i So ELEMENTS OF MAMMALIAN ANATOMY. 



^Y^' 



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 lateral 

 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). 



