Motor System — Muscles and Skeleton 111 



to the axis of the column suggests the direction of the dominant strain 

 exerted upon the process. A complete vertebral column (Fig. 107), 

 viewed with special attention to the processes of its vertebrae, is vir- 

 tually a three-dimensional diagram of the mechanical stresses sustained 

 by its various parts. 



Except in fishes, the first one or two vertebrae of the column are 

 more or less modified in relation to the support and movement of the 

 skull. The first vertebra, called the atlas (in ancient Greek mythology 

 a giant named "Atlas" bore the world upon his shoulders), articulates 

 with the one or two rounded prominences, condyles, carried by the 

 occipital bones at the rear of the skull. In reptiles, birds, and mammals, 

 the atlas is a bony ring consisting mainly of a neural arch and having 

 only a vestige of a centrum (Fig. 108A). In the embryo, the atlas de- 

 velops a centrum consisting (as is usual in other vertebrae) of two 

 parts, a larger pleurocentrum and a small hypocentrum. But in the 

 later embryo, the pleurocentrum becomes separated from its own verte- 

 bra and fuses onto the anterior face of the centrum of the second verte- 

 bra, leaving only the hypocentrum as permanent property of the atlas. 

 The annexed pleurocentrum forms a knoblike process (Fig. 108B) 

 which projects forward into the lower part of the large aperture in 

 the ring-shaped atlas — i.e., into the place where the first centrum 

 should be. Known as the odontoid process, it serves as a pivot on 

 which the atlas (and the skull with it) may rotate. The second vertebra 

 is accordingly called the axis (or epistropheus). 



Numerically, vertebrae usually correspond to the myomeres to 

 which the column is immediately adjacent but, in position, the verte- 

 brae and the myomeres alternate (Fig. 109). The vertebrae are inter- 

 segmental. The middle of a centrum is opposite a myoseptum. This 

 arrangement is necessary, at least for such short muscles as retain the 

 primitive segmentation (e.g., as in fishes). To cause bending at a verte- 

 bral joint, a muscle must be attached to the two vertebrae involved in 

 the joint. An exceptional condition is found in the tail of sharklike 

 fishes and a few others, where the number of vertebrae is doubled — 

 i.e., within a given region there are twice as many vertebrae as myo- 

 meres. The vertebrae are complete and typical in structure although 

 they may be shorter than those elsewhere in the column. This condi- 

 tion, known as "diplospondyly," doubtless increases the flexibility 

 of the tail — important because most fishes use the tail as a stern pro- 

 peller, bending it sharply back and forth laterally. 



At the anterior end of the vertebral column, the central nervous 

 cord contained within its neural arches continues forward and expands 



