154 WILLIAM F. ALLEN 



The main points of the previous section may be summarized 

 as follows: (a) For certain regions of the body in Polistotrema 

 there is a certain specific distribution of the sensory and motor 

 components in the spinal nerves. Those in the extreme caudal 

 region have the most primitive arrangement of their fibers and 

 closely resemble the condition found in Petromyzon. (b) The 

 characteristics which are common to all of the spinal nerves are 

 as follows. Each spinal nerve has one dorsal or sensory root 

 and two ventral or motor roots (exceptions to be noted for the 

 last three nerves). The sensory root fibers enter the latero- 

 dorsal surface of the spinal cord as a single bundle and leave the 

 inner surface of their ganglia to enter the neural arch on a level 

 wdth the central axis of the spinal cord. The two motor roots 

 take origin from a number of rootlets scattered along the latero- 

 ventral surface of the spinal cord for a distance of nearly a seg- 

 ment They emerge from the membranous neural canal through 

 separate foramina on a level with the dorsal surface of the noto- 

 chord, and soon unite on the outer surface of the notochord, or 

 below to form a ventral motor ramus A dorsal motor ramus 

 usually takes origin as a branch from the motor ramus ventralis 

 close to its origin from the union of the motor roots or in a few 

 instances it may arise from either or both of the motor roots 

 outside the neural arch. It distributes itself to the inner sur- 

 face of each myotome. The spinal ganglia are more or less 

 spindle-shaped and somewhat flattened in a vertical plane. 

 From their two poles dorsal and ventral sensory rami take origin. 

 They follow at first the inner siu"face of the intermuscular septa 

 of the myotomes. Occasional ganglion cells, isolated or in 

 clusters, occur along the peripheral course of these fibers, (c) 

 The following characteristics will hold for all of the true cephalic 

 spinal nerves down to a region a little below the anus. All of 

 the motor components are cephalad of their corresponding sen- 

 sory components, and all of the ventral motor rami join their 

 respective sensory rami from in front, opposite the notochord. 

 Upon reaching the dorsal border of the myotomes the main 

 portion of the dorsal sensory fibers bend laterally and ventrally 

 to form the R. cutaneous superior, a few fibers, however, con- 



