A text-book of veterinary anatomy

THE ARTICULATIONS OR JOINTS

An articulation or joint is formed by the union of two or more bones or cartilages by other tissue. Bone is the fundamental part of most joints; in some cases a bone and a cartilage, or two cartilages, form a joint. The uniting medium is chiefly fibrous tissue or cartilage, or a mixture of these. Union of parts of the skeleton by muscles (Synsarcosis), as in the attachment of the thoracic limb in the horse, will not be considered in this section.

Joints may be classified—(a) anatomically, according to their mode of development, the nature of the uniting media, and the form of the joint surfaces; (b) physiologically, with regard to the amount and kind of movement or the absence of mobility in them; (c) by a combination of the foregoing considerations.

The classification of joints is still in a very unsatisfactory state, and, unfortunately, the same term is used in various senses by different authors. The two main subdivisions proposed by Hepburn are: (1) Those in which the uniting medium is coextensive with the opposed joint surfaces, and in which a direct union of these surfaces is thereby effected. (2) Those in which the uniting medium has undergone interruption in its structural continuity, and in which a cavity of greater or less extent is thus formed in the interior of the joint. This distinction is of considerable importance clinically.

Three chief subdivisions of joints are usually recognized—viz., synarthroses, diarthroses, and amphiarthroses.

SYNARTHROSES

In this group the segments are united by fibrous tissue or cartilage, or a mixture of the two in such a manner as practically to preclude movement; hence they are often termed fixed or immovable joints. There is no joint cavity. Many of these joints are temporary, the uniting medium being invaded by the process of ossification, with a resulting ankylosis or synostosis. The chief classes in this group of joints are as follows:

(1) Suture.—This term (Sutura) is applied to those joints in the skull in which the adjacent bones are closely united by fibrous tissue—the sutural ligament. In many cases the edges of the bones have irregular interlocking margins, forming the sutura serrata, e. g., the frontal suture. In others the edges are beveled and overlap, forming the sutura squamosa, e. g., the parieto-temporal suture. If the edges are plane or slightly roughened, the term sutura harmonia is applied to the joint, e. g., the nasal suture.

(2) Synchondrosis.—In these the two bones are united by cartilage, e. g., the joint between the basioccipital and the sphenoid bone. Very few of these joints are permanent.

(3) Symphysis.—This term is usually limited to a few median joints which connect symmetrical parts of the skeleton, e. g., symphysis pelvis, symphysis mandibulæ. The uniting media are cartilage and fibrous tissue. In some cases a cleft-like rudimentary joint cavity occurs.

(4) Gomphosis.—This term is applied to the implantation of the teeth in the alveoli.

These joints are characterized by the presence of a joint cavity and by their mobility. They are often called movable or true joints. The structures which enter into their formation are:

1. The joint surfaces (Facies articulares), which are usually more or less expanded. They are in most cases smooth, and vary much in form. They are formed of specially dense bone, which differs histologically from ordinary compact substance. In certain cases (vide Osteology) the surface is interrupted by non-articular cavities known as synovial fossæ.

2. The articular cartilages (Cartilagines articulares), usually hyaline in type, form a covering over the articular surfaces of the bones. They vary in thickness in different joints; they are thickest on the areas of the greatest pressure, and usually tend to accentuate the curvature of the bone, i. e., on a concave surface the peripheral part is the thickest, while on a convex surface the central part is the thickest. The articular cartilages are non-vascular, very smooth, and have a bluish tinge in the fresh state. They diminish the effects of concussion and greatly reduce friction.

