176 ELEMENTS OF MAMMALIAN ANATOMY. 
The subarachnoidal space lies between the arachnoid and 
pia mater. The pia mater is the delicate vascular mem- 
brane following so closely the folds and clefts of the brain 
that it becomes apparent only when lifted carefully by the 
forceps. 
In order to study the brain satisfactorily each student 
should have a specimen hardened according to directions 
on page 10, and also should be permitted to examine a 
series of sections cut transversely about a centimeter thick. 
The brain of a calf or sheep serves the learner’s purpose 
better than that of a cat, as the parts are larger, and they 
are also easier to procure. An additional advantage is also 
found in the fact that there is some difference between the 
brains of the cat and calf, and the student must therefore 
rely more on his own observations. 
External Features.—The brain is composed of five 
parts: the medulla oblongata, or myelencephalon; the pons 
Varolii and cerebellum, forming the metencephalon; the 
corpora quadrigemina and crura cerebri, composing the 
mesencephalon; the diencephalon, including the optic 
thalami and other parts bounding the third ventricle; and 
the ftelencephalon, or cerebral hemispheres. The medulla 
oblongata, sometimes called the bulb or stem of the brain, 
is the expansion of the spinal cord as it passes through the 
foramen magnum. The pons Varolii is the bridge of 
transverse fibers seen on the ventral aspect of the brain just 
craniad of the medulla (Fig. 93). The cerebellum or 
little brain lies on the dorsal side of the medulla and is 
partly covered by the caudal part of the cerebrum. The 
mesencephalon is not visible externally on the dorsal aspect, 
but may be seen immediately craniad of the cerebellum by 
cutting away the cerebrum (Fig. 96). The diencephalon 
is visible externally only on the ventral aspect of the uncut 
brain, where it forms the floor of the third ventricle. It 
