result that these parts possess greater pliancy and freedom of movement. The intervertebral fibrocartilages are adherent, by their surfaces, to thin layers of hyaline cartilage which cover the upper and under surfaces of the bodies of the vertebræ; in the lower cervical vertebræ, however, small joints lined by synovial membrane are occasionally present between the upper surfaces of the bodies and the margins of the fibrocartilages on either side. By their circumferences the intervertebral fibrocartilages are closely connected in front to the anterior, and behind to the posterior, longitudinal ligaments. In the thoracic region they are joined laterally, by means of the interarticular ligaments, to the heads of those ribs which articulate with two vertebræ.   

Structure of the Intervertebral Fibrocartilages.Each is composed, at its circumference, of laminæ of fibrous tissue and fibrocartilage, forming the annulus fibrosus; and, at its center, of a soft, pulpy, highly elastic substance, of a yellowish color, which projects considerably above the surrounding level when the disk is divided horizontally. This pulpy substance (nucleus pulposus), especially well-developed in the lumbar region, is the remains of the notochord. The laminæ are arranged concentrically; the outermost consist of ordinary fibrous tissue, the others of white fibrocartilage. The laminæ are not quite vertical in their direction, those near the circumference being curved outward and closely approximated; while those nearest the center curve in the opposite direction, and are somewhat more widely separated. The fibers of which each lamina is composed are directed, for the most part, obliquely from above downward, the fibers of adjacent laminæ passing in opposite directions and varying in every layer; so that the fibers of one layer are directed across those of another, like the limbs of the letter X. This laminar arrangement belongs to about the outer half of each fibrocartilage. The pulpy substance presents no such arrangement, and consists of a fine fibrous matrix, containing angular cells united to form a reticular structure.  The intervertebral fibrocartilages are important shock absorbers. Under pressure the highly elastic nucleus pulposus becomes flatter and broader and pushes the more resistant fibrous laminæ outward in all directions.  Articulations of Vertebral Arches.The joints between the articular processes of the vertebræ belong to the arthrodial variety and are enveloped by capsules lined by synovial membranes; while the laminæ, spinous and transverse processes are connected by the following ligaments.

Ligaments.Ligaments are composed mainly of bundles of white fibrous tissue placed parallel with, or closely interlaced with one another, and present a white, shining, silvery appearance. They are pliant and flexible, so as to allow perfect freedom of movement, but strong, tough, and inextensible, so as not to yield readily to applied force. Some ligaments consist entirely of yellow elastic tissue, as the ligamenta flava which connect together the laminæ of adjacent vertebræ, and the ligamentum nuchæ in the lower animals. In these cases the elasticity of the ligament is intended to act as a substitute for muscular power. 

The Articular Capsules.The articular capsules form complete envelopes for the freely movable joints. Each capsule consists of two strataan external (stratum fibrosum) composed of white fibrous tissue, and an internal (stratum synoviale) which is a secreting layer, and is usually described separately as the synovial membrane.   18  The fibrous capsule is attached to the whole circumference of the articular end of each bone entering into the joint, and thus entirely surrounds the articulation.  

The synovial membrane invests the inner surface of the fibrous capsule, and is reflected over any tendons passing through the joint cavity, as the tendon of the Popliteus in the knee, and the tendon of the Biceps brachii in the shoulder. It is composed of a thin, delicate, connective tissue, with branched connective-tissue corpuscles. Its secretion is thick, viscid, and glairy, like the white of an egg, and is hence termed synovia. In the fetus this membrane is said, by Toynbee, to be continued over the surfaces of the cartilages; but in the adult such a continuation is wanting, excepting at the circumference of the cartilage, upon which it encroaches for a short distance and to which it is firmly attached. In some of the joints the synovial membrane is thrown into folds which pass across the cavity; they are especially distinct in the knee. In other joints there are flattened folds, subdivided at their margins into fringe-like processes which contain convoluted vessels. These folds generally project from the synovial membrane near the margin of the cartilage, and lie flat upon its surface. They consist of connective tissue, covered with endothelium, and contain fat cells in variable quantities, and, more rarely, isolated cartilage cells; the larger folds often contain considerable quantities of fat.   

