GENERAL ANATOMY OF THE NERVOUS SYSTEM 05 



At the extreme anterior end each of the hemispheres is 

 prolonged into a brush-like development of nerve-fibers 

 which comes into relation with the nasal lining on the 

 same side. These extensions are usually termed the 

 olfactory nerves and are counted as the first in the cranial 

 series. No other nerves come into direct connection with 

 the cerebrum; even the optic nerves, which are so much 

 more important in determining the reactions of the more 

 intelligent animals, join the parts just behind the hemi- 

 spheres and affect them only after the impulses have been 

 relayed or forwarded. The optic nerves are second in the 

 series as we count from before backward. Other cranial 

 nerves emerge at short intervals behind the optic. It will 

 be better not to enumerate them now, but to speak of them 

 with direct reference to the human brain a little later. 



All the divisions which have been described as having 

 places in the organization of the brain of the frog can be 

 recognized in the brains of the higher vertebrates. Their 

 proportionate development we shall find to vary widely 

 from what has been pictured. There is one region only 

 hinted at in the brain of the frog which comes to be a large 

 feature in many cases. This is the cerebellum,. Its posi- 

 tion in the frog is indicated by a slight dorsal growth 

 at the anterior margin of the fourth ventricle. 



We shall now find it helpful to compare with the brain 

 of the frog that of the rabbit, which will show a state of 

 things intermediate between what has been described 

 above and the human condition which must be outlined 

 presently. If we ignore differences of absolute scale 

 we shall see at a glance that the cerebral hemispheres in 

 the rabbit are more massive than in the frog. We shall see 

 also that the cerebellum has become a part of the brain 

 only less conspicuous than the hemispheres themselves. 

 We can still discover between the cerebellum and the 

 cerebrum the surface of the midbrain, but this section 

 which was so prominent in the frog has become much less 

 so. It is evident that a moderate additional increase in 

 the bulk of the cerebrum and cerebellum would bring 



