THE PHENOMENON OF CONDUCTION. 83 



on p. 152 that suggest an explanation less opposed to current 

 views than that offered by Bayliss. 



Cells of Origin of the Anterior and Posterior Root Fibers. 

 The efferent fibers of the anterior root arise as axons or axis cylinder 

 processes from nerve cells in the gray matter of the cord at or near 

 the exit of the root. The motor fibers to the voluntary muscles 

 arise from the large cells of the anterior horn of gray matter; the 

 fibers to the plain muscle and glands, autonomic fibers according 

 to Langley's nomenclature, take their origin from spindle-shaped 

 nerve cells lying in the so-called lateral horn of the gray matter.* 

 According to the accepted belief regarding the nutrition of nerve 

 fibers, any section or lesion involving these portions of the gray mat- 

 ter or the anterior root will be followed by a complete degeneration 

 of the efferent fibers. In the case of the fibers to the voluntary 

 muscles this degeneration will extend to the muscles and include 

 the end-plates. In the case of the autonomic fibers the degenera- 

 tion will extend to the peripheral ganglia in which they terminate, 

 involving, therefore, the whole extent of what is called the pre- 

 ganglionic fiber (see the chapter on the autonomic nerves and the 

 sympathetic system). The posterior root fibers have their origin 

 in the nerve cells contained in the posterior root ganglia. These 

 cells are unipolar, the single process given off being an axis cylinder 

 process or axon. It divides into two branches, one passing into 

 the cord by way of the posterior root, the other toward the periph- 

 eral tissues in the corresponding spinal nerve in which they form the 

 peripheral sensory nerve fibers. It follows that a section or lesion 

 of the posterior root will result in a degeneration of the branch 

 entering the cord, this branch having been cut off from its nutri- 

 tive relationship with its cells of origin. The degeneration will in- 

 volve the entire length of the branch and its collaterals to their 

 terminations among the dendrites of other spinal or bulbar neurons 

 (see the chapter on the spinal cord). After a lesion of this sort 

 the stump of the posterior root that remains in connection with 

 the posterior root ganglion maintains its normal structure. On the 

 other hand, a section or lesion involving the spinal nerve will be 

 followed by a degeneration of all the fibers, efferent and afferent, 

 lying to the peripheral side of the lesion, since these fibers are cut 

 off from connection with their cells of origin, while the fibers in the 

 central stump of the divided nerve will retain their normal structure. 



Afferent and Efferent Fibers in the Cranial Nerves. The 

 first and second cranial nerves, the olfactory and the optic, contain 

 only afferent fibers, which arise in the former nerve from the olfac- 

 tory epithelium in the nasal cavity, in the latter from the nerve 

 cells in the retina. The third, fourth, and sixth nerves contain 

 only efferent fibers which arise from the nerve cells constituting 

 * Herring, "Journal of Physiology," 29, 282, 1903. 