Closely associated with synovial membrane, and therefore conveniently described in this section, are the mucous sheaths of tendons and the mucous bursæ. Mucous sheaths (vaginæ mucosæ) serve to facilitate the gliding of tendons in fibroösseous canals. Each sheath is arranged in the form of an elongated closed sac, one layer of which adheres to the wall of the canal, and the other is reflected upon the surface of the enclosed tendon. These sheaths are chiefly found surrounding the tendons of the Flexor and Extensor muscles of the fingers and toes as they pass through fibroösseous canals in or near the hand and foot.  

Bursæ mucosæ are interposed between surfaces which glide upon each other. They consist of closed sacs containing a minute quantity of clear viscid fluid, and may be grouped, according to their situations, under the headings subcutaneous, submuscular, subfacial, and subtendinous.

Development of the Joints. The mesoderm from which the different parts of the skeleton are formed shows at first no differentiation into masses corresponding with the individual bones. Thus continuous cores of mesoderm form the axes of the limb-buds and a continuous column of mesoderm the future vertebral column. The first indications of the bones and joints are circumscribed condensations of the mesoderm; these condensed parts become chondrified and finally ossified to form the bones of the skeleton. The intervening non-condensed portions consist at first of undifferentiated mesoderm, which may develop in one of three directions. It may be converted into fibrous tissue as in the case of the skull bones, a synarthrodial joint being the result, or it may become partly cartilaginous, in which case an amphiarthrodial joint is formed. Again, it may become looser in texture and a cavity ultimately appear in its midst; the cells lining the sides of this cavity form a synovial membrane and thus a diarthrodial joint is developed.   

The tissue surrounding the original mesodermal core forms fibrous sheaths for the developing bones, i. e., periosteum and perichondrium, which are continued between the ends of the bones over the synovial membrane as the capsules of the joints. These capsules are not of uniform thickness, so that in them may be recognized especially strengthened bands which are described as ligaments. This, however, is not the only method of formation of ligaments. In some cases by modification of, or derivations from, the tendons surrounding the joint, additional ligamentous bands are provided to further strengthen the articulations.   

In several of the movable joints the mesoderm which originally existed between the ends of the bones does not become completely absorbeda portion of it persists and forms an articular disk. These disks may be intimately associated in their development with the muscles surrounding the joint, e. g., the menisci of the knee-joint, or with cartilaginous elements, representatives of skeletal structures, which are vestigial in human anatomy, e. g., the articular disk of the sternoclavicular joint.

Classification of Joints The articulations are divided into three classes: synarthroses or immovable, amphiarthroses or slightly movable, and diarthroses or freely movable, joints.   

Synarthroses (immovable articulations).Synarthroses include all those articulations in which the surfaces of the bones are in almost direct contact, fastened together by intervening connective tissue or hyaline cartilage, and in which there is no appreciable motion, as in the joints between the bones of the skull, excepting those of the mandible. There are four varieties of synarthrosis: sutura, schindylesis, gomphosis, and synchondrosis.   

Sutura.Sutura is that form of articulation where the contiguous margins of the bones are united by a thin layer of fibrous tissue; it is met with only in the skull. When the margins of the bones are connected by a series of processes, and indentations interlocked together, the articulation is termed a true suture (sutura vera); and of this there are three varieties: sutura dentata, serrata, and limbosa. The margins of the bones are not in direct contact, being separated by a thin layer of fibrous tissue, continuous externally with the pericranium, internally with the dura mater. The sutura dentata is so called from the tooth-like form of the projecting processes, as in the suture between the parietal bones. In the sutura serrata the edges of the bones are serrated like the teeth of a fine saw, as between the two portions of the frontal bone. In the sutura limbosa, there is besides the interlocking, a certain degree of bevelling of the articular surfaces, so that the bones overlap one another, as in the suture between the parietal and frontal bones. When the articulation is formed by roughened surfaces placed in apposition with one another, it is termed a false suture (sutura notha), of which there are two kinds: the sutura squamosa, formed by the overlapping of contiguous bones by broad bevelled margins, as in the squamosal suture between the temporal and parietal, and the sutura harmonia, where there is simple apposition of contiguous rough surfaces, as in the articulation between the maxillæ, or between the horizontal parts of the palatine bones. Schindylesis is that form of articulation in which a thin plate of bone is received into a cleft or fissure formed by the separation of two laminæ in another bone, as in the articulation of the rostrum of the sphenoid and perpendicular plate of the ethmoid with the vomer, or in the reception of the latter in the fissure between the maxillæ and between the palatine bones.   

