184 ELEMENTS OF MAMMALIAN ANATOMY. 
fibers known as commissures, two of which are visible on 
the ventral surface of the brain, and the others may be. 
seen in a sagittal section (Fig. 95). 
The pons Varolii is the commissure on the ventral aspect 
of the medulla. Its fibers pass into the cerebellum on 
either side, forming the middle peduncle or crus cerebelli 
ad pontem. The optic commissure or optic chiasm is 
formed by the crossing of the optic nerves, craniad of the 
tuber cinereum. Some of the fibers originating in the cells 
of the retina of one eye pass by this commissure directly 
to the cells in the retina of the other eye, while a second 
set passes from the eye to the optic tract on the opposite 
side of the brain, and still a third set, originating in one 
corpus quadrigeminum, passes by the optic commissure 
direct to the opposite corpus quadrigeminum (Figs. 95, 
104). 
The corpus callosum is the largest commissure of the 
brain. It joins the two cerebral hemispheres, and forms 
the roof of the lateral ventricles. This broad plate of 
fibers (Figs. 95, 96, 97, 98), which may be seen at the 
bottom of the great longitudinal fissure by pressing the 
hemispheres slightly apart, is about one millimeter thick and 
three centimeters wide. Laterally the fibers radiate in all 
directions to the gray matter of the cortex. The ventral 
bend of the median cranial portion of the callosum is the 
genu or knee. The caudal border is the spleniwm. 
The fornix lies ventral to the callosum (Figs. 95 and 97) 
and consists of a median plate of fibers, the body, two 
posterior columns, and two anterior columns or pillars. 
The median plate or body of the fornix sends some fibers 
into the ventral surface of the callosum. From the cranial 
border of the body near the median line the two anterior 
columns, or pillars, descend in a curve, forming the 
cranial boundary of the third ventricle as far ventrad as 
