196 ELEMENTS OF MAMMALIAN ANATOMY. 



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. 92, 93, 94, 95), 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 cephalic portion of the 

 callosum is the genu or knee. The caudal border is the 

 splenium. 



The fornix lies ventral to the callosum (Figs. 92 and 

 94) 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 

 cephalic border of the body near the median line the 

 two anterior columns, or pillars, descend in a curve, 

 forming the cephalic boundary of the third ventricle as 

 far ventrad as the anterior commissure. Here the two 

 columns diverge slightly from the median line, but con- 

 tinue their descent, curving caudad to their termination 

 in the corpora albicantia. The posterior columns, or 

 crura, descend from the caudal border of the body, 

 curving laterad into the median cornu of the lateral ven- 

 tricle, and gradually unite with the cornu ammonis. 

 The cornu ammonis, or hippocampus major, is a thickened 

 projecting fold of the wall of the median cornu (Fig. 93). 

 The free lateral margin of the posterior pillar or crus of 

 the fornix is the fimbria, or tcznia hippocampi. Adjacent 

 to the median line in either hemisphere, a thick lamina of 

 matter, the septum lucidum, stretches from the cephalic 

 part of the fornix dorsad to the callosum. The very 

 narrow cavity formed by the adherence of the margins 



