'84 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 splenium. 



The jornix 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 



