64 THE ANATOMY OP VEETEBEATED ANIMALS. 



occipital, and temporal lobes — while, on the outer side of the 

 corpus striatum, a central lobe (the insula of Reil) lies in 

 the midst of these. The lateral ventricles are prolonged 

 into the frontal, occipital, and temporal lobes, and acquire 

 what are termed their anterior, posterior, and descending, 

 cornua. 



Fui-thermore, while, in the lower vertebrates, the surface 

 of the cerebral hemispheres is smooth ; in the higher, it be- 

 comes complicated by ridges and furrows, the gyri and sulci, 

 which follow partictilar patterns. The superficial vascular 

 layer of connective tissue which covers the brain, and is 

 called pia mater, dips into these sulci : but the arachnoid, 

 or delicate serous membrane, which, on the one hand, covers 

 the brain, and, on the other, lines the cranium, passes from 

 convolution to convolution without entei-ing the sulci. The 

 dense periosteal membrane which lines the interior of the 

 skull, and is itself lined by the parietal layer of the arach- 

 noid, goes by the name of the dura mater. 



The general nature of the modifications obsei-vable in the 

 brain as we pass from the lower to the higher inammalia 

 is very well shown by the accompanying figures of the brain 

 of a Rabbit, a Pig, and a Chimpanzee (Figs. 21 & 22). 



In the Rabbit, the cerebral hemispheres leave the cere- 

 bellum completely exposed when the brain is viewed from 

 above. There is but a mere rudiment of the Sylvian fissure 

 at Sy, and the three principal lobes, frontal {A), occipital {B), 

 and temporal [C), are only indicated. The olfactory nerves 

 are enormous, and pass by a broad smooth tract, which 

 occupies a great space in the lateral aspect of the brain, 

 into the natiform protuberance of the temporal lobe (C). 



In the Pig, the olfactory nerves and tract are hardly less 

 conspicuous ; but the natiform protu.berance is more sharply 

 notched off, and begins to resemble the unciform gyrus in 

 the higher Mammalia, of which it is the homologue. The 

 temporal gyri (C*), though still very small, begin to enlarge 

 downwards and forwards over this. The upper part of the 

 cerebral hemisphere is much enlarged, not only in the 

 frontal, but also in the occipital region, and to a great extent 



