IV. 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 (Suture). — 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 seam remaining as witness 

 of the original condition. 



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

 bra' and the sacro-iliac and pubic bones are closely bound together by disks 

 of fibro-cartilage. This material, which is very strong, but yielding and 

 elastic, acts as a butler to deaden the effect of jars, 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 spinal 

 column can be thought of as an elastic staff; the capacity for movement 

 differs greatly in different regions, however, partly on account of differences 

 in the thickness of the intervertebral disks as compared with the antero- 

 posterior and lateral diameters of the bodies of the vertebrae, and more espe- 

 cially on account of the method of contact of the superior and inferior verte- 

 bral processes. In the cervical region the disks are thick and the diameter 

 of the vertebras is small, and this permits of considerable bending in all 

 directions and a certain amount of rotation. In the dorsal region a slight 

 amount of bending from side to side and a slight amount of rotation are pos- 

 sible ; but backward bending is inhibited by contact of the articular processes, 

 and forward bending is prevented by the strong articular ligaments. In the 

 lumbar region bending in all directions is more free, but rotation is made 

 impossible by the interlocking of the articular processes. 1 



3. Union of Fibrous Bands (Syndesmosis). — Some of the bones, as those 

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

 -.iiiii' time that they bind the bones together, admit of a certain amount of 



1 Kick : (.'oiiipfiiiliiim tier Phygiologie drs Menxchen, "Wien, 1891. 

 •Ill 



