THE NEEVOUS SYSTEM. 



I. THE CENTRAL NERVOUS SYSTEM. 



{Re-written hi/ the translator.) 



A. The Spinal 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 

 Jilmu terminale. 



The spinal cord is flattened dorso-ventrally, and is constricted 

 at a point impars media, Reissner) somewhat anterior to its 

 middle : in consequence of this constriction the cord has two 

 enlargements; an anterior smaller, and a posterior larger [intn- 

 mescentiae anterior v. posterior), from wliich arise the nerves of the 

 brachial and lumbar plexuses respectively. At about the sixth 

 or seventh vertebra, the hinder enlargement diminishes rather 

 abruptly to form the so-called conus mednllaris ; this is continued 

 into the Jilimi terminale, which enters the cavity of the urostyle. 

 The hinder portion of the lumbar nerves forms a cauda equina, 

 the constituent nerves of which surround thejilum terminale. 



A dorsal longitudinal fissure {sulcus longitiulinalis 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 [sulcus lonyitudinalis inferior) is 

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

 be traced in the hinder part of the conus medtillaris or in thejilum, 

 termi7iale. 



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

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



