THE NERVOUS SYSTEM 233 



learis (IV), and ahducens (VI) are wholly motor. The tri- 

 geminal (V), facial (VII), glossopharyngeal (IX), Vagus (X), 

 accessorius (XI) (mainly motor), and hypoglossus (XII) contain 

 both motor and sensory fibers. 



The dissection of the cranial nerves is very difficult. A head, 

 containing a brain hardened by a formalin injection, should 

 be placed in 500 c.c. of 5 per cent, nitric acid, which will decal- 

 cify the bone in about a week. After washing out the acid by 

 soaking the specimen in running water twenty-four hours, the 

 dissector may with much care follow the nerves peripherad from 

 their origin at the base of the brain. The vagus nerve must, 

 of course, be traced in an entire specimen, where it may be 

 easily followed in the neck region along with the carotid artery, 

 whence it passes to the lungs and stomach (Fig. 72). 



Some of the sensory nerve roots bear ganglia, the largest 

 of which is the semilunar (Gasserian) ganglia, more than a half 

 centimeter in diameter, forming a knot on the sensory root of 

 the trigeminal, within the cranial cavity (Fig. 104). 



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. 104 and 113). 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 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 laminai of several of the arches of the vertebra?, so that 

 the roof may be removed from the vertebral canal (Fig. 23). 



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 



