70 THE NERVOUS SYSTEM OF THE FROG 



A special set of nerves in connection with the bloodvessels 

 and viscera forms the sympathetic nervous system. 



For the dissection of the nervous system specimens should be 

 taken which have been in strong spirit for two or three days, and 

 in which the brain has been exposed to the action of the spirit by 

 removal of the roof of the skull. 



A. The Central Nervous System. 



This is divisible into an anterior portion the brain lying 

 in the cavity of the cranium ; and a posterior portion the 

 spinal cord which lies in the neural canal of the vertebral 

 column. There is no sharp line of demarcation between the 

 two portions, which are directly continuous with each other. 



If the brain and spinal coi-d have not already been exposed, 

 clear away the dorsal muscles from both sides of the spine : cut 

 through the occipito-atlantal membrane, flexing the frog's head 

 slightly to make the membrane tense, and being careful not to 

 injure the brain beneath it. Introduce one blade of the scissors 

 into tJie cranial cavity, with the flat surface of the blade parallel 

 to t/te back of the frog, and keeping as close to the roof of the skull 

 as possible. Cut carefully through the side walls of the skull, 

 first on one side and then on the other. Turn the roof of the 

 skull forwards with forceps, and remove it altogether. 



Similarly cut through and remove the neural arches of the 

 vertebras one by one, from before backwards. 



Examine and draw the central nervous system in situ, showing 

 its several parts. 

 I. The Brain. 



a. The dorsal surface of the brain : note from before 

 backwards the following parts, removing the pigmented 

 membrane (pia mater) covering the several parts as you 

 come to them. 



i. The olfactory lobes, which form the most anterior 

 portion of the brain, are united together in the 

 median plane : they give off the olfactory nerves 

 from their anterior ends, and are separated behind 

 by slight constrictions from the hemispheres. 

 ii. The cerebral hemispheres are a pair of smooth 

 ovoid bodies which touch each other in the median 

 plane but are not fused together. 



