CH. XVII.] 



NERVE-CENTRES. 



189 



font Vtr. 



of S/tiivxl Cord 



t -/ -_\ Lit Dorsal 



> / 3\ tfriwra 



olfactory and the optic) arise from the cerebrum. The remaining 



ten pairs arise from the district of grey matter called the floor of 



the fourth ventricle 



or its immediate 



neighbourhood ; this 



tract of grey matter 



is situated at the 



lower part of the 



brain where it joins 



the spinal cord ; this 



portion of the brain 



is called the Bulb or 



Medulla oblongata. 



The spinal nerves 

 are arranged in pairs, 

 3 1 in number. Their 

 general structure and 

 functions we have 

 already studied (pp. 

 169 1 72). 



The more intimate 

 structure of the brain 

 and spinal cord we 

 shall consider at 

 length in subsequent 

 chapters. For the 

 present we shall deal 

 with some of the 

 general aspects of the 

 nerve - centres, both 

 as regards structure 

 and function. 



Both brain and 

 spinal cord consist of 

 two kinds of tissue, 



Fig. 195. View of the cere- 

 bro-spinal axis of the 

 nervous syntem. The 

 rijfht half of the cranium 

 and trunk of the body 

 has been removed by a 

 vertical section ; the 

 membrane* of the brain 



and spinal cord have alo been removed, and the roots and first part of On- fifth iiml 

 ninth cranial, and of all the spinal nerves of the right side, have been diiwected out 

 and laid separately on the wall of the skull and on the several vertebra opposite to 

 the place of their natural exit from the cranio-spinal cavity. (After Bourgery. 



U H'rrExtvunit 

 tf SfutMl Curd 



\ 1st Lumbar 

 -\ Vcrfrtr* 



K* 



