THE SKELETAL SYSTEM 49 



articulations of bones are of two types — immovable articulations 

 or synarthroses, and movable articulations, diarthroses, or joints. 

 In the former, motion is either absent or, at least, greatly restricted. 

 In the latter, it is definitely provided for through the presence of 

 joint structures. Synarthrosis may be formed by ligamentous 

 union, distinguished as syndesmosis. This is exemplified between 

 carpal or tarsal bones and between the radius and the ulna. The 

 articulation of the bones of the skull (except in the basal region as 

 indicated below), which are fitted together by more or less uneven 

 edges or surfaces, with usually only a small amount of fibrous 

 tissue continuous with the periosteum between, is known as suture, 

 which is thus a special case of syndesmosis. Cartilage union, or 

 synchondrosis, occurs in certain situations, as in the basal region of 

 the skull. Union by fibrocartilage, or symphysis, is found in the 

 articulation of the two sides of the pelvis (symphysis pubis), and 

 in that of the two halves of the mandible. 



In a joint (Fig. 26), the apposed surfaces of the bones are 

 accurately modelled in relation to each other, and are, moreover, 

 covered by layers of cartilage, the articular cartilages, which form 

 joint cushions. Between the two surfaces is a space, the cavity of 

 the joint, containing a viscid material, the synovia, which serves 

 for lubrication. The space is enclosed by a connective tissue 

 capsule continuous with the perichondrium of the articular cartilage 

 or with the periosteum of the bone. Within the capsule, perichon- 

 drium is present at the edges of the articular cartilages but thins 

 out and is absent over the surfaces which actually rub together. 

 The synovia is secreted by the inner or synovial layer of the capsule. 

 The strength of the joint depends largely on the enclosing capsule, 

 but it is usually greatly increased by the presence of accessory 

 ligaments. In the more complex joints, such as that of the knee 

 (Fig. 26, B), interarticular cartilages (menisci) are enclosed between 

 the bone surfaces, and the latter are connected directly by short 

 ligamentous cords. The various ligaments of a joint permit free 

 motion of the bones, but only up' to a certain point, which varies 

 according to the functional needs of each particular joint. 



Several differences are observable in joints according to the 

 form of the apposed surface and the kind of motion provided for. 

 Thus in the ball-and-socket joint or enarthrosis, exemplified by 



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