subject determine its shape. As an example of this it has been found that the mobility of the metacarpo-phalangeal joint of the thumb in a large number of working men is less than it is in a large number of women who use needles and thread, or in a large number of medical students who use pens and scalpels, and that the slightly movable thumb has quite a differently shaped articular surface from the freely movable one (see J. Anat. and Phys. xxix. 446). R. Fick, too, has demonstrated that the concavity or convexity of the joint surface depends on the position of the chief muscles which move the joint, and has enunciated the law that when the chief muscle or muscles are attached close to the articular end of the skeletal element that end becomes concave, while, when they are attached far off or are not attached at all, as in the case of the phalanges, the articular end is convex. His mechanical explanation is ingenious and to the present writer convincing (see Handbuch der Gelenke, by R. Fick, Jena, 1904). Bernays, however, pointed out that the articular ends were moulded before the muscular tissue was differentiated (Morph. Jahrb. iv. 403), but to this Fick replies by pointing out that muscular movements begin before the muscle fibres are formed, and may be seen in the chick as early as the second day of incubation.
The freely movable joints (true diarthrosis) are classified as follows:—
(1) Gliding joints (Arthrodia), in which the articular surfaces are flat, as in the carpal and tarsal bones.
(2) Hinge joints (Ginglymus), such as the elbow and interphalangeal joints.
(3) Condyloid joints (Condylarthrosis), allowing flexion and extension as well as lateral movement, but no rotation. The metacarpo-phalangeal and wrist joints are examples of this.
(4) Saddle-shaped joints (Articulus sellaris), allowing the same movements as the last with greater strength. The carpo-metacarpal joint of the thumb is an example.
(5) Ball and socket joints (Enarthrosis), allowing free movement in any direction, as in the shoulder and hip.
(6) Pivot-joint (Trochoides), allowing only rotation round a longitudinal axis, as in the radio-ulnar joints.
Embryology.
Joints are developed in the mesenchyme, or that part of the mesoderm which is not concerned in the formation of the serous cavities. The synarthroses may be looked upon merely as a delay in development, because, as the embryonic tissue of the mesenchyme passes from a fibrous to a bony state, the fibrous tissue may remain along a certain line and so form a suture, or, when chondrification has preceded ossification, the cartilage may remain at a certain place and so form a synchondrosis. The diarthroses represent an arrest of development at an earlier stage, for a part of the original embryonic tissue remains as a plate of round cells, while the neighbouring two rods chondrify and ossify. This plate may become converted into fibro-cartilage, in which case an amphiarthrodial joint results, or it may become absorbed in the centre to form a joint cavity, or, if this absorption occurs in two places, two joint cavities with an intervening meniscus may result. Although, ontogenetically, there is little doubt that menisci arise in the way just mentioned, the teaching of comparative anatomy suggests that, phylogenetically, they originate as an ingrowth from the capsule pushing the synovial membrane in front of them. The subject will be returned to when the comparative anatomy of the individual joints is reviewed. In the human foetus the joint cavities are all formed by the tenth week of intra-uterine life.
Anatomy
Joints of the Axial Skeleton.
The bodies of the vertebrae except those of the sacrum and coccyx are separated, and at the same time connected, by the intervertebral disks. These are formed of alternating concentric rings of fibrous tissue and fibro-cartilage, with an elastic mass in the centre known as the nucleus pulposus. The bodies are also bound together by anterior and posterior common ligaments. The odontoid process of the axis fits into a pivot joint formed by the anterior arch of the atlas in front and the transverse ligament behind; it is attached to the basioccipital bone by two strong lateral check ligaments, and, in the mid line, by a feebler middle check ligament which is regarded morphologically as containing the remains of the notochord. This atlanto-axial joint is the one which allows the head to be shaken from side to side. Nodding the head occurs at the occipito-atlantal joint, which consists of the two occipital condyles received into the cup-shaped articular facets on the atlas and surrounded by capsular ligaments. The neural arches of the vertebrae articulate one with another by the articular facets, each of which has a capsular ligament. In addition to these the laminae are connected by the very elastic ligamenta subflava. The spinous processes are joined by interspinous ligaments, and their tips by a supraspinous ligament, which in the neck is continued from the spine of the seventh cervical vertebra to the external occipital crest and protuberance as the ligamentum nuchae, a thin, fibrous, median septum between the muscles of the back of the neck.
The combined effect of all these joints and ligaments is to allow the spinal column to be bent in any direction or to be rotated, though only a small amount of movement occurs between any two vertebrae.
The heads of the ribs articulate with the bodies of two contiguous thoracic vertebrae and the disk between. The ligaments which connect them are called costo-central, and are two in number. The anterior of these is the stellate ligament, which has three bands radiating from the head of the rib to the two vertebrae and the intervening disk. The other one is the interarticular ligament, which connects the ridge, dividing the two articular cavities on the head of the rib, to the disk; it is absent in the first and three lowest ribs.
The costo-transverse ligaments bind the ribs to the transverse processes of the thoracic vertebrae. The superior costo-transverse ligament binds the neck of the rib to the transverse process of the vertebra above; the middle or interosseous connects the back of the neck to the front of its own transverse process; while the posterior runs from the tip of the transverse process to the outer part of the tubercle of the rib. The inner and lower part of each tubercle forms a diarthrodial joint with the upper and fore part of its own transverse process, except in the eleventh and twelfth ribs. At the junction of the ribs with their cartilages no diarthrodial joint is formed; the periosteum simply becomes perichondrium and binds the two structures together. Where the cartilages, however, join the sternum, or where they join one another, diarthrodial joints with synovial cavities are established. In the case of the second rib this is double, and in that of the first usually wanting. The mesosternal joint, between the pre- and mesosternum, has already been given as an example of a symphysis.
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. In many Mammals the middle fasciculus of the stellate ligament is continued right across the ventral surface of the disk into the ligament of the opposite side, and is probably serially homologous with the ventral arch of the atlas. A similar ligament joins the heads of the ribs dorsal to the disk. To these bands the names of anterior (ventral) and posterior (dorsal) conjugal ligaments have been given, and they may be demonstrated in a seven months’ human foetus (see B. Sutton, Ligaments, London, 1902). 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.
The Jaw Joint, or temporo-mandibular articulation, 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 articularis (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 vertebrates 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 Ruminants the lateralizing movement is the chief one. Sometimes, as in the Ornithorhynchus, the meniscus is absent.