3. The joint capsule (Capsula articularis) is, in its simplest form, a sac, the margins of which are attached around the articulating surfaces. It consists of two layers—an external one, composed of fibrous tissue (Stratum fibrosum), and an internal one, the synovial layer or membrane (Stratum synoviale). The fibrous layer, sometimes termed the capsular ligament, is attached either close to the margins of the articular surfaces or at a variable distance from them. Its thickness varies greatly in different situations: in certain places it is extremely thick, and sometimes cartilage or bone develops in it; in other places it is practically absent, the capsule then consisting only of the synovial membrane. Parts of the capsule may undergo thickening and so form ligaments, which are not separable, except artificially, from the rest of the capsule. The synovial layer lines the joint cavity except where this is bounded by the articular cartilages; it stops normally at the margin of the latter. It is a thin membrane, and is richly supplied by close networks of vessels and nerves. It frequently forms folds (Plicæ synoviales) and villi (Villi synoviales), which project into the cavity of the joint. The folds commonly contain pads of fat which fill up interstices and vary in form and position in various phases of movement. The synovial membrane secretes a fluid, the synovia, which resembles white-of-egg and lubricates the joint.^[28] In many places the membrane forms extra-articular pouches, which facilitate the play of muscles and tendons.

The joint cavity (Cavum articulare) is inclosed by the synovial membrane and the articular cartilages. Normally, it is, strictly speaking, only a potential cavity, which contains nothing but a small amount of synovia.

The student must guard against a false conception of the joint cavity which may result from dissections and diagrams, in which an actual cavity of considerable extent appears to exist. A correct idea of the intimate apposition of the parts is best obtained from the study of frozen sections. It is also instructive to examine joints which have been injected so as to distend the capsule fully. It is then seen that the cavity is often of much greater potential extent than one might suppose, and that the capsule is often very irregular in form, i. e., forms a variety of sacculations.

The foregoing are constant and necessary features in all diarthroses. Other structures which enter into the formation of these joints are ligaments, articular discs or menisci, and marginal cartilages.

4. Ligaments.—These are strong bands or membranes, usually composed of white, fibrous tissue, which bind the bones together. They are pliable, but practically inelastic. In a few cases, however, e. g., ligamenta flava, ligamentum nuchæ, they are composed of elastic tissue. They may be subdivided, according to position, into periarticular and intraarticular. Periarticular ligaments are frequently blended with or form part of the fibrous capsule; in other cases they are quite distinct. Strictly speaking, intraarticular ligaments, though within the fibrous capsule, are not in the joint cavity; the synovial membrane is reflected over them. The term seems justifiable, however, on practical grounds. Ligaments which connect directly opposed surfaces of bones are termed interosseous. The special names are based usually on their position, form, and attachments, e. g., lateral or collateral, cruciate, sacro-iliac, etc. In many places muscles, tendons, and thickenings of the fasciæ function as ligaments and increase the security of the joint. Atmospheric pressure and cohesion play a considerable part in keeping the joint surfaces in apposition.

5. Articular discs or menisci are plates of fibro-cartilage or dense fibrous tissue placed between the articular cartilages, and divide the joint cavity partially or completely into two compartments. They render certain surfaces congruent, e. g., femoro-tibial joint, allow greater range or variety of movement, and diminish concussion.

6. A marginal cartilage (Labrum glenoidale) is a ring of fibro-cartilage which encircles the rim of an articular cavity. It enlarges the cavity and tends to prevent fracture of the margin.

Vessels and Nerves.—The arteries form anastomoses around the larger joints, and give off branches to the extremities of the bones and to the joint capsule. The synovial membrane has a close-meshed network of capillaries; the latter form loops around the margins of the articular cartilages, but do not usually enter them. The veins form plexuses. The synovial membrane is also well supplied with lymphatics. Nerve-fibers are especially numerous in and around the synovial membrane and there are special nerve-endings, e. g., Pacinian bodies and the articular end-bulbs described by Krause.

Movements.—The movements of a joint are determined chiefly by the form and extent of the joint surfaces and the arrangement of the ligaments. They are usually classified as follows:

1. Gliding.—This refers to the sliding of one practically plane surface on another, as in the joints between the articular processes of the cervical vertebræ.

2. Angular Movements.—In these cases there is movement around one or more axes. Motion which diminishes the angle included by the segments forming the joint is termed flexion, while that which tends to bring the segments into line with each other is called extension.