Gomphosis.Gomphosis is articulation by the insertion of a conical process into a socket; this is not illustrated by any articulation between bones, properly so called, but is seen in the articulations of the roots of the teeth with the alveoli of the mandible and maxillæ.   

Synchondrosis.Where the connecting medium is cartilage the joint is termed a synchondrosis. This is a temporary form of joint, for the cartilage is converted into bone before adult life. Such joints are found between the epiphyses and bodies of long bones, between the occipital and the sphenoid at, and for some years after, birth, and between the petrous portion of the temporal and the jugular process of the occipital.

The anterior longitudinal ligament is a broad and strong band of fibers, which extends along the anterior surfaces of the bodies of the vertebræ, from the axis to the sacrum. It is broader below than above, thicker in the thoracic than in the cervical and lumbar regions, and somewhat thicker opposite the bodies of the vertebræ than opposite the intervertebral fibrocartilages. It is attached, above, to the body of the axis, where it is continuous with the anterior atlantoaxial ligament, and extends down as far as the upper part of the front of the sacrum. It consists of dense longitudinal fibers, which are intimately adherent to the intervertebral fibrocartilages and the prominent margins of the vertebræ, but not to the middle parts of the bodies. In the latter situation the ligament is thick and serves to fill up the concavities on the anterior surfaces, and to make the front of the vertebral column more even. It is composed of several layers of fibers, which vary in length, but are closely interlaced with each other. The most superficial fibers are the longest and extend between four or five vertebræ. A second, subjacent set extends between two or three vertebræ while a third set, the shortest and deepest, reaches from one vertebra to the next. At the sides of the bodies the ligament consists of a few short fibers which pass from one vertebra to the next, separated from the concavities of the vertebral bodies by oval apertures for the passage of vessels.  

The Posterior Longitudinal Ligament (ligamentum longitudinale posterius; posterior common ligament). The posterior longitudinal ligament is situated within the vertebral canal, and extends along the posterior surfaces of the bodies of the vertebræ, from the body of the axis, where it is continuous with the membrana tectoria, to the sacrum. It is broader above than below, and thicker in the thoracic than in the cervical and lumbar regions. In the situation of the intervertebral fibrocartilages and contiguous margins of the vertebræ, where the ligament is more intimately adherent, it is broad, and in the thoracic and lumbar regions presents a series of dentations with intervening concave margins; but it is narrow and thick over the centers of the bodies, from which it is separated by the basivertebral veins. This ligament is composed of smooth, shining, longitudinal fibers, denser and more compact than those of the anterior ligament, and consists of superficial layers occupying the interval between three or four vertebræ, and deeper layers which extend between adjacent vertebræ.

The ligamentum nuchæ is a fibrous membrane, which, in the neck, represents the supraspinal ligaments of the lower vertebræ. It extends from the external occipital protuberance and median nuchal line to the spinous process of the seventh cervical vertebra. From its anterior border a fibrous lamina is given off, which is attached to the posterior tubercle of the atlas, and to the spinous processes of the cervical vertebræ, and forms a septum between the muscles on either side of the neck. In man it is merely the rudiment of an important elastic ligament, which, in some of the lower animals, serves to sustain the weight of the head.  

The interspinal ligaments thin and membranous, connect adjoining spinous processes and extend from the root to the apex of each process. They meet the ligamenta flava in front and the supraspinal ligament behind. They are narrow and elongated in the thoracic region; broader, thicker, and quadrilateral in form in the lumbar region; and only slightly developed in the neck.  

