NERVOUS SYSTEM 865 



quadrigemina, only the anterior pair being connected with the optic 

 nerves, the posterior pair being connected with the sense of hearing. , 



An important point in the understanding of certain brain structures 

 is the knowledge that the pallium causes a folding so that the original 

 postero-ventral end of the cerebrum, lateral to the pyriform lobe, is 

 pushed below and to the outside of the lateral parts of the hemispheres, 

 the fissure of Sylvius marking the, place of folding. It is at the bottom 

 of this fissure that the island of Reil is found, which is only the covered 

 part of the sides of the hemispheres. 



All higher forms of mammals have the hemispheres arranged in 

 many convolutions. This permits an increase in surface without neces- 

 sitating a great bulk increase. However, some animals with less mental 

 ability apparently have more convolutions than the more highly organ- 

 ized, so that it cannot definitely be said that greater convolutions neces- 

 sarily carry greater mental power.* 



The mammalian cerebellum, while better developed than that of 

 reptiles, is not as highly developed as that of birds. 



Ornithorhynchus (Fig. 472) has the most primitive brain of all 

 mammals. It is small, the cerebral hemispheres are smooth and lack all 

 convolutions. This animal is aquatic in its habits, living in stagnant 

 water and feeding chiefly on mollusks, crustaceans, and worms which it 

 secured by scooping up the muddy bottom with its bird-like bill. 

 THE ORGANS OF SPECIAL SENSE 



It will be remembered that one of the outstanding characteristics 

 of living matter is its irritability. Contractility is usually added to 

 irritability when living protoplasm is discussed. It has already been 

 shown that various functions in the body may be carried on when the 

 entire nerve supply to that portion has been destroyed. We may there- 

 fore say, that while irritability and contractility are essential parts of 

 living matter, the irritability which causes contractility need not be 

 brought about by a definite system of nerves, although the nerves do 

 carry the stimulus from one part of the body to another, thus coordi- 

 nating the various parts and permitting them to work together for some 

 common end. 



In all higher forms, there are external organs of special sense, such 

 as the nose, the eye, and the ear. In some of the lower forms such as 

 the earthworm, we found, that while there are no definite eyes, the earth- 

 worm nevertheless responded to light thrown upon its body by moving 

 out of -the way as rapidly as it could. We. know from this experiment 

 that the earthworm is sensitive to light, and that therefore there are 

 definite sensory regions, more or less specialized, in its skin by which 

 it can receive a stimulus from light. 



.It -w.ould profit an animal little to be. able to receive a stimulus if 

 it could not in some way move itself .toward or away from such stimu- 

 lus. The muscles., by -which an animal may move out of harm's way or 



* Echidna has more brain convolutions per body- weight than man. 



