NERVOUS SYSTEM' 179 



The nervous system can be well studied in transverse 

 sections of adults, or by examination of young specimens 

 mounted whole. If fresh specimens can be obtained, the 

 entire nervous system can be isolated by placing them in 

 twenty per cent, nitric acid for three days, then washing 

 thoroughly and leaving in water for a day. By this treatment 

 the connective tissue is so softened and loosened that on shak- 

 ing the bottle the nervous system is readily and completely 

 separated from all the other parts. It should then be trans- 

 ferred to alcohol, stained, and mounted in the usual manner. 



1. The central nervous system or spinal cord is of nearly 



uniform diameter along the greater part of its length : 

 it diminishes slightly in size towards the anterior 

 end, and much more markedly at the posterior end, 

 which is very slender. 



The olfactory lobe is a small conical process 

 from the dorsal surface of the anterior end of the 

 cord, slightly to the left of the median plane. 



The central canal lies much nearer the ventral 

 than the dorsal surface : it is very small except at 

 the extreme front end, where it expands to form the 

 ventricle, from the anterior end of which a small 

 diverticulum extends into the olfactory lobe. 



At intervals along the spinal cord are spots of 

 black pigment, contained in cells in the floor of the 

 central canal. 



2. The peripheral nervous system consists of nerves of two 



kinds : (1) nerves which arise by single roots, and, 

 except in the case of the first pair, from the dorsal 

 surface of the central nervous system ; (2) nerves 

 arising by multiple roots, and from the ventral 

 surface of the cord. 



a. Nerves arising by single roots. These apparently 

 correspond to the dorsal or sensory roots of the 

 spinal nerves of other vertebrates: they differ 

 from these, however, in having no ganglia, and 



N2 