The Intertransverse Ligaments (ligamenta intertransversaria).The intertransverse ligaments are interposed between the transverse processes. In the cervical region they consist of a few irregular, scattered fibers; in the thoracic region they are rounded cords intimately connected with the deep muscles of the back; in the lumbar region they are thin and membranous.

The movements permitted in the vertebral column are: flexion, extension, lateral movement, circumduction, and rotation.   16  In flexion, or movement forward, the anterior longitudinal ligament is relaxed, and the intervertebral fibrocartilages are compressed in front; while the posterior longitudinal ligament, the ligamenta flava, and the interand supraspinal ligaments are stretched, as well as the posterior fibers of the intervertebral fibrocartilages. The interspaces between the laminæ are widened, and the inferior articular processes glide upward, upon the superior articular processes of the subjacent vertebræ.

Amphiarthroses (slightly movable articulations).  In these articulations the contiguous bony surfaces are either connected by broad flattened disks of fibrocartilage, of a more or less complex structure, as in the articulations between the bodies of the vertebræ; or are united by an interosseous ligament, as in the inferior tibiofibular articulation. The first form is termed a symphysis, the second a syndesmosis.  

Diarthroses (freely movable articulations).This class includes the greater number of the joints in the body. In a diarthrodial joint the contiguous bony surfaces are covered with articular cartilage, and connected by ligaments lined by synovial membrane. The joint may be divided, completely or incompletely, by an articular disk or meniscus, the periphery of which is continuous with the fibrous capsule while its free surfaces are covered by synovial membrane.

The varieties of joints in this class have been determined by the kind of motion permitted in each. There are two varieties in which the movement is uniaxial, that is to say, all movements take place around one axis. In one form, the ginglymus, this axis is, practically speaking, transverse; in the other, the trochoid or pivot-joint, it is longitudinal. There are two varieties where the movement is biaxial, or around two horizontal axes at right angles to each other, or at any intervening axis between the two. These are the condyloid and the saddle-joint. There is one form where the movement is polyaxial, the enarthrosis or ball-and-socket joint; and finally there are the arthrodia or gliding joints.   

Ginglymus or Hinge-joint.In this form the articular surfaces are moulded to each other in such a manner as to permit motion only in one plane, forward and backward, the extent of motion at the same time being considerable. The direction which the distal bone takes in this motion is seldom in the same plane as that of the axis of the proximal bone; there is usually a certain amount of deviation from the straight line during flexion. The articular surfaces are connected together by strong collateral ligaments, which form their chief bond of union. The best examples of ginglymus are the interphalangeal joints and the joint between the humerus and ulna; the kneeand ankle-joints are less typical, as they allow a slight degree of rotation or of side-to-side movement in certain positions of the limb.   

Trochoid or Pivot-joint (articulatio trochoidea; rotary joint).Where the movement is limited to rotation, the joint is formed by a pivot-like process turning within a ring, or a ring on a pivot, the ring being formed partly of bone, partly of ligament. In the proximal radioulnar articulation, the ring is formed by the radial notch of the ulna and the annular ligament; here, the head of the radius rotates within the ring. In the articulation of the odontoid process of the axis with the atlas the ring is formed in front by the anterior arch, and behind by the transverse ligament of the atlas; here, the ring rotates around the odontoid process.   

Condyloid Articulation (articulatio ellipsoidea).In this form of joint, an ovoid articular surface, or condyle, is received into an elliptical cavity in such a manner as to permit of flexion, extension, adduction, abduction, and circumduction, but no axial rotation. The wrist-joint is an example of this form of articulation.   

Articulation by Reciprocal Reception (articulatio sellaris; saddle-joint).In this variety the opposing surfaces are reciprocally concavo-convex. The movements are the same as in the preceding form; that is to say, flexion, extension, adduction, abduction, and circumduction are allowed; but no axial rotation. The best example of this form is the carpometacarpal joint of the thumb.   

