THE NERVOUS SYSTEM 1 27 



C. SPINAL CORD AND SPINAL NERVES 



(Figs. 24, 32) 



The spinal cord is essentially a continuation of the brain 

 into the spinal column, in which the proportion of white to gray 

 material becomes progressively less from the cranial portion 

 caudad. In the brain the gray substance surrounds the white, 

 but as the nerve fibers pass to the spinal cord the gray matter 

 occupies the central part, mainly the spinal crescent (capital 

 H) which is formed by tw^o columns (dorsal and ventral horns) 

 joined by a central portion (commissura grissea). The white 

 substance in the spinal cord is formed mainly of medulated 

 nerve fibers and neuroglia, while the gray substance is 

 formed mainly by cell bodies, dendrites, and some nonmedu- 

 lated axons. 



The spinal nerves leave the spinal cord in about 37 pairs; 

 8 cervical, 12 thoracic, 8 lumbar, 3 sacral, and 6-8 caudal, 

 through the intervertebral foramina. The dorsal and ventral 

 roots of the three sacral nerves really have separate foramina 

 (Fig. 7, VF). A typical spinal nerve leaves the spinal cord 

 by two roots, or rami, which are covered by a common sheath 

 beginning a very short distance outside of the intervertebral 

 foramen. The dorsal root (radix posterior) forms an enlarge- 

 ment (dorsal ganglion), usually within the intervertebral 

 foramen, while the ventral root (radix anterior) does not have 

 a ganglion, but is larger than the dorsal and is connected with 

 the autonomic (sympathetic) nervous system by a short 

 communicating branch (ramus communicans) . The student 

 should observe these structures in one of the spinal nerves of 

 the thoracic group. 



Method: Remove the flesh from about five thoracic vertebrae 

 and with the point of the bone cutters expose the dorsal sur- 

 face of the spinal cord by removing the arch of two or three 

 vertebrae. 



