260 



THE CAT. 



'CHAP. ix. 



and of smaller portions which connect these together and with the 

 spinal cord, of which the hrain (as has heen already observed) is, as 

 it were, the greatly enlarged, anterior termination. 



The largest portion of the hrain hy far is the cerebrum ; which is 

 made up of two great masses termed hemispheres^ placed side by 

 side, and forming the anterior, upper, and lateral parts of the brain. 

 Each cerebral hemisphere is considered to principally consist of two 

 main parts or lobes. The more anterior of these is called the 

 frontal lobe, and it includes nearly the anterior half of each hemi- 

 sphere. The other is the temporal lobe, which forms a lateral and 

 inferior prominence, which lies in the " internal temporal fossa " of 

 the inside of the skull.* The posterior, inner, and upper portion of 

 each hemisphere may be regarded as a slightly and indistinctly 

 developed posterior lobe. The hemispheres are united with the 

 hinder part of the brain mainly as follows : The spinal cord on 

 entering the skull becomes modified and takes the name of the 

 medulla oblongata, and this is the hindmost part of the base of the 

 brain. Continuing forwards the medulla divides into two large 

 branches, the crura, which pass respectively one into each cerebral 

 hemisphere, and thus connect them with the spinal column. On 

 the dorsal surface of this continuation of the spinal cord into the 

 skull, is placed the cerebellum (or second largest portion of the brain), 

 while on its ventral surface is a prominent mass of transversely 

 disposed fibres the pom Varolii which, as it were, wraps round 

 the anterior end of the medulla on its under surface, and covers in, 

 ventrally, its divergence into the crura, which thus appear to issue 

 from above the anterior margin of the pons. The brain contains 

 within it certain cavities, which, with one exception, are the greatly 

 enlarged and complexly shaped continuation forwards of that 

 minute canal (the " canalis centralis ") which we have seen to 

 traverse the spinal cord for its whole length. The different por- 

 tions of this curiously expanded cavity within the brain are termed 

 ventricles, and they are lined by a delicate epithelial membrane 

 termed the ependyma. This ventricular cavity extends forwards 

 beneath the cerebellum and above the pons Yarolii, and as it is 

 mainly bounded below (in front of the pons Varolii) by the 

 crura and certain other structures between them, so it is bounded 

 above (between the cerebellum and the cerebrum) by a variously 

 formed layer of brain substance, which will be described further on, 

 and which constitutes a minor bond of union between the cerebrum 

 and the parts behind it. From the anterior and lower part of each 

 hemisphere there proceeds forwards a body which consists of a 

 cylindrical prolongation of brain substance, ending in a rounded 

 expansion. These two bodies are called the olfactory lobes.} They 

 lie within the olfactory fossa of the cranium. 



* Sec ante, p. 83. 



t In human anatomy they are often 

 called the olfactory nerves. But in fact 



the true olfactory nerves come from 

 them. 



