70 THE NERVOUS SYSTEM OF THE FROG 



A special set of nerves in connection with the bloodvessels 

 and viscera forms the sympathetic nervous system. 



F<yr the dissection of the nervous system specimens should be 

 taken vihich have been in strong spirit for 1/wo or three days, amd 

 in wMch 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 ; aaid 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 cord have not already been exposed, 

 clear away the dorsal m,uscles from, both, sides of the spine : cut 

 through the occipito-atla/ntal mxmhra/ne, flexing the frog's head 

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

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

 into the cranial cavity, with the flat suirface of the blade parallel 

 to the back of the frog, and keeping as dose to the roof of the skull 

 as possible. Cut ca/refuUy through the side walls of the sk^dl, 

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

 shuU forvjards with forceps, amd remove it altogether. 



Simila/rly cut through amd remove the newral a/rches of the 

 vertebrcB one by one, from before backwards. 



Exa/mine and d/raw the central nervous system in situ, showing 

 its several pa/rts. 

 I. The Brain. 



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

 backwards the following parts, removing the pigmented 

 membra/ne (pia mater) covering the several parts as you 

 corns 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. 



