THE NERVOUS SYSTEM, 

 I. THE CENTRAL NERVOUS SYSTEM. 



(Re-written ly the translator.) 



A. THE SPIXAL CORD (Fig. 97). 



1. External form. The spinal cord is, in comparison with the 

 brain, somewhat small ; the two organs are directly continuous, 

 and present no distinct line of demarcation : the point of origin 

 of the first spinal nerve is, therefore, arbitrarily accepted as the 

 anterior limit of the spinal cord ; posteriorly it terminates in the 

 jihim terminate. 



The spinal cord is flattened dorso- vent rally, and is constricted 

 at a point (pars media, Reissner) somewhat anterior to its 

 middle: in consequence of this constriction the cord has two 

 enlargements ; an anterior smaller, and a posterior larger (intii- 

 mescentiae anterior v. posterior)^ from which arise the nerves of the 

 brachial and lumbar plexuses respectively. At about the sixth 

 o seventh vertebra, the hinder enlargement diminishes rather 

 abruptly to form the so-called conns medttOaru ; this is continued 

 into the Jitum terminal?, which enters the cavity of the urostyle. 

 The hinder portion of the lumbar nerves forms a cancla equiua, 

 the constituent nerves of which surround thejf/fMi terminate. 



A dorsal longitudinal fissure (sidcus longitudinal'^ superior) is 

 well marked in the middle line of the dorsal surface of the pos- 

 terior enlargement ; anteriorly and posteriorly it rapidly fades away, 

 its position being merely indicated by a small amount of connective- 

 tissue and a blood-vessel. 



The ventral longitudinal fissure (%ulcm longitudinally inferior) is 

 well marked throughout the length of the cord. Neither fissure can 

 be traced in the hinder part of the conns medidlaris or in faejilum 

 terminate. 



Ten pairs of nerves arise from the spinal cord, each nerve arising 

 by two roots, a ventral and a dorsal (anterior and posterior), from 



