78 THE PHYSIOLOGY OF MUSCLE AND NERVE. 



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 an- 

 terior roots. This is shown by the fact that they do not degenerate 

 when the root is cut between the ganglion and the cord, as they 

 should do if they originated from cells in the cord. Bayliss's own 

 explanation of this curious fact is that the fibers in question are 

 ordinary afferent fibers, but that they are capable of a double ac- 

 tion: they can convey sensory impulses from the blood-vessels to 

 the cord according to the usual type of sensory fibers, but they 

 can also convey efferent impulses, antidromic impulses as he desig- 

 nates them, to the muscles of the blood-vessels. In other words, 

 for this special set of fibers he attempts to re-establish the view 

 held by physiologists before the time of Bell, namely, that one 

 and the same fiber transmits normally both afferent and efferent 

 impulses. An exception so peculiar as this to an otherwise general 

 rule cannot be accepted without hesitation. It is possible that 

 future work may give 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.f 

 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- 



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

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



