BRAIN OF THE MAMMAL 1 



1. Examine the dorsal surface of a Sheep's brain. Note the large 

 anterior cerebrum, which consists of a right and left cerebral hemi- 

 sphere. Posterior to the latter is the median, unpaired cerebellum, 

 lying above the medulla which merges into the spinal cord. The 

 brain is enclosed in three membranes : (a) the dura mater, which lines 

 the interior of the skull ; (b) the arachnoid, which is the membrane you 

 see, and (c) the pia mater, which lies below the arachnoid and is very 

 closely applied to the brain tissue. Note that the outer surface of the 

 cerebrum (cerebral cortex) is arranged in ridges (gyri), between 

 which are depressions (sulci) of varying depths. 



2. Examine the ventral surface of your specimen. Note: (a) the 

 cerebral hemispheres; (6) the olfactory lobes (often destroyed in 

 removing the brain from the skull), from each of which (c) olfactory 

 nerve fibers arise ; (d) the optic chiasma, from which (e) the optic nerves 

 arise ; (/) the pituitary body, under and posterior to the optic chiasma, 

 and lying on (g) the thickened, ventral wall (crura cerebri) of the mid- 

 brain ; (h) the prominent, transverse band of fibers (pons varolii) with 

 (0 a root of the important fifth cranial nerve (trigeminal) arising just 

 posterior and on each side; 0") the eighth cranial nerves (auditory) 

 which arise close to the ventral edge of the cerebellum just posterior to 

 the fifth cranial nerve. Make a drawing from the ventral surface to show 

 the structures as observed. 



3. Examine the cut surface of a brain which has been sectioned in a 

 median longitudinal plane. Note the general arrangement and the large 

 cerebral hemisphere which lies anterior and dorsal to the mid-brain. 

 Identify the following structures in addition to those noted above : (a) the 

 corpus callosum, which consists of a fibrous plate connecting the two 

 cerebral hemispheres; (6) the corpora quadrigemina, which lie just 

 anterior to the cerebellum and constitute the dorsal wall of the mid-brain ; 

 (c) the small spherical pineal body lying in an indentation anterior to 

 the corpora quadrigemina ; and (d) the cavity (fourth ventricle) in the 

 medulla, lying underneath the cerebellum. 



4. Make a drawing of the half-brain from the cut surface, showing as 

 many as possible of the structures which you have found in the preceding 

 paragraphs. 



iB. pp. 209 211 ; 220-221. 

 347 



