THE NERVOUS SYSTEM. 2 19 



Each nerve is connected to the cord by a ventral and 

 dorsal root (Figs. 90 and 98). 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 com- 

 posed of fibers transmitting impulses to the central 

 nervous system. A ganglion about the size of a pin- 

 head is located on the dorsal root immediately proximad 

 of its junction with the ventral root, within the inter- 

 vertebral 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 vertebrae, 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. 101 and 102). 



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 sym- 

 pathetic 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 branches of the sixth, seventh, and eighth cervical 



