THE PHENOMENON OF CONDUCTION. 85 



roots in the same regions is without effect upon these peripheral 

 tissues. One apparent exception, however, has been noted. A 

 number of observers have found that stimulation of the peripheral 

 end of the divided posterior roots (fifth lumbar to first sacral) 

 causes a vascular dilatation in the hind limb. The matter has 

 been particularly investigated by Bayliss,* who gives undoubted 

 proof of the general fact. At the same time he shows that the 

 fibers in question are not efferent fibers from the cord passing out 

 by the posterior instead of the anterior roots. This is shown by 

 the fact that they do not degenerate when the root is cut between 

 the ganghon and the cord, as they should do if they originated from 

 cells in the cord. A possible explanation of this exception in terms 

 of the orthodox views in regard to afferent and efferent fibers is in- 

 dicated by the facts and schemata given on p. 150. 



Cells of Origin of the Anterior and Posterior Root Fibers. — 

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

 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, f 

 The posterior root fibers have their origin in the nerve cells con- 

 tained 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 peripheral tissues in the corre- 

 sponding spinal nerve in which it forms one of the peripheral sen- 

 sory nerve fibers. (See Fig. 60.) 



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 

 their nuclei of origin in the midbrain and pons. The fifth nerve 

 resembles the spinal nerves in that it has two roots, one containing 

 afferent and the other efferent fibers. The efferent fibers, consti- 

 tuting the small root, arise from nerve cells in tlie pons and mid- 

 brain, the afferent fibers arise from the nerve cells in the Gasserian 

 ganglion. This ganglion, being a sensory ganglion, is constituted 

 like the posterior root ganglia. Its nerve cells give off a single 



* Bayliss, "Journal of Physiology," 26, 173, 1901, and 28, 276, 1902. 

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