With reference to the joints of the lower parts of the limbs, it seems advisable to employ the terms dorsal and volar or plantar flexion, since these joints can be “overextended.” Similarly, the terms dorsal and ventral flexion should be applied to the corresponding movements of the spinal column. The meaning of the term lateral flexion is evident. These movements are all rotations around axes which are approximately either transverse or vertical. Depression, elevation, and lateral movement of the lower jaw fall in this category.

3. Circumduction.—This designates movements in which the distal part of the limb describes a circle or a segment of one. In man such movement is easily performed, but in quadrupeds it is possible to a limited degree only, and is to be regarded usually as an indication of disease.

4. Rotation.—As a matter of convenience, this term is reserved to indicate rotation of one segment around the longitudinal axis of the other segment forming the joint. It is seen typically in the atlanto-axial joint.

5. Adduction and abduction designate respectively movement of a limb toward and away from the median plane, or of a digit toward and away from the axis of the limb.

Classification.—This is based on the form of the joint surfaces and the movements which occur. The following chief classes may be recognized:

1. Arthrodia, or gliding joint. In these the surfaces are practically flat, admitting of gliding movement. Examples: carpo-metacarpal joints; joints between the articular processes of the cervical and thoracic vertebræ.

2. Ginglymus, or hinge-joint. In this class the joint surfaces consist usually of two condyles, or of a segment of a cylinder or cone, which are received by corresponding cavities. In typical cases the movements are flexion and extension, i. e., around a single transverse axis. Examples: occipito-atlantal and elbow joints.

3. Trochoid, or pivot joint. In these the movement is limited to rotation of one segment around the longitudinal axis of the other. Example: atlanto-axial joint.

4. Enarthrosis, or ball-and-socket joint. These are formed by a surface of approximately spherical curvature, received into a corresponding cavity. They are multiaxial, and allow of the greatest variety of movement, e. g., flexion, extension, rotation, abduction, adduction, circumduction. Examples: hip and shoulder joints.^[29]

AMPHIARTHROSES

These joints, as the name indicates, share some characters with both of the preceding groups. In them the segments are directly united by a plate of fibro-cartilage, and usually by ligaments also. The amount and kind of movement are determined by the shape of the joint surfaces and the amount and pliability of the uniting medium.^[30] These joints are all medial in position, and are best illustrated by the joints between the bodies of the vertebræ. There is usually no joint cavity, but in certain situations a rudimentary one exists.

THE ARTICULATIONS OF THE HORSE

Joints and Ligaments of the Vertebræ

The movable vertebræ form two sets of articulations, viz., those formed by the bodies, and those formed by the articular processes of adjacent vertebræ; the former are termed intercentral, and the latter, interneural. Associated with these are ligaments uniting the arches and processes; some of these are special, i. e., confined to a single joint, while others are common, i. e., extend along the entire vertebral column or a considerable part of it. The joints between the atlas and axis and between the former and the skull require separate consideration.

INTERCENTRAL ARTICULATIONS

These are amphiarthroses, formed by the junction of the extremities of the bodies of adjacent vertebræ. The articular surfaces in the cervical region consist of a cavity on the posterior end of the body of the anterior vertebra, and a corresponding convexity or head of the succeeding vertebra. In the other regions the surfaces are much flattened. The uniting media are:

1. The intervertebralfibro-cartilages (Fibrocartilagines intervertebrales). Each of these is a disc which fits into the space between the bodies of two adjacent vertebræ, to which it is intimately attached. The discs are thinnest in the middle of the thoracic region, thicker in the cervical and lumbar regions, and thickest in the coccygeal region. Each consists of a dense fibrous peripheral part (Annulus fibrosus), and a soft pulpy central part (Nucleus pulposus).

