THE CEREBRAL HEMISPHERES 



943 



The velar veins (veins of (talai), one on either side close to the median line, 

 running in the fold of the velum interpositum, are formed by the union of the 

 tenial, striatal, and middle cornual veins. The two velar veins unite to form a 

 common trunk which empties into the straight sinus. 



The Hippocampus and Fornix. The hippocampus and the fornix merit special 

 description. The hippocampus, as seen in the middle cornu, is a white eminence 

 about 5 cm. (2 inches) in length, of a curved elongated form, enlarging cephalad 

 and tapering caudad as the hippocampal fissure decreases in depth. The enlarged 

 extremity is marked by alternate elevations and depressions, usually three in 

 number, the hippocampal digitations; because of its resemblance to a lion's paw it 

 is sometimes called the pes leonis or pes hippocampi. The white appearance 

 of the ventricular aspect of the hippocampus is due to a stratum of white substance, 

 the alveus, made up of myelinic axones from hippocampal cells and continued 

 into the fimbria. The fimbria is folded so that its sharp margin is directed 



Choroid plexus 



Bulb of posterior cornu 



Hippocampus minor 



Fissure of. 

 Sylvius 



Eminentia collateralis 

 Fimbria .. 

 Hippocampus major 

 FIG. 696. Posterior and descending cornua of ieft lateral ventricle exposed from the side. 



toward the cavity of the middle cornu; eventually its fibres will be seen to enter 

 into the formation of the fornix. The formation of the hippocampus is best 

 observed in a coronal section (Fig. 682). In this view it is seen to be a peculiarly 

 folded margin of the cerebral cortex, corrugated by the intrusion of the hippo- 

 campal and fimbriodentate fissures. Morphologically it is a vestigial sub- 

 merged portion of the rhinencephalon, as a part of which it has already been 

 described (p. 930). 



The fornix (Figs. 697, 698) is really a paired structure consisting of bilaterally 

 symmetrical halves composed of longitudinally directed fibres which arch on each 

 side from the region of the uncus to the corpus albicans. The two lateral parts 

 join each other in the mesal plane along the summit of the arch to form the body 

 of the fornix (corpus fornicis). Frontad they diverge slightly as they proceed 

 toward the corpus albicans; caudad they diverge more widely. The paired 

 diverging portions are called respectively the anterior and posterior pillars of the 

 fornix. The fibres of each half fornix arise from the pyramidal cells in the hippo- 



