SPINAL NERVES IN VERTEBRATES 139 



mammal. It will be seen in Amphioxus that the motor and 

 sensory elements are T\ddely separated, a motor nerve appearing 

 opposite each muscle plate, and a sensory nerve at the inter- 

 muscular septum. The corresponding nerve roots of the two 

 sides do not enter and leave the spinal cord in the same plane. 

 A section passing through a motor root on one side would in all 

 probability pass through the sensory root on the opposite side. 

 The arrangement shown in figure 30 for mammals will hold for 

 all forms from Amphibia up and is too well-known to require 

 more than the mere statement, that the dorsal (sensory) fibers 

 which enter the spinal cord in the same plane as the correspond- 

 ing ventral (motor) fibers leave, intermingle with the latter after 

 passing through a spinal ganglion, and the mixed fibers form the 

 three characteristic rami, dorsal, ventral, and communicans 

 (fig. 30, R.D., R.V., and R.C.). 



Upon closer examination of figure 29 it mil be seen that the 

 fibers of the ventral root, radix anterior {V.R.), after leaAdng 

 the ventro-lateral surface of the spinal cord and penetrating the 

 neural arch, spread out on the inner surface of the myotomes 

 somewhat after the manner of a cone. In my haematoxylin 

 sections the light marginal areas bounded by a dotted line in 

 figure 29 have the appearance of connective tissue more than 

 nerve fibers. A similar appearance may have led Balfour' (80) 

 to affirm that the ventral roots, accurately described by Schneider 

 in 1879, are mere processes of the muscle plate. This cone-like 

 distribution of the motor fibers to the myotomes could be de- 

 scribed also as forming a motor ramus dorsalis and a motor 

 ramus ventrahs (R.D.M. and R.V.M.). 



As will be seen from figures 28 and 29 and from the descrip- 

 tions of Schneider (79), Hatschek ('92), Dogiel ('02), Johnston 

 ('05), Miss Kutchin and others, the sensory fibers of Amphioxus 

 are collected by dorsal and ventral rami. These trunks follow 

 the outer surface of the intermuscular septum, and when a little 

 above or below the level of the dorsal surface of the spinal cord 

 they penetrate the connective tissue septum to unite on or near 

 the inner surface of the septum, in forming a short dorsal root, 

 radix posterior {D.R.), which pierces the neural arch to enter 



