THE NERVOUS SYSTEM 



559 



and become concentrated into two parallel strands dorsal to the spinal 

 cord. The crest is continuous throughout the entire length of the central 

 nervous system, except for an interruption in the head region between 

 the fifth and seventh nerves. As development proceeds, each strand 

 of neural crest cells migrates en masse ventrad. In the trunk region 

 the cells become more and more concentrated and metamerically arranged 

 as a series of ganglionic masses median to the myotomes. For each 

 myotome, there develops both a somatic motor root and a dorsal sensory 

 ganglion. Within the sensory ganglia, two kinds of cells become differen- 

 tiated. Toward the center, most of the cells become nerve cells, from 

 each of which processes grow in two directions, one process into the dorsal 



A B 



Fig. 461. — A, diagram of early spinal cord; B, later, showing increase in size and 

 consequent ventral fissure, c. central canal; e, ectoderm; /, floor plate; g, anlage of 

 spinal ganglion; nc, neural crest; r, roof plate; 5, sulcus limitans; v, ventral fissure. 

 (From Kingsley's "Comparative Anatomy of Vertebrates.") 



column of the tube and the other ventrally to join the somatic motor 

 nerves which have already formed. The resulting mixed nerve gives 

 off dorsal and ventral rami, to skin and muscles. The relations of the 

 fibers of the sensory roots have already been described. Most of the 

 peripheral cells of the ganglion become neurolemma cells. Possibly 

 all neurolemma cells of sensory nerves are so derived. 



The development of the neural crest in the head region differs in 

 some details from that of the spinal nerves just described. Olfactory 

 nerve fibers, except those in the nervus terminalis, develop not from 

 the neural crest but from the olfactory epithelium; and, since the optic 

 nerve is strictly a fiber tract of the brain, its development also is not from 

 the neural crest. The oculomotor nerve is somatic motor, like the 

 trochlearis, abducens, and hypoglossus, and its development is like that 

 of the ventral root of a spinal nerve. The trigeminal is a mixed nerve 

 with visceral motor fibers and with sensory fibers developed from the 

 neural crest. There is evidence that some of the neural crest cells of all 

 the visceral arches enter into the formation of connective tissue of the 

 arches. The sensory elements of the facial and acoustic come from a 

 common ganglion. Indeed the acoustic nerve develops as a branch 

 of the seventh and only secondarily acquires an independent root. The 



