378 ELEMENTAR Y ANA TOM Y. [LESS. 



growth from the middle of the base of its hemisphere, and 

 forms the axis around which the whole hemisphere is de- 

 veloped. The " Island of Reil" lies in the outer surface of 

 each corpus striatum. 



The optic thalami are thickenings in the outer walls of the 

 third ventricle. 



The corpora mammillaria are the anterior inferior ends of 

 the fornix, being the roots of what are called its " anterior 

 pillars." 



The roof of the fourth ventricle is formed of the ependyma 

 alone, the pia mater and arachnoid being reflected over the 

 postero-inferior surface of the cerebellum. 



The " temporal lobe " of the cerebrum is developed early ; 

 the " occipital lobe " is a subsequent outgrowth from the 

 hinder part of the hemisphere. 



At first the cerebral hemispheres are smooth, the various 

 convolutions arising successively as development proceeds. 



The corpora quadrigemina are at first represented by but 

 a single hollow prominence on each side. 



10. A survey of the brain of Vertebrates generally, shows 

 us that this organ in man attains a very notable degree ot 

 SIZE and complexity. Yet the convolutions of the cerebrum 

 may be still more numerous than in him, as we see in the 

 Whales ; and the absolute size of the organ in the same 

 animals exceeds that of man very considerably. 



It might be supposed that the mass of the brain in man, 

 as compared with the weight of his body, is at a maximum ; 

 but it is not so, for even in some small Birds the proportional 

 weight of the brain is more than twice as great as in man. 



Nevertheless, considering man's actual bulk, the relative 

 size of his brain is very large, for, speaking generally, the 

 larger the body the less the relative size of the brain ; so that 

 we may safely assert that no animal with kistiulk of body has 

 a brain nearly so large relatively as man has. 



11. The CEREBRAL HEMISPHERES in man, in that they are 

 excessively convoluted, agree with those of the largest ani- 

 mals of man's own class, but in the smaller members of even 

 his own order (e.g. some American Monkeys) they may be 

 almost smooth, as is the case in all below Mammals. They 

 may be broader relatively than in man, as in Cetaceans, and 

 in a less degree in Seals. They may be more truncated 

 anteriorly (as in Ruminants), or more pointed (as in the 

 Rabbit). 



They may, by rare exception, project back beyond the 



