THE FORE-BRAIN OR PROSENCEPHALON 



831 



corpus callosum, the rostrum. It is bounded medially by the anterior portion 

 of the septum pellucidum, and laterally by the head of the caudate nucleus. Its 

 apex reaches the posterior surface of the genu of the corpus callosum. 



The posterior cornu (cornu posterius; postcornii) (Figs. 737, 738) passes into the 

 occipital lobe, its direction being backward and lateralward, and then medialward. 

 Its roof is formed by the fibers of the corpus callosum passing to the temporal and 

 occipital lobes. On its medial wall is a longitudinal eminence, the calcar avis 

 (hippocampus minor], which is an involution of the ventricular wall produced by 

 the calcarine fissure. Above this the forceps posterior of the corpus callosum, 

 sweeping around to enter the occipital lobe, causes another projection, termed the 

 bulb of the posterior cornu. The calcar avis and bulb of the posterior cornu are 

 extremely variable in their degree of development; in some cases they are ill- 

 defined, in others prominent. 





FIG. 737. Central part and anterior and posterior cornua of lateral ventricles exposed from above. 



The inferior cornu (cornu inferior; descending horn; middle horn; medicornu) (Fig. 

 739), the largest of the three, traverses the temporal lobe of the brain, forming 

 in its course a curve around the posterior end of the thalamus. It passes at first 

 backward, lateralward, and downward, and then curves forward to within 2.5 cm. 

 of the apex of the temporal lobe, its direction being fairly well indicated on the 

 surface of the brain by that of the superior temporal sulcus. Its roof is formed 

 chiefly by the inferior surface of the tapetum of the corpus callosum, but the tail 

 of the caudate nucleus and the stria terminalis also extend forward in the roof of 

 the inferior cornu to its extremity; the tail of the caudate nucleus joins the 



