MUSCLES.] ANATOMY 833 an amphiarthrodial joint. 6, 6, the two bones; c, c, the plate of cartilage on the articular sur face of each bone ; Fc, the inter mediate flbro - cartilage ; /, /, the external ligaments. stoppage of the growth of the skull in breadth, and, by way of compensation, great increase in its length, so as to produce a very elongated and somewhat boat-shaped cranium. The movable joints are divided into the amphiarthrodial and the diarthrodial joints. An amphiarthrosis or half- joint has only a feeble range of movement. It consists of two bones, each of which has its articular surface covered by a plate of cartilage, and which, plates are firmly connected to gether by an intermediate disc of fibro-cartilage. The centre of this Fl . i 5 .-yerticai section through disc is soft, or may even be hollowed out into a cavity, lined by a smooth synovial membrane, and containing a little fluid. Ligamentous bands, continuous with the periosteum invest ing the bones, invest the nbro-cartilage, and assist in bind ing the bones together. The best examples of amphi arthrodial joints are furnished by the articulations between the bodies of the true vertebrae. A diarthrosis admits of more or less perfect movement. In it the two articular surfaces are each covered by a plate of encrusting cartilage, the free surface of which is smooth and polished ; between these surfaces is a cavity containing a glairy fluid, the synovia, for lubricating ., the smooth surfaces of the cartil age and facilitating the movements o O of the joint. This cavity is en closed by ligaments, which are attached to the bones, and the inner surface of these ligaments is lined by a synovial membrane which secretes the synovia. Some- Fic.iG.-yertica]sectionthron B h times a plate or meniscus of fibro- cartilage is interposed between, without, however, being attached to the encrusting cartilages of a diarthrodial joint, so as more or less perfectly to subdivide the cavity enclosed by the ligaments into two spaces. The articular surfaces of diarthrodial joints are retained in apposition with each other, some times by investing ligaments, at others by surrounding muscles and tendons; at others by atmospheric pressure, aided by the adhesive character of the interposed synovia. The form of the articular or mov able surfaces varies very materi ally in different examples of these joints, and the modifications in form determine the direction of the movements of the joints. In some, as the carpal and tarsal joints, the surfaces are almost flat, so that they glide on each other ; the movement is comparatively slight, and about an axis perpen dicular to the moving surfaces : these are called plane- surfaced joints. In other joints the articular surfaces may be regarded as produced by the rotation of a straight or curved line about an axis lying in the same plane ; these are called rotation joints, and they present various modifica tions according to the direction and relation of the rotat ing line to the axis. One form of a rotation joint is the a diarthrodial joint. 6, 6, the two bones; c, c, the plate of cartilage on the articular sur face of each bone ; /, I, the in vesting ligament, the doited line within which represents the synovial membrane. The letter s is placed in the cavity of the joint. f i FIG. 17. Vertical section through a diarthrodial joint, in which the cavity is subdivided into two by an interposed fibre-car tilage or meniscus, Fc. The other letters as in Fig. 16. pivot joint, in which the movement takes place about the axis of one of the bones, which is the axis of rotation of the joint ; examples of this joint are found in the joint between the atlas and odontoid process of the axis and the radio-ulnar joint. Another form is the gvnglymus or hinge joint, in which the axis of rotation of the joint is perpendicular to the axis of the two bones ; the move ments of the hinge are called flexion when the anglo between the two bones is diminished, and extension when the angle is increased. An important modification of the ginglymus is the screwed-surf aced joint, examples of which are found in the elbow and ankle ; here the plane of flexion is not perpendicular, but oblique to the axis of the joint. The saddle-shaped and oblong joints are also modi fied hinges, but allow motion about two axes ; in the oblong both axes are on the same side of the joint ; but in the saddle-shaped there is an axis of rotation on each side of the joint. The best example of the saddle-shaped is found between the metacarpal bone of the thumb and the trape- ziam; of the oblong between the fore-arm and the carpus. In the ball-and-socket joint a spheroidal head fits into a cup, and rotation takes place about any diameter of the sphere; the joint therefore is multi-axial; the hip and shoulder joints are the best examples. Some joints, in which the forms of the articular surfaces are more complex, are called composite; in them the movements of a hinge and of a ball-and-socket joint may be combined; the knee may be cited as an example of this form of articulation. In a large number of movable joints only portions of the opposite articular surfaces are in contact with each other at a given time ; but, as the joint describes its path of movement, different parts of the surfaces come into contact with each other successively, and it is not unusual to find the articular surface both of the cartilage and the sub jacent bone mapped out into distinct areas or facets, which are adapted to corresponding facets on the opposite arti cular surface in particular positions of the joint. When the corresponding facets on opposite articular surfaces break contact with each other, the space between becomes occupied by synovia, or in some joints, more especially the knee, by folds of synovial membrane enclosing clumps of fat, which have been called synovial pads. In the simple hinge, in that with screwed surfaces, in the oblong and composite joints, the principal ligaments are situated at the sides of the joint, and are called lateral ; they not only prevent lateral displacement of the bones, but, by a tightening of their fibres, check excessive movement for wards or backwards during flexion and extension. In the saddle-shaped and ball-and-socket joints, the joint is in cluded within a bag-like ligament called capsular. In the pivot joints the cavity in which the pivot fits is completed by a transverse or a ring-shaped ligament. The MUSCLES are the organs which, by their contraction Muscles, or shortening, move the bones on each other at the joints. The muscles constitute the flesh of the body. They are so arranged as to be capable not only of moving the various bones on each other, but the entire body from place to place. Hence the muscles are organs both of motion and locomotion. As they can be brought into action at the will of the individual, they are called voluntary muscles. Some of the muscles are engaged in the movement of other structures than the bones, such as the eye-ball, tongue, cartilages of the larynx, &c. About 400 muscles are usually enumerated, and the names applied to them express either their position, or relative size, or shape, or direction, or attachments, or mode of action. The word muscle is itself derived from the Latin musculiis, a little mouse, from a fancied resemblance between that animal and some of the most simply formed muscles. It is customary to dis tinguish in a muscle a central part, or belly, and two es-
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