NERVOUS SYSTEM. 



47 



central nervous system, they are often spoken of as afferent 

 roots, while for analogous reasons the ventral roots are 

 termed efferent. 1 



These roots differ also in their mode of development. Cer- 

 tain features of the origin of the dorsal root are still in dispute, 

 but the following statements are pretty generally accepted. At 

 the time of closure of the neural tube a thin sheet of cells is 

 visible on either side of the line of closure between the epider- 

 mis and the tube. By un- 

 equal growth this sheet of 

 cells, or neural crest, becomes 

 converted into segments, 

 each segment developing into 

 a ganglion of a dorsal root. 

 Apparently fibres grow out 

 from this ganglion to enter 

 the cord, while others grow 

 peripherally to connect with 

 the sense organs. No such 



crest is formed for the ventral root ; but the fibres forming this 

 are connected with the ganglion cells of the cord, and they 

 lengthen with the growth of the animal, so as to connect with 

 the muscles, glands, etc. 



Each spinal nerve soon divides into two chief branches, 

 a ramus dorsalis supplying the dorsal region, and a ramus ven- 

 tralis distributed upon the sides and ventral surface. This latter 

 also gives off a ramus intestinalis to the viscera. These latter 

 connect with the sympathetic system, a pair of longitudinal 

 nerve cords with ganglia (derived from the spinal ganglia) lying 

 near the junction of the mesentery with the dorsal wall of the 

 coelom. This system supplies the digestive tract, the vascular 

 system and many glands, and in certain ichthyopsida it may 

 extend into the head. 



Typically the spinal nerves follow the myocommata or septa 

 between the muscle plates (to be described later), but in all 

 forms above fishes in the region of the limbs several of the ven- 



FIG. 48. Diagram of embryonic spinal 

 cord with neural crest, C. 



1 Later studies show that we must distinguish visceral and somatic motor fibres ; vis- 

 ceral and somatic sensory tracts. (See the section on cranial nerves.) 