The fibrous ring consists of laminæ of fibrous tissue and fibro-cartilage, which pass obliquely between the two vertebræ and alternate in direction, forming an X-shaped arrangement. The central part of the ring is largely cartilaginous, and gradually assumes the character of the pulpy center. The latter is very elastic and is compressed, so that it bulges considerably from the surface of sections; it consists of white and elastic fibers, connective-tissue cells, and peculiar clear, transparent cells of various sizes. It is a remnant of the notochord. There are joint cavities in the cervical intercentral joints, and in those between the last cervical and the first thoracic, and between the last lumbar and the sacrum. In the latter the cavity is coextensive with the extremities of the bodies; in the former, it is usually not so extensive.

2. The inferior common ligament (Ligamentum longitudinale ventrale) lies on the ventral surface of the bodies of the vertebræ and the intervertebral fibro-cartilages, to which it is firmly attached. It begins about the fourteenth or fifteenth thoracic vertebra, and is at first a narrow, thin band. Further back it becomes gradually thicker and wider, and terminates on the pelvic surface of the sacrum by spreading out and blending with the periosteum. It is strongest in the lumbar region, where the tendons of the crura of the diaphragm fuse with it.

3. The superior common ligament (Ligamentum longitudinale dorsale) lies on the floor of the vertebral canal from the dens or odontoid process to the sacrum. It is narrow over the middles of the vertebral bodies, and widens over the intervertebral fibro-cartilages, to which it is very firmly attached.

This ligament is in relation with the spinal veins on either side, and in the middle of each vertebra a transverse anastomotic branch passes under the ligament.

INTERNEURAL ARTICULATIONS

Each typical vertebra presents two pairs of articular processes, which form diarthroses with the two adjacent vertebræ. The articular surfaces are extensive, flat, and oval in the cervical region, small and flat in the thoracic region, while in the lumbar region the anterior ones are concave and the posterior convex. The joint capsule is strong and ample in the cervical region, in conformity with the large size and greater mobility of these joints in the neck. In the thoracic and lumbar regions the capsule is small and close. These joints are arthrodia.

Associated with these joints are the ligamenta flava, which connect the arches of adjacent vertebræ. They are membranous and consist largely of elastic tissue.

The supraspinous ligament extends medially from the occipital bone to the sacrum. From the withers backward it consists of a strong cord of fibrous tissue, attached to the summits of the vertebral spines. In the neck and withers it is remarkably modified to form the ligamentum nuchæ, which requires more extended notice.

The ligamentum nuchæ is a powerful elastic apparatus, the principal function of which is to assist the extensor muscles of the head and neck. It extends from the occipital bone to the withers, where it is directly continuous with the supraspinous ligament. It consists of two parts—funicular and lamellar. The funicular part (Pars occipitalis) arises from the external occipital protuberance and the fossa below it, and is inserted into the summits of the vertebral spines at the withers. Two bursæ are usually found under it in the adult. The supra-atloid bursa lies between the ligament and the dorsal arch of the atlas. The supraspinous bursa occurs usually over the fourth thoracic spine, but may be over the third and may extend to the fifth.^[31] In the neck the funicular part consists for the greater part of two bands closely applied and attached to each other. Near and at the withers it broadens greatly, forming an expansion three to five inches (ca. 8 to 12.5 cm.) in width, the lateral margins of which are thin and overlie the trapezius and rhomboideus muscles. Behind the higher spines it becomes narrower and thinner, and is continued by the white fibrous lumbo-dorsal ligament.^[32] A mass of fat and elastic tissue lies upon the ligament as far back as the withers. It varies greatly in amount in different subjects, and is most developed in stallions of draft breeds, in which it forms the basis of the “crest.” The lamellar portion (Pars cervicalis) consists of two laminæ separated medially by a layer of loose connective tissue. Each lamina is formed of digitations which arise from the second and third thoracic spines and from the funicular portion, are directed downward and forward, and end on the spines of the cervical vertebræ, except the first and last. The digitation which is attached to the spine of the axis is very thick and strong. Behind this they diminish in size and strength; that to the sixth cervical is quite thin and feeble, or may be absent.

The interspinous ligaments (Ligamenta interspinalia) extend between the spines of contiguous vertebræ. In the cervical region they are narrow elastic bands, and in the thoracic and lumbar regions they consist of white fibers directed obliquely downward and forward.

