108 THE CEREBRUM. 



the posterior border is the thickest part of the corpus callosum. 

 It is on that account called the pad, or splenium. 



The anterior border is bent downward and then backward, 

 sweeping through 180 degrees of flexion (Fig. 29). It tapers 

 down to a sharp edge, called the rostrum. A very thin sheet-like 

 extension of the rostrum, called the lamina rostralis, proceeds 

 backward from the beak and becomes continuous with the lamina 

 terminalis at the base of the brain. Running downward on either 

 side is a low ridge, continuous with the striae longitudinales, 

 which constitutes the gyrus subcallosus. Each gyrus subcallosus, 

 after passing across the anterior perforated substance, ends in 

 the uncus of the hippocampal gyrus. The transverse fibers of 

 the rostrum, in the hemisphere, form the floor of the anterior horn 

 of the lateral ventricle. 



Genu and Truncus (Fig. 29). The down-turned anterior part 

 of the corpus callosum is the genu. It joins the rostrum to the 

 main body, the truncus. The genu forms part of the anterior 

 boundary of the cerebral cavity; the truncus forms the roof. 

 Fibers uniting the frontal lobes of the cerebrum pass through the 

 genu, and in the hemisphere, bound the anterior horn of the 

 lateral ventricle above and in front. Those fibers arching for- 

 ward and forming the roof of the anterior horn are called the 

 forceps minor. The forceps major, composed of fibers from the 

 splenium which bend backward into the occipital lobe, lies in the 

 roof and inner wall of the posterior horn and produces the eminence 

 called the bulb (Fig. 38). 



Each lateral extremity of the corpus callosum is overhung by 

 the gyrus cinguli, which covers the lateral longitudinal stria. 

 Inclosed between the gyrus cinguli and corpus callosum is the 

 callosal fissure (ventricle of the callosum). The lateral extremity 

 of the corpus callosum, within the cerebral hemisphere, inter- 

 mingles with the superior lamina of the internal capsule and 

 thus stretches entirely across the fore-brain cavities (Figs. 30 

 and 42). 



The boundaries of the general cavity of the fore-brain may be 

 given as follows: 

 Roof (base of wedge) 



