THE NERVOUS SYSTEM. 207 



The Spinal Nerves. There are forty pairs of nerves 

 connected with the spinal cord. They issue from the 

 vertebral canal through the intervertebral foramina. Each 

 nerve is connected to the cord by a ventral and dorsal root 

 (Figs. 93 and 102). The former is also known as the 

 motor root, since its fibers are almost entirely motor, while 

 the latter is the sensory root, as it is composed of fibers 

 transmitting impulses to the central nervous system. A 

 ganglion about the size of a pinhead is located on the dorsal 

 root immediately proximad of its junction with the ventral 

 root, within the intervertebral foramen. This anatomy can 

 be displayed by cutting away the dorsal muscles on either 

 side of the column, and then, with the bone-cutters, severing 

 the laminae of several of the arches of the vertebras, so that 

 the roof may be removed from the vertebral canal (Fig. 

 22). 



Immediately beyond the intervertebral foramen each 

 nerve gives off a dorsal branch to the muscles of the back, 

 and a small connecting twig to the sympathetic system. 

 The main nerve is then spoken of as the ventral branch. 

 These main nerves or ventral branches, in various regions 

 of the trunk, anastomose with each other, forming plexuses. 

 In the region of the neck there is formed the cervical plexus; 

 in the region of the shoulder, the brachial plexus; in the 

 region of the loins, the lumbar plexus; and in the region of 

 the sacrum, the sacral plexus (Figs. 105 and 106). 



There are eight cervical nerves, the first of which does 

 not make its exit through the intervertebral foramen, as do 

 all the other spinal nerves, but traverses a foramen in the 

 atlas. The first five cervical nerves, the three posterior 

 cranial nerves, and branches from the sympathetic trunk, 

 form the cervical plexus. These five cervical nerves 

 supply mainly the structures of the neck. 



There are thirteen pairs of thoracic nerves. The ventral 