The intertransverse ligaments (Ligamenta intertransversaria) are membranes which connect adjacent transverse processes in the lumbar region.

INTERTRANSVERSE ARTICULATIONS

These joints (peculiar to equidæ) are diarthroses formed by the transverse processes of the last two lumbar vertebræ and the alæ of the sacrum. A similar joint between the fourth and fifth lumbar processes is frequently present. The articular surfaces have an elongated oval form, the anterior one being concave and the posterior one convex. The capsule is tight, and is reinforced ventrally.

SACRAL AND COCCYGEAL ARTICULATIONS

In the foal the bodies of the five sacral vertebræ form joints which resemble somewhat those in the posterior part of the lumbar region. These joints are invaded by the process of ossification early, so that the consolidation of the sacrum is usually complete, or nearly so, at three years.

The coccygeal vertebræ are united by relatively thick intervertebral fibro-cartilages, which have the form of biconcave discs. Special ligaments are not present, but there is a continuous sheath of fibrous tissue. The movement in this region is extensive and varied. In old horses the first coccygeal vertebra is often fused with the sacrum.

MOVEMENTS OF THE VERTEBRAL COLUMN

The movements of the spine, exclusive of those at the atlanto-axial joint, are dorsal, ventral, and lateral flexion, and rotation. The range of movement at a single joint is very small, but the sum of the movements is considerable. The movements are freest in the cervical and coccygeal regions. Rotation is extremely limited in the thoracic and lumbar regions.

ATLANTO-AXIAL ARTICULATION

This is a trochoid or pivot joint, of a rather peculiar character. The articular surfaces are: (1) On the lateral masses of the atlas, two somewhat saddle-shaped facets, which are usually confluent ventrally; (2) on the axis, reciprocal saddle-shaped surfaces which extend upon the odontoid process and are confluent on its ventral aspect. It will be observed that the joint surfaces are not at all accurately adapted to each other, so that only limited areas are in contact at any time.

The joint capsule is attached around the margins of the articular surfaces. It is loose and ample enough laterally to allow extensive movement.

The superior atlanto-axial ligament (Ligamentum interarcuale) is membranous and reinforces the capsule dorsally.

The interspinous ligament (Ligamentum inter-spinale) consists of two elastic bands which extend from the dorsal arch of the atlas to the spine of the axis.

The inferior atlanto-axial ligament (Ligamentum dentis externum) arises from the ventral tubercle of the atlas and is attached by two branches on the ventral spine of the axis.

The odontoid ligament (Ligamentum dentis internum) is short, very strong, and somewhat fan-shaped. It extends from the rough concave dorsal surface of the dens or odontoid process, widens in front, and is attached to the transverse rough area on the inner surface of the ventral arch of the atlas.

Movements.—The atlas and the head rotate upon the axis; the axis of rotation passes through the center of the odontoid process and body of the axis.

THE ATLANTO-OCCIPITAL ARTICULATION

This joint may be classed as a ginglymus. The articular surfaces of this joint are: (1) On the atlas, two deep oval cavities; (2) the corresponding condyles of the occipital bone.

The joint surfaces are oblique, coming very close to the median line ventrally, but separated by a considerable interval dorsally. Posteriorly, a triangular rough area cuts into the middle of each articular surface on the atlas.

There are two roomy joint capsules, which sometimes communicate ventrally, especially in old subjects.

The superior atlanto-occipital membrane extends from the dorsal arch of the atlas to the upper margin of the foramen magnum. It is blended with the capsules and contains many elastic fibers.

The inferior atlanto-occipital membrane extends from the ventral arch of the atlas to the lower margin of the foramen magnum. It is narrower and thinner than the superior membrane, and also fuses with the joint capsules.

The lateral ligaments are two short bands which are partially blended with the capsules. Each is attached to the border of the wing of the atlas near the intervertebral foramen, and to the outer surface of the paramastoid or styloid process of the occipital bone.

