98 Elementury Manual of Zoology. 
on the same side, These constitute what is known as the sympathetic 
nerve system. 
The more complete dissection of the nervous system can best be per- 
formed on a frog which has lain some time in alcohol, as this hardens the 
nerve tissue.. Most of the important features, however, can be made 
out without this preliminary. 
Turn the frog over and dissect off the skin from the back and head. 
Cut away the muscles along the vertebral column on either side, 
and dissect out the flat scapular bones above the shoulders. Find the big 
opening (foramen magnum) at tbe base of the skull, where the spinal 
cord passes into the head. Stick the point of your scissors into it and 
carefully cut away the top of the head, so as to expose tbe brain. 
Great care is necessary in doing this not to injure nerve tissue. When 
the brain 4s fairly exposed cnt away the vertebral arches in the same 
manner down tle back, so as to expose the spinal cord. Pick away the 
darkish membrane (pia mater) which covers the white nerve matter, and 
make out the following parts, beginning from in front :— 
(1) Two elongated lobes, each with a constriction in the middle. 
They lie side by side and occupy the greater part of the 
length of the brain case. The slender portions in front 
of the constrictions are the olfactory lobes, while the 
swollen portions bebind the constrictions are the cerebral 
hemispheres. 
(2) Behind the cerebral hemispheres a broad flat portion (thala- 
mencephalon) ; upon the front part gf this, between the 
ends of the cerebral hemispheres, lies the small pineal 
body, which you may have some difficulty in making out. 
(3) Behind the thalamencephalon two large rounded masses 
situated side by side. ‘Ihese are the optie lobes. 
(4) Behind the optic lobes a narrow transverse band of nerve- 
matter. This is the cerebellum. 
(5) Behind the cerebellum is the expanded anterior portion of the 
spinal cord. “It is known as the medulla oblongata, 
(6) Behind, the medulla oblongata is continued into the spinal 
cord. ‘lrace this backwards as far as you can, noticing 
the roots of the nerves that are given off from it on either 
side. 
Lift up the olfactory lobes in front, bi@aking through the nerves 
(olfactory) which they give off to the nasal cavities'in front. Notice 
the large optic nerves which are given off below to the eyes, and behind 
these, in the middle line, the small prominence which is known as the 
pituitary body. Pull out the spinal cord as far down as possible, 
noticing the roots of the nerves which it gives off on either side, and the 
groove which extends down its ventral surface, 
