XII. PHYSIOLOGY OF SPECIAL MUSCULAR 

 MECHANISMS. 



A. THE ACTION OP LOCOMOTOR MECHANISMS. 



The Articulations. The form, posture, and movements of vertebrates 

 are largely determined by the structure of the skeleton and the method of 

 union of the bones of which it is composed. There are two hundred bones in 

 the human skeleton, and they are so connected together as to be immovable, 

 or to allow of many varieties and degrees of motion. There are four prin- 

 cipal methods of articulation : 



1. Union by Bony Substance (Sutures). This form of union occurs 

 between the bones of the skull. These bones, which at birth are independent 

 structures connected by fibrous tissue, gradually grow together and make 

 a continuous whole, only a more or less distinct searn remaining as witness 

 of the original condition. 



2. Union by Fibro-Cartilages (Symphyses). The bodies of the verte- 

 brae and the pelvic bones are closely bound together by disks of fibre-cartilage. 

 This material, which is very strong, but yielding and elastic, permits of a 

 slight amount of movement when the force applied is considerable, and restores 

 the bones to their original position on the removal of the force. The inter- 

 vertebral disks act, moreover, as elastic cushions or buffers to deaden the 

 effect of sudden jars. 



3. Union of Fibrous Bands (Syndesmoses). Some of the bones, as of 

 the carpus and tarsus, are connected by interosseous ligaments which, at the 

 same time that they bind the bones together, admit of a certain amount of 

 play, the extent of the movement varying with the character of the surfaces 

 and the length of the ligaments. 



4. Union by Joints. The adjacent surfaces of most of the bones are so 

 formed as to permit of close contact and freedom of movement in special 

 directions. The parts of the bones entering into the joint are clothed with 

 very smooth cartilage, and the joint-surfaces are lubricated by synovial fluid, 

 a viscid liquid secreted by a delicate membrane which lines the fibrous capsule 

 by which the joint is surrounded. The joint-capsule is firmly attached to the 

 bones at the margin of the articular cartilages, and, at the same time that it 

 completely surrounds and isolates the joint-cavity, helps to bind the bones 

 together. The bones are further united by strong ligaments, in some cases 

 within and in other cases without the capsule. These ligaments are so placed 

 that they are relaxed in certain positions of the joints and tightened in others ; 



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