74 THE NERVOUS SYSTEM OF THE FROG. 
These two functions are fulfilled by different nerves, which 
may accordingly be distinguished as (a) afferent or sensory 
nerves, conveying impulses ¢o the central parts, and (6b) efferent 
or motor nerves, conveying impulses from the brain or cord. 
A special set of nerves in connection with the bloodvessels 
and viscera form 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. 
Divisible into an anterior portion—the brain—lying in the 
cavity of the cranium; and a posterior portion—thespinal 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. 
The central nervous system appears at a very early period in 
the development of the frog as a longitudinal groove extending 
the whole length of the dorsal surface. By fusion of its lips 
the groove becomes converted into a tube, which soon separates 
from the skin. This tube, which at first consists of almost 
unmodified epidermis, becomes converted by thickening of. its 
walls and by further changes into the central nervous system ; 
the modifications being greater at the anterior end or brain 
than in the posterior part or spinal cord, but the tubular 
character being retained throughout life along the whole length. 
If the brain and spinal cord have not been already 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 the cranial cavity, with the flat surface of the blade parallel 
to the 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 wt altogether. 