Spherical joitns /Enarthrosis/ (ball-and-socket joints).Enarthrosis is a joint in which the distal bone is capable of motion around an indefinite number of axes, which have one common center. It is formed by the reception of a globular head into a cup-like cavity, hence the name ball-and-socket. Examples of this form of articulation are found in the hip and shoulder. 

Gliding joint is a joint which admits of only gliding movement; it is formed by the apposition of plane surfaces, or one slightly concave, the other slightly convex, the amount of motion between them being limited by the ligaments or osseous processes surrounding the articulation. It is the form present in the joints between the articular processes of the vertebræ, the carpal joints, except that of the capitate with the navicular and lunate, and the tarsal joints with the exception of that between the talus and the navicular.

The Kind of Movement. Admitted in Joints the movements admissible in joints may be divided into four kinds: gliding and angular movements, circumduction, and rotation. These movements are often, however, more or less combined in the various joints, so as to produce an infinite variety, and it is seldom that only one kind of motion is found in any particular joint.   Gliding movement is the simplest kind of motion that can take place in a joint, one surface gliding or moving over another without any angular or rotatory movement. It is common to all movable joints; but in some, as in most of the articulations of the carpus and tarsus, it is the only motion permitted. This movement is not confined to plane surfaces, but may exist between any two contiguous surfaces, of whatever form.  

Angular movement occurs only between the long bones, and by it the angle between the two bones is increased or diminished. It may take place: forward and backward, constituting flexion and extension; or toward and from the median plane of the body, or, in the case of the fingers or toes, from the middle line of the hand or foot, constituting adduction and abduction. The strictly ginglymoid or hinge-joints admit of flexion and extension only. Abduction and adduction, combined with flexion and extension, are met with in the more movable joints; as in the hip, the shoulder, the wrist, and the carpometacarpal joint of the thumb.   

Circumduction is that form of motion which takes place between the head of a bone and its articular cavity, when the bone is made to circumscribe a conical space; the base of the cone is described by the distal end of the bone, the apex is in the articular cavity; this kind of motion is best seen in the shoulder and hip-joints.  

Rotation is a form of movement in which a bone moves around a central axis without undergoing any displacement from this axis; the axis of rotation may lie in a separate bone, as in the case of the pivot formed by the odontoid process of the axis vertebræ around which the atlas turns; or a bone may rotate around its own longitudinal axis, as in the rotation of the humerus at the shoulder-joint; or the axis of rotation may not be quite parallel to the long axis of the bone, as in the movement of the radius on the ulna during pronation and supination of the hand, where it is represented by a line connecting the center of the head of the radius above with the center of the head of the ulna below.  

Ligamentous Action of Muscles.The movements of the different joints of a limb are combined by means of the long muscles passing over more than one joint. These, when relaxed and stretched to their greatest extent, act as elastic ligaments in restraining certain movements of one joint, except when combined with corresponding movements of the otherthe latter movements being usually in the opposite direction. Thus the shortness of the hamstring muscles prevents complete flexion of the hip, unless the knee-joint is also flexed so as to bring their attachments nearer together. The uses of this arrangement are three fold: It coordinates the kinds of movements which are the most habitual and necessary, and enables them to be performed with the least expenditure of power. It enables the short muscles which pass over only one joint to act upon more than one. It provides the joints with ligaments which, while they are of very great power in resisting movements to an extent incompatible with the mechanism of the joint, at the same time spontaneously yield when necessary. In the thoracic region, notably in its upper part, all the movements are limited in order to reduce interference with respiration to a minimum. The almost complete absence of an upward inclination of the superior articular surfaces prohibits any marked flexion, while extension is checked by the contact of the inferior articular margins with the laminæ, and the contact of the spinous processes with one another. The mechanism between the seventh cervical and the first thoracic vertebræ, which limits extension of the cervical region, will also serve to limit flexion of the thoracic region when the neck is extended. Rotation is free in the thoracic region: the superior articular processes are segments of a cylinder whose axis is in the mid-ventral line of the vertebral bodies. The direction of the articular facets would allow of free lateral flexion, but this movement is considerably limited in the upper part of the region by the resistance of the ribs and sternum.   


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