Comparative Anatomy. For the convexity or concavity of the vertebral centra in different classes of vertebrates, see SKELETON : Axial. The intervertebral disks first appear in the Crocodilia, the highest existing order of reptilia. The ligamentum nuchae is a strong elastic band in the Ungulata which supports the weight of the head. In the Carnivora it only reaches as far forward as the spine of the axis.
This occurs between the sigmoid cavity of the temporal bone and the condyle of the jaw. Between the two there is an interarticular fibro-cartilage or meniscus, and the joint is surrounded by a capsule of which the outer part is the thickest. On first opening the mouth, the joint acts as a hinge, but very soon the condyle begins to glide forward on to the eminentia artic ularis (see SKULL) and takes the meniscus with it. This gliding movement between the meniscus and temporal bone may be separately brought about by protruding the lower teeth in front of the upper, or, on one side only, by moving the jaw across to the opposite side.
Comparative Anatomy. The joint between the temporal and mandibular bones is only found in Mammals ; in the lower verte brates the jaw opens between the quadrate and articular bones. In the Carnivora it is a perfect hinge ; in many Rodents only the antero-posterior gliding movement is present ; while in the Rumi nants the lateralizing movement is the chief one. Sometimes, as in the Ornithorhynchus, the meniscus is absent.
The sterno-clavicular artic ulation is a gliding joint with an intervening meniscus, and allows slight upward and downward and forward and backward move ments. There is a well-marked capsule, of which the anterior part is strongest. The two clavicles are joined across the top of the presternum by an interclavicular ligament.
The acromio-clavicular articulation is also a gliding joint, but allows a swinging or pendulum movement of the scapula on the clavicle. The upper part of the capsule is strongest, and from it hangs down a partial meniscus into the cavity.
Comparative Anatomy. Bland Sutton regards the inter-clavicu lar ligament as a vestige of the interclavicle of Reptiles and Mono tremes. The menisci are only found in the Primates, but it must be borne in mind that many Mammals have no clavicle, or a very rudimentary one.
This is a good example of the ball and socket variety. Its most striking characteristic is mobility at the expense of strength. The small size of the glenoid cavity in com parison with the head of the humerus, and the great laxity of the capsule, favour this, although the glenoid cavity is slightly deepened by a fibrous lip, round its margin. The coracoid and acromial processes of the scapula, with the coraco-acromial liga ment between them, serve as an overhanging protection to the joint, while the biceps tendon runs over the head of the humerus, inside the capsule, though surrounded by a sheath of synovial membrane. Were it not for these two extra safeguards the shoulder would be even more liable to dislocation than it is. In side the front of the capsule are three folds of synovial membrane, called gleno-humeral folds.
Comparative Anatomy. In the lower Vertebrates the shoulder is adapted to support rather than prehension and is not so freely movable as in the Primates. The tendon of the biceps has evi dently sunk through the capsule into the joint, and even when it is intra-capsular there is usually a double f old connecting its sheath of synovial membrane with that lining the capsule. In Man this has been broken through, but remains of it persist in the superior gleno-humeral fold. The middle gleno-liumeral fold is the vestige of a strong ligament, which steadies and limits the range of movement of the joint in many lower Mammals.
This is a hinge, though its transverse axis of movement is not quite at right angles to the central axis of the limb, but is lower internally than externally. This tends to bring the forearm towards the body when the elbow is bent. The elbow is a great contrast to the shoulder, as the trochlea and capitellum of the humerus are closely adapted to the sigmoid cavity of the ulna and head of the radius (see SKELETON : Appendicular) ; con sequently movement in one plane only is allowed, and the joint is a strong one. The capsule is really continuous. The joint cavity communicates freely with that of the superior radio-ulnar articu lation.