52 THE FROG 
the olfactory nerves, then turn the brain back, cutting through 
the nerves as they come into view. Great care is necessary to 
avoid tearing off the exceedingly delicate cranial nerves. Im- 
merse the nervous system, ventral surface up, in water, and 
observe: 
(a) The ventral longitudinal fissure of the spinal cord. 
(6) On the ventral side of the thalamencephalon the optic 
chiasma formed by the crossing of the optic nerves (second pair 
of cranial nerves) ; the slightly bi-lobed infundibulum posterior 
to the chiasma; posteriorly the infundibulum is continuous with 
the pituitary body or hypophysis which is generally torn off 
when the brain is removed from the cranium. 
Compare the brain of the frog with figures or a model of the 
brain of man. What parts are relatively larger in the human 
brain? 
With a sharp scalpel, razor, or scissors carefully slice off the 
roof (dorsal wall) of the brain until the cavities are exposed 
throughout its entire extent. In the cerebral hemispheres make 
out a pair of cavities, the first and second ventricles; do they 
extend into the olfactory lobes? The unpaired cavity (third 
ventricle) of the thalamencephalon connects in front with the 
first and second by transverse passages, the foramina of Monro. 
In the optic lobes are a pair of optic ventricles, which, since they 
do not occur in mammals, are not numbered. The optic ven- 
tricles communicate with a median cavity, the aqueduct of 
Sylvius or iter e tertio ad quartum ventriculum, which, as the 
latter name implies, connects the third ventricle with the fourth 
ventricle situated in the medulla. 
Study prepared slides showing cross-sections through different 
regions of the brain. With the aid of figures showing sagittal 
and horizontal sections (see Holmes’ Biology of the Frog, figures 
83 and 84), determine from what parts of the brain the cross- 
sections are taken. Make outline drawings of sections through 
four different regions, and label all the parts shown. 
2. The peripheral nervous system. Of the cranial nerves, the 
olfactory and optic nerves are readily observed during the study 
of the brain. On account of the difficulty attending their dis- 
section in the frog, the smaller cranial nerves are omitted; they 
