32 THE INDIVIDUAL ORGANISM 



by the mandible, or lower jaw, and 8 by each of the paired maxillae that 

 form the upper jaw. 



The vertebral column is joined to the skull by a peculiar hinge joint, 

 in which two rounded projections (occipital condyles) on the base of the 

 skull fit into corresponding depressions in the uppermost vertebra and 

 permit a limited rocking motion between the two. This provides the 

 up-and-down nodding motion of our heads. The comparatively restricted 

 rotary motion, used in turning the head to either side, is provided by a 

 pivot joint between the first vertebra and the second. Both motions are 

 limited and restrained by strong ligaments as well as by the muscles of 

 the neck, thus protecting the delicate spinal cord from injury. The spinal 

 cord extends through an opening in the base of the skull into the neural 

 canal of the vertebral column, and would be pinched or twisted by exces- 

 sive movement of the head upon the backbone. 



The whole vertebral column consists of some 33 bones, the vertebrae, 

 arranged in a linear series from the skull to slightly below the pelvic 

 girdle. All the vertebrae are built upon the same general plan, but they 

 are variously modified in different parts of the column. In each vertebra 

 the large central opening, called the neural canal, is bounded ventrally 1 

 by the dorsal face of a large bony cylinder, the centrum, and on all other 

 sides by the neural arch. Projecting from the neural arch are a large dorsal 

 neural spine, a pair of transverse processes, and paired anterior and 

 posterior articular processes. 



Each vertebra is separated from the one above and the one below by 

 thick cartilaginous pads between the centra and by smooth, cartilage- 

 covered sliding joints where the articular processes are in contact. The 

 movement between any two adjacent vertebrae is very slight, being- 

 limited by strong, ligamentous bindings and intricately interlaced mus- 

 cles. Nevertheless the summation of the slight movements between 

 adjacent vertebrae gives considerable flexibility to the spinal column as a 

 whole. 



Notwithstanding the basic similarity between all the vertebrae, the 

 work they have to perform in the several body regions is correlated with 

 differences in proportion and in the details of their structure. Five verte- 

 bral regions may be distinguished. The neck region consists of 7 cervical 

 vertebrae; then comes the thoracic or chest region, with 12 thoracic verte- 



1 In describing anatomical features it is convenient to be able to do so regardless of 

 the position of the body. Venter means "belly," dorsum means "back"; cephalon 

 means "head," and cauda means "tail." Hence "ventral" or "ventrally" means of or 

 toward the belly, regardless of whether the posture is erect as in man or horizontal 

 as in four-footed animals. "Dorsal" pertains to the back, "cephalic" to the head 

 region, and "caudal" to the tail region, or to directions toward those regions. "Upper" 

 and "lower," "front" and "rear" are unsatisfactory terms for anatomical use, since 

 they depend upon external orienting criteria. 



