180 ELEMENTS OF MAMMALIAN ANATOMY. 
two small rounded white bodies, the corpora albicantia. 
The tuber cinereum, a slightly elevated mass of gray matter 
behind the optic chiasm, bears on its surface the funnel- 
shaped stalk, the infundibulum, to which the hypophysis is 
attached. If the two latter parts are removed, there is 
seen a small elongated aperture through the tuber cinereum 
into the third ventricle (Figs. 93 and 95). 
The optic commissure, or optic chiasm, is the commissure 
formed by the crossing of the optic nerves just craniad of 
the tuber cinereum. The prolongation of the optic nerves 
dorsad from the optic commissure forms the optic tracts, 
partly covered by the temporal lobes. On either side of 
the median fissure just craniad of the optic chiasm is a 
somewhat triangular area known as the anterior perforated 
space because of the numerous vessels that enter the brain 
in this region. Laterally this space is bounded by the 
lateral olfactory tract or lateral root of the olfactory nerve, 
which presents the appearance of a band of white fibers 
extending from the olfactory lobe into the temporal lobe. 
The medial or inner root of the olfactory nerve is seen 
adjacent to the median ventral line craniad of the anterior 
perforated space. The olfactory lobes project from the 
cranial ventral portion of the cerebrum and give origin to 
the first pair of cranial nerves. 
Internal Structure.—The canalis centralis, a small canal 
extending throughout the center of the spinal cord, enlarges 
in the region of the brain, forming four cavities or ven- 
tricles communicating with each other by narrow channels. 
The brain is therefore to be considered as a hollow struc- 
ture. The first and second ventricles, also known as Jateral 
ventricles, occupy the cerebral hemispheres (Fig. 94). The 
third and fourth ventricles lie in the median line, and are 
therefore well seen in a sagittal section of the brain (Fig. 
95). 
