NERVOUS TISSUE 



133 



cells shown in the diagram the cord contains ramifying association and commissural 

 cells. Thus stimulation at one point on the surface of the animal may cause 

 coordinated muscular contractions in different parts of the body. As Retzius has 

 pointed out, if the neuro-epithelial cells should withdraw into the interior of the 

 animal, leaving their branching process in the epidermis, the conditions in verte- 

 brates would be closely paralleled. 



The development and structure of the central nervous system and the 

 sense organs will be considered in a later chapter. The following account 

 deals first with the development of the spinal nerves, the spinal sympathetic 

 system, and the cerebral nerves; and secondly, with the adult structure 

 of these parts, including the ganglia, nerve trunks, and nerve endings. 



DEVELOPMENT OF THE SPINAL NERVES. 



The formation of the medullary groove (or neural groove) as a longi- 

 tudinal trough in the ectoderm, and its conversion into the medullary tube 





V 



FIG. 125. THE DEVELOPMENT OF THE NERVOUS SYSTEM AS SEEN IN CROSS SECTIONS OF RABBIT EMBRYOS: 

 A, 7$ DAYS; B, 8J DAYS; C, 9 DAYS; D, ioj DAYS; E, IA DAYS. 



c. c. f Central cavity; d. r., dorsal root; d. ra., dorsal ramus; ep., ependymal layer; g. c., ganglion cells; 

 g. 1., gray layer; m. g., medullary groove; m. t., medullary tube; o. b., oval bundle; s. g., sympathetic 

 ganglion; sp. g., spinal ganglion; s. ra., sympathetic ramus; v. r., ventral root; v. ra., ventral ramus; 

 w. 1., white layer. 



by the coalescence of its dorsal edges, have been described in a previous 

 section (p. 37). The anterior part of the tube expands to form the brain; 

 the posterior part becomes the relatively slender spinal cord. 



At about the time when the medullary tube separates from the epi- 

 dermal ectoderm, some cells become detached from the medial dorsal 

 portion of the tube and pass down on either side of it, as shown in Fig. 125, 

 C and D. These cells constitute the neural crest. They multiply by 

 mitosis and accumulate in paired masses, corresponding in number with 



