Nerves, Spinal Cobd, Beain and Othee Ganglia 87 



Fig. 71. Sections of the spinal cord in the lower cervical, mid-thoracic, and mid- 

 lumbar regions. (Quain's Anatomy.) 



On the right side of each section conducting tracts are indicated. M, marginal 

 bundle, or Lissauer's tract. P-M, (in the lumbar section), septo-marginal tract. 



THE SPINAL AND CRANIAL NERVES. 



The spinal nerves issue from the cord in pairs, a pair for each articula- 

 tion of the spinal column. The nerve on each side has two roots, that is, 

 it is made up out of two sets of fibers, one set efferent, issuing from the 

 anterior horn of the gray matter of the cord, and the other (afferent) 

 entering the cord on the posterior side. The spinal ganglion lies on the 

 posterior root and the two roots are united in the intraspinal foramen just 

 beyond the ganglion, forming a single nerve. 



There are 31 pairs of spinal nerves, which are named, according to the 

 position of their origins in the spinal cord, cervical (8), thoracic (12), 

 lumbar (5), sacral (5), and coccygeal (1). The nerves are num- 

 bered in each group downwardly; thus, the uppermost spinal nerve is the 

 ' 1st cervical ', the next, the ' 2nd cervical ', the ninth, the ' 1st thoracic '. 



The nerves issuing above the 1st cervical are called cranial nerves. 

 When a nerve is referred to by number with no region assigned, ' cranial ' 

 is always meant. Two of the cranial nerves (I and VIII) are afferent 

 only; two (XI and XII) are efferent only; One (II) is almost ex- 

 clusively afferent, but contains a few efferent fibers: and the other seven 

 are mixed, i. e., they contain both afferent and efferent fibers in consider- 

 able numbers. 



The cranial nerves in order, as they are conventionally numbered, are 

 given in the following list : 



I. The olfactory nerve (afferent). This is not a ' nerve ' in the usual 

 sense, but a number of nerves not collected into a bundle, running from 

 the olfactory membrane, in the nasal cavity, to the olfactory bulb, from 

 which the connections are continued to the brain stem through the olfac- 

 tory tract. [Fig. 60.] 



II. The optic nerve (principally afferent) is composed almost alto- 

 gether of axons from the ganglion cells in the retina of the eye. These 

 pass back to the optic chiasm, where the fibers from the left half of the 

 eye continue in the left optic tract, and the fibers from the right half con- 

 tinue, in the right optic tract, back to the external geniculate body, the 

 frulvinar of the thalamus, and the superior corpora quadrigemina. From 

 the first two of these primary visual centers, fibers pass to the visual 

 cortex. From the third, fibers go to the oculo-motor center which controls 

 the contractions of the eye-muscles. Fibers also pass outward to the eye 

 in the optic nerve from the primary centers. Some of these are from the 

 oculo-motor center, but there are also fibers from the other two centers. 



