Nervous System and Sense Organs 197 



but in mammals they are tiny and insignificant. They are overshadowed 

 by the large dorsal part, the cerebrum. In the mammalian brain, the 

 olfactory lobes are actually ventral in position rather than anterior. 



The cerebrum is the largest and most conspicuous part of the mam- 

 malian brain and fills a large share of the cranium. This condition 

 contrasts with that found among the lower vertebrates such as the fish 

 and frog, where the cerebrum is a small inconspicuous lobe. In mam- 

 mals, this lobe has become so large that the remaining portions of the 

 brain are considerably different in their relationships to one another than 

 in the fish or frog. The increasing size of the cerebrum (Fig. 59) is cor- 



FISH 

 Perch 



AMPHIBIAN 



Tuad 



REPTILE 

 Snake 



BIRD 



Pigeon 



MAMMAL 

 Cat 



-olfactory nerve 

 olfactory lobe 

 cerebral Jv/< 

 emispherfy Y 



pineal v 

 body ■ 



ptic 

 lobe 



erebellurri^ 



medulla /\ y 

 •oblongata \ 



olfactory lobe 



cerebral 

 hemisphere 



cerebellum--- 



medulla ,. 

 oblonga'ta 



P"ig. 59. — A comparative series of vertebrate brains, dorsal views. 

 (From Guyer: Animal Biology, Harper & Brothers.) 



related with the increasing intelligence of the various vertebrate groups. 

 Man, theoretically the most thoughtful of mammals, has the most highly 

 developed cerebrum. From these observations and experimental work, 

 it is clear that the cerebrum is concerned with what is termed "higher 

 thinking." 



The cerebrum is made up of the two cerebral hemispheres sep- 

 arated from one another by a deep median groove. In mammals each 

 hemisphere is in turn composed of several lobes which are named ac- 

 cording to the skull bone under which they are located. Thus there are 

 the frontal, the parietal, the occipital, and the temporal lobes. The outer 

 portion of the cerebrum, the cortex, is composed of gray matter ; that is, 

 the nerve cell bodies are located there. The surface of the cortex is in- 



