E. NERVOUS SYSTEM. 



THE following elements, which are all derived from the epiblast, 

 may be distinguished in nervous tissues: (1) Ganglion cells, 

 provided with processes, and supported by a connective-tissue 

 framework, the neuroglia; and (2) Fibres, entering into or aris- 

 ing from the former, and serving as conductors of sensory or 

 motor impulses. Each fibre maybe invested by a delicate cover- 

 ing or sheath, the neurilemma (primitive sheath, or sheath of 

 Schwann). 



The nervous system may be divided into three main parts, the 

 central (brain and spinal cord), peripheral, and sympathetic 

 systems. The central part is the first to arise, and is formed as 

 a direct product of the epiblast; the other two become established 

 later. 



I. THE CENTRAL NERVOUS SYSTEM. 



1. THE SPINAL CORD. 



The first indication of the central nervous system is a furrow 

 (medullary groove) which appears on the dorsal side of the 

 embryo, and which gradually becomes converted into a tube by the 

 meeting of its edges : this tube then becomes separated from the 

 epiblast and gives rise to the hollow medullary cord (cp. p. 7), 

 the walls of which are at first comparatively thin ; it consists of 

 a more expanded anterior, and a longer and more slender posterior 

 section. From the former arises the brain, from the latter the 

 spinal cord. 



In an early stage of development the lumen of the medullary 

 cord is as a rule continuous posteriorly with that of the primitive 

 intestine (neurenteric canal). This connection, however, soon dis- 

 appears, and the spinal cord then consists of a cylindrical or more 

 or less flattened hollow cord, by no means filling the neural canal : 

 its walls become gradually more and more thickened, until finally 

 only a very narrow lumen, the central canal, remains; this is 

 lined by ciliated epithelium. 



K 