Movements.—These are chiefly flexion and extension. A small amount of lateral oblique movement is also possible.

Articulations of the Thorax

COSTO-VERTEBRAL ARTICULATIONS

Each typical rib forms two joints with the vertebral column, one by its head, and one by its tubercle. They are termed respectively costo-central and costo-transverse joints.

I. The costo-central articulations (Articulationes capitulorum) are trochoid or rotatory joints, formed by the junction of the head of the rib with the bodies of two adjacent vertebræ and the intervertebral fibro-cartilage. The two facets on the head of the rib are separated by a non-articular groove, and correspond to the two concave facets (Foveæ costales) on the vertebral bodies. The capsules are rather tight, and are covered by the accessory ligaments, which are as follows: 1. The radiate ligament (Ligamentum capituli costæ radiatum) extends ventrally from the neck of the rib to spread out on the vertebral bodies and the intervertebral fibro-cartilage. 2. The conjugal ligament (Ligamentum conjugale)—absent from the first joint—is attached to the groove on the head of the rib, passes transversely into the vertebral canal, and divides under the superior common ligament into two branches; one of these is attached to the body of the anterior vertebra; the other is continued across to the head of the opposite rib, and is attached to the intervertebral fibro-cartilage. The joint cavity is divided into two compartments by the conjugal ligament. 3. The ligament of the neck of the rib (Ligamentum colli costæ) is a strong band which crosses the joint dorsally. It is attached on the vertebra above the costal facet and on the neck of the rib.

II. The costo-transverse articulations. These occur between the facets on the tubercles of the ribs and those on the transverse processes of the vertebræ. They are gliding joints. The capsule is reinforced by the superior costo-transverse ligament (Ligamentum costo-transversarium dorsale), a distinct strong band which arises on the transverse process and ends on the non-articular part of the tubercle. It is covered by the levator costæ muscle, and begins to be quite distinct at the fifth joint.

The cavity for the head of the first rib is formed by concave facets on the bodies of the last cervical and first thoracic vertebræ. The conjugal ligament is absent, but the ligament of the neck is short and strong. The radiate ligament is very strong, and consists of two bands. In the case of the last two or three ribs the costo-central and costo-transverse joints are confluent, and the various structures are correspondingly modified.

Movements.—The chief movement is rotation around an axis which connects the centers of the head and tubercle of the rib. The movement is very limited in the anterior part of the series of joints, but very considerable in the posterior part.

In the case of the first rib, the movement is evidently extremely limited. The facet for the tubercle of the rib is deeply concave, and the axis of rotation is almost transverse. Further back the facets on the transverse processes become flat, and the axis of rotation gradually approaches a longitudinal direction. This, in connection with the mobility of the ventral ends of the asternal ribs and their elasticity, allows a great increase here in the range of movement, the effect of which is to enlarge (chiefly) the transverse diameter of the thorax.

COSTO-CHONDRAL ARTICULATIONS

The costo-chondral junctions are synarthroses. The rib has a concave surface which receives the convex end of the cartilage. They are united by the continuity of the strong periosteum and perichondrium.

CHONDRO-STERNAL ARTICULATIONS

These joints (Articulationes sternocostales) are diarthroses formed by the junction of the cartilages of the sternal ribs with the sternum. The articular ends of the cartilages (except the first) are somewhat enlarged, and present surfaces of cylindrical curvature. The articular surfaces on the sternum for the first pair of cartilages are placed close together on the dorsal border of the cariniform cartilage (Manubrium sterni); the other seven are placed laterally at the junction of the segments. The capsules are strong and tight; the first pair of joints has a common capsule, and the cartilages articulate with each other medially. The lower ends of the first pair of ribs are firmly attached to each other by dense fibrous tissue, which is prolonged forward along the upper margin of the cariniform cartilage and is continuous behind with the sternal ligament. Each of the other capsules is reinforced dorsally by the superior costo-sternal ligament (Ligamentum sternocostale radiatum), composed of radiating fibers which blend with the sternal ligament. The movement is rotation around a nearly vertical axis, except in the case of the first pair of joints.

