8 o 
MAMMALIAN ANATOMY 
Other Important Features Shown in the Sagittal Section.— 
The exposed wall of the longitudinal fissure formed by the medial 
surface of the cerebral hemisphere with its various gyri, the most 
definite and conspicuous of which is the gyrus cinguli which 
follows the dorsal curve of the corpus callosum; the dorsal and 
ventral outpushings of the third ventricle, which form the pineal 
body (epiphysis) and the hypophysis respectively (the latter, 
however, usually broken off in removing the brain from the cranial 
cavity); the massa intermedia, a soft mass lying in the middle of 
the third ventricle and appearing here as a cut area section, circular 
in outline; the cut surface of the cerebellum, with its tree-like 
arrangement (arbor vitae) of white nerve substance (bundles of 
nerve fibers) surrounded by the gray cortex; the commissures, i.e., 
bundles of fibers connecting the two halves and hence cut trans¬ 
versely here. These are ( a ) the corpus callosum, which connects 
the two cerebral hemispheres, ( b ) a portion of the fornix just 
ventral to the posterior end of the corpus callosum, (c) the optic 
chiasma, (d) the anterior commissure, very small, dorsal to the 
optic chiasma, and connected with it by the lamina terminalis, 
(e) the posterior commissure at the dorso-posterior limit of the 
third ventricle, (/) the pons. 
Draw the median sagittal section showing the cavities and other 
features , and making very clear the distinction between external 
surfaces, internal surfaces , and cut surfaces. 
2. Dissection of the Cerebral Hemispheres. 
Throughout this dissection draw details to show fact$ which you 
learn. 
For this, and the subsequent dissections, use preferably a 
w r hole brain, supplementing the material as needed by the half 
brains obtained by making the median section. Orient the dis¬ 
section by frequent reference to the external features of the brain 
and to the median sagittal section. Carefully cut away the 
dorsal surface of the cerebral hemispheres by making thin hori¬ 
zontal slices with a sharp wet razor or scalpel. While making this 
dissection note the distribution of white and gray matter, and 
latter forming a thick external layer, the cerebral cortex. Note 
that there are small bundles of fibers which extend beneath the 
