SUPPORT, MOTION, AND SENSATION 353 



The Spinal Cord 



The medulla ohlonsata is {'outiiniecl almost im])erceptibly over into 

 the spinal cord, which extends in adult man from the foramen magnum 

 of the skull posteriorly through the vertebral column for seventeen to 

 eighteen inches. The spinal cord is, roughly speaking, the size of the 

 Uttle finger, or about 0.4 of an inch in diameter. Two enlargements 

 occur in it, one in the region of the shoulder-blades, and the other 

 below the small of the back, respectively knowm as the cervical and 

 lumbar enlargements. 



The internal structure of the cord is quite characteristic. In cross 

 section, the central gray matter somewhat re.sembles the letter "H," 

 the position of the gray and white matter being apparently reversed 

 from their ])osition in the brain. As a matter of fact, in both cord 

 and brain the gray matter is disposed inside close around the cavity 

 that extends throughout the whole central nervous system. Outside 

 this central gray matter are the transmission fibers which app(>:ir 

 white. In the cerebellum and cerebrum of the brain there is super- 

 imposed an outer layer of gray matter that constitutes the centers of 

 adjustment. This secondary gray layer is .so pronounced in the brain 

 that it gives rise to the popular impression of a reversal in the arrange- 

 ment of white and gray matter between the cord and brain. The gray 

 matter is composed of a ventral, or anterior, and a dorsal, or posterior, 

 column, divided into these two parts by the tran.sverse bar of the "H." 

 The white matter may also be subdivided into three parts on either 

 side, a ventral, lateral, and dorsal funiculus. 



The Spinal Nerves 



The nerves in this group, like the cranials, are paired, there being 

 31 pairs in man. Each nerve, moreover, is "mixed," that is, it is 

 composed of a dorsal or sensory root containing receptor neurons, 

 carrying messages toward the brain, and a ventral or jnotor root bearing 

 effector neurons, which carry messages away from the brain to muscles 

 and glands. It will be noted that some of the cranial nerves, unlike 

 the spinal nerves, have lost this original ability to transmit messages 

 both ways and have been reduced to one-way traffic, for example, the 

 three pairs of eye-muscle nerves (III, IV, and VI) handle only outgoing 

 impulses, wdiile the auditory nerve (VIII) can only transmit .stimuli 

 inward toward the brain. From the point in a mixed nerve where the 

 incoming and the outgoing roots fuse are typically given off the 