INTERCHONDRAL LIGAMENTS

The eighth and ninth costal cartilages are firmly united by fibrous tissue. The chondro-xiphoid ligament attaches the ninth costal cartilage to the xiphoid cartilage. The remaining cartilages are rather loosely attached to each other by elastic tissue.

STERNAL ARTICULATIONS

In the new-born foal the sternum has seven bony segments (Sternebræ), united by persisting cartilage. The last two segments coalesce within a few weeks after birth. In old subjects there is more or less ossification of the intersegmental cartilage, which may lead to partial fusion of adjacent segments, especially posteriorly. The sternal ligament (Ligamentum sterni proprium internum) lies on the thoracic surface of the sternum. It arises on the first segment, and divides opposite the second chondro-sternal joint into three bands. The median band passes backward and spreads out on the last segment and the xiphoid cartilage. The lateral branches—thicker and wider—lie along the lateral borders above the chondro-sternal joints, and end at the cartilage of the eighth rib; they are covered by the transversus thoracis muscle.

The Articulations of the Skull

TEMPORO-MANDIBULAR ARTICULATION

This joint (Articulatio mandibularis) is the only diarthrosis formed between bones of the skull.

The articular surfaces are dissimilar in form and size. That on the squamous temporal bone is concavo-convex, and the long axis is directed outward and somewhat forward; it consists of a glenoid cavity, which is continued upon the postglenoid process behind, and a condyle in front. The mandible presents a transversely elongated condyle.

The articular disc is placed between the joint surfaces, which it renders congruent. Its upper and lower surfaces are molded upon the temporal and mandibular surfaces respectively, and its circumference is attached to the joint capsule; thus it divides the joint cavity into upper and lower compartments, the former being the more roomy.

The joint capsule is strong and tight. It is reinforced by two ligaments. The external ligament (Ligamentum laterale) extends obliquely across the anterior part of the outer surface of the capsule, from which it is not distinctly separable. The posterior ligament (Ligamentum posterius) is an elastic band which is attached above to the postglenoid process, and below to a line on the posterior face of the neck of the mandible.

Movements.—The chief movements take place around a transverse axis passing through both joints. Associated with this hinge-like action is slight gliding movement, as in opening and shutting the mouth. When the mouth is shut, the condyle lies under the glenoid cavity. When the mandible is depressed, the condyle moves forward under the articular eminence of the temporal bone, carrying the disc with it. In protrusion and retraction of the lower jaw the gliding movement just described occurs without the hinge-like rotation of the condyle. These movements are similar in both joints. In the lateral movements (as usually performed in mastication) the action consists of rotation of the condyles around a vertical axis, while the disc glides forward on one side and backward on the other.

THE SYNARTHROSES OF THE SKULL

Most of the bones of the skull are united with the adjacent bones by sutures; a few are united by cartilage. The difference in the uniting medium depends on the fact that most of these bones are developed in membrane, but some are preformed in cartilage. Most of these joints are temporary, and are obliterated at various periods during development and growth. Their importance rests on the fact that so long as they persist, continuous growth is possible. They are usually designated according to the bones which enter into their formation, e. g., spheno-squamous, naso-frontal, internasal, etc. Special names (borrowed from human anatomy) are sometimes used; thus the interparietal, the parieto-occipital, and the parieto-frontal sutures are often called the sagittal, lambdoid, and coronal respectively.

Detailed description of the sutures has not sufficient clinical value to justify much addition to the statements made in the osteology in this connection. The obliteration or closure of the sutures is, however, worthy of brief mention. The cranial sutures are usually all closed at seven years, but the apex only of the petrous temporal is fused with the occipital and squamous temporal. Most of the facial sutures are practically closed at ten years, although complete synostosis may in some be delayed for years or may not occur at all; the nasal suture, for example, often persists even in advanced age, so far as its anterior part is concerned.

