OF ANIMALS. 33 



longata, and somewhat under the posterior lobes of the cerebrum, some- 

 what in the situation of the cerebellum in the next class, are equal in 

 size to one-sixth of the whole brain. They have each a cavity in the 

 middle, which make the fourth and fifth ventricles, and these commu- 

 nicate with, or enter at, the communication of the third with the sixth, 

 so that all those six ventricles communicate with each other. 



The cerebellum is a prominent pyramidal body, standing on the pos- 

 terior and upper part of the medulla oblongata, behind and somewhat 

 between the posterior lobes of the cerebrum and in contact with them : 

 it is more convoluted than the cerebrum, which convolutions are some- 

 times similar to the human. 



Of the Sixth Class, or Quadrupeds. — The brain in this class is, in 

 general, larger than in the preceding, and the parts more compacted, 

 the whole mass being brought into nearly a globular figure. 



The cerebellum is more immediately under the cerebrum, and the 

 convolutions in the cerebrum are deeper 1 . 



The nates and testes are four small bodies, with no visible cavities, 

 which are not seen externally, but lie at the posterior end of the third 

 ventricle. 



The ventricles are only four in number 2 . The two lateral ones com- 

 municate under the lower edge of the septum lucidum, and are pretty 

 large ; beginning in the anterior lobe of the cerebrum by a blunt end 

 pretty far forward, going directly back, and when got some consider- 

 able way, bending outward and downward, then forward and still 

 down- and also inward, and ending nearly under their origins. In them 

 lie the corpora striata, the thalami nervorum opticorum, the plexus cho- 

 roides, and the fornix. The third ventricle is directly under the fornix, 

 and communicates forwards by a small opening with the infundibulum, 

 which goes down to the pituitary gland ; behind, it communicates with 

 the fourth ventricle, which is partly in the medulla oblongata. 



The cerebrum and cerebellum end by four peduncles in the tuber- 

 culum annulare, and the medulla oblongata goes out from it ; at the 

 going out of wbich are four pyramidal bodies, viz. the corpora olivaria, 

 and corpora pyramidaha. 



In the brain the cortical substance is on the outside, in the medulla 

 spinalis within ; in some it is in one line running down, in others two. 



1 [This applies only to the orders Cetacea, Ungulata, Carnivora, and Qiiadrwmana, 

 associated together as ' Gyrencephala ' in my "Cerebral System of the Mammalia," 

 Proceedings of the Linnean Society, 1857.] 



2 [The interspace between the layers of the septum lucidum is now regarded as 

 constituting a fifth ventricle, and is peculiar to the Mammalia.] 



D 



