60 THE ANATOMY OJ VERTEBRATED 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 yentricles are prolonged 

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

 what are termed their anterior, pjosterior^ and descendiyig 

 cornua. 



Furthermore, 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 particular patterns. The superficial vascular lay- 

 er 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 entering the sulci. The dense perios- 

 teal membrane which lines the interior of the skull, and is 

 itself lined by the parietal layer of the arachnoid, goes by the 

 name of the dura mater. 



The general nature of the modifications observable in the 

 brain as we pass from the lower to the higher mammalia is 

 very well shown by the accompanying figures of the brain of 

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



In the Rabbit, the cerebral hemispheres leave the cerebel- 

 lum 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 {JB), and tem- 

 poral ( G), are only indicated. The olfactory nerves are enor- 

 mous, 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 protuberance is more sharply 

 notched off, and begins to resemble the unciform gjTus in the 

 higher 3Iammalia, of which it is the homologue. The tem- 

 poral gyri ( C), though still very small, begin to enlarge down- 

 ward and forward 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 hides the cere- 

 helium when the brain is viewed from above. What in the 

 \labbit was a mere angulation at Sy, in the Pig has become a 

 .ong sulcus — the Sylvian fissure, the lips of which are formed 

 by a gyrus, the Sylvian, or angidar, gyrus. Two other sets 

 of gyri, more or less parallel with this, are visible upon the 

 outer surface of the hemisphere; and at the entrance of the 