The principal synchondroses are: (1) That between the basioccipital and the body of the sphenoid (Synchondrosis sphenooccipitalis); (2) that between the presphenoid and postsphenoid (Synchondrosis intersphenoidalis); (3) those between the parts of the occipital bone (Synchondroses intraoccipitales). The first is usually ossified at four years, the second at three years, and the occipital bone is consolidated at two years.

The symphysis of the lower jaw (Symphysis mandibulæ) ossifies at one to six months.

THE HYOIDEAN ARTICULATIONS

The temporo-hyoid articulation is an amphiarthrosis, in which the dorsal angle of the proximal end of the great cornu (Stylo-hyal) is attached by a short bar of cartilage to the hyoid process of the petrous temporal bone. The cartilage (Arthrohyal) is about half an inch (ca. 1 to 1.5 cm. ) in length. The chief movement is hinge-like, the axis of motion passing transversely through both joints.

The intercornual articulation is an amphiarthrosis formed by the junction of the distal extremity of the great cornu with the proximal end of the small cornu (kerato-hyal). They are united by a very short piece of cartilage, in which there is usually a small nodule of bone in the young subject. This nodule, the epihyal or middle cornu, is usually fused with the great cornu in the adult. The chief movement here is also hinge-like, the angle between the cornua being increased or diminished.

The basi-cornual articulation is a diarthrosis formed by the junction of each small cornu (kerato-hyal) with the body (basi-hyal). The small cornu has a concave facet which articulates with the convex facet on either end of the dorsal surface of the body. The capsule is ample enough to allow considerable movement, which is chiefly hinge-like. The movements of the hyoid bone are concerned chiefly in the acts of mastication and swallowing. In the latter the distal parts of the hyoid bone are moved forward and upward, carrying the root of the tongue and the larynx with them, and then return to their former position.

The Articulations of the Thoracic Limb

In the absence of the clavicle the thoracic limb forms no articulation with the trunk, unless we regard as such the union by muscles. The movement of the shoulder on the chest-wall is chiefly rotation around a transverse axis passing through the scapula behind the upper part of the spine.

THE SHOULDER JOINT

The shoulder or scapulo-humeral joint (Articulatio scapulo-humeralis) is formed by the junction of the distal end of the scapula with the proximal end of the humerus. The articular surfaces are: (1) On the scapula, the glenoid cavity; (2) on the humerus, the head. Both surfaces are approximately spherical and similar in curvature, but the humeral surface is much more extensive than that of the scapula.

The joint capsule is ample enough to allow the bones to be drawn apart about an inch (ca. 2 to 3 cm.); but this requires a very considerable amount of force unless air is admitted into the joint cavity. The fibrous layer (or capsular ligament) is not attached to the margin of the joint surfaces, but at a distance of one to two centimeters from it. It is strengthened in front by two diverging elastic bands, which arise on the scapular tuberosity and end on the lips of the bicipital groove. A pad of fat is interposed between the capsule and the tendon of the biceps.

Ligaments are absent from this joint, but the muscles and tendons around it afford remarkable security, so that dislocation seldom or never occurs. The large extent of the head of the humerus is also of importance in this regard.

The principal muscles which are attached around the joint and act as ligaments are: externally, the supraspinatus, infraspinatus, and teres minor; internally, the subscapularis; in front, the biceps and supraspinatus; behind, the triceps.

Movements.—While it is a typical enarthrosis in structure, and capable of the various movements of the ball-and-socket joint, the chief normal movements are flexion and extension. In the position of rest the angle formed between the scapula and humerus posteriorly is about 110° to 120°; in flexion it is reduced to about 80°, and in extension it is increased to about 145°. Adduction and abduction are very restricted, the former being limited chiefly by the infraspinatus, the latter by the subscapularis and the low insertion of the superficial pectoral muscles. Rotation is somewhat freer, but does not exceed 33°, when all the muscles are removed (Franck).

THE ELBOW JOINT

This, the cubital articulation (Articulatio cubiti), is a ginglymus formed between the distal extremity of the humerus and the proximal ends of the bones of the forearm.