82 ANATOMY OF THE RABBIT 



the basis of the cerebral hemispheres. It contains, as divisions of 

 the primary cavity, a pair of cavities, the lateral ventricles. Origi- 

 nally, the whole of the telencephalon was concerned with olfactory 

 functions, but it has been invaded by an increasing number of 

 fibres from more posterior parts bearing non-olfactory nerve 

 impulses. In the frog, a small part of it has already become free 

 from olfactory connections and in the higher vertebrates the non- 

 olfactory part becomes increasingly preponderant. In the mam- 

 malian brain, the olfactory portion, or rhinencephalon, is more or 

 less definitely marked off from the rest. This portion, sometimes 

 termed olfactory lobe or olfactory brain, includes the olfactory 

 bulb and the pyriform lobe, already mentioned, with a number of 

 related parts. 



A primitive cerebral hemisphere like that of the frog or the 

 young mammalian embryo comprises distinguishable dorsal and 

 ventral halves, designated respectively pallial and basal. In the 

 adult mammal, the basal portions have become massive and the 

 pallium has spread partly over their lateral and ventral surfaces 

 as well as constituting the dorsal wall of the hemisphere. The 

 cells of the pallium have formed a highly specialized superficial 

 layer, the cerebral cortex. 



The unpaired portion of the prosencephalon is considered as 

 belonging in part to the telencephalon and in part to the dience- 

 phalon. Its cavity, the third ventricle, is connected with each 

 lateral ventricle through an interventricular foramen. Its anterior 

 wall is formed by a transverse connection of the cerebral hemispheres, 

 the lamina terminalis. In all vertebrates this portion of the brain 

 is remarkable for the manner in which its wall is differentiated. 

 The ventral portion extends downward as a slender funnel-like 

 structure, the infundibulum, the tip of the latter being attached 

 to the pituitary body or hypophysis and its base being connected 

 with a small grey elevation, the tuber cinereum. Its cavity is the 

 recessus infundibuli. Immediately in front of the infundibulum 

 the optic tracts cross each other on the ventral surface of the brain, 

 forming the optic chiasma, and immediately behind it the floor is 

 thickened, forming externally a pair of rounded protuberances, 

 the mamillary bodies. In the brain of the rabbit, the latter bodies 

 are fused so that superficially they consist of a larger median 



