442 'Journal of Comparative Neurology and Psychology^ 



role, that in man and the apes they are reduced to a small structure, 

 while in the aquatic mammals they are completely absent. No 

 one will deny that the various degrees of development of the parts 

 of the palaeencephalic olfactory apparatus furnish most impor- 

 tant problems for psychology. 



I should hke to show you by means of two more examples how 

 the development of the palaeencephalon is dependent upon the 

 demands of the outside world. The roof of the mid-brain, which 

 receives upon the one hand the optic nerves and upon the other 

 hand secondary sensory tracts, is much more strongly developed 

 in birds and fishes than in any other vertebrates, but in blind 

 animals it may be atrophied. In cases where we find such 

 anatomical atrophy, we should be stimulated to investigate the 

 capacity for sight. Then it appears that in animals (Proteus) which 

 are entirely blind certain tracts of other sensory mechanisms are 

 especially strongly developed. Their mode of operation presents 

 still other problems. 



The size of the cerebellum is so completely dependent upon life 

 habits that in some sedentary animals it has completely disap- 

 peared, while in weak swimmers (eel, flounder) it is very small, 

 but in the strong swimmers and fliers it attains enormous size. 

 In so nearly related animals as the land and water chelonians the 

 former have a cerebellum less than half as large as that of the latter. 

 Much futile work on the physiology of the cerebellum would have 

 been spared us if w^e had regarded these facts of comparative 

 anatomy. 



Finally, let me refer again to the important apparatus of the 

 lateral line sense of the fish. Because of the obviousness of 

 its end organs in the skin, this sense fortunately has found many 

 investigators and now we know through the investigations of 

 FucHS, and of Huber that this entire apparatus enables the 

 animal to detect changes of pressure in the water, particularly 

 the resistance which it encounters in swimming. Here anatomy 

 has led to physiological investigation. 



The palaeencephalon alone is present in the bony fishes. The 

 activities which depend upon it we will designate as palceencephalic 

 activities. Since in all other vertebrates, with the appearance of 

 the neencephalon, quite diff'erent — neencephalic — activities make 

 their appearance, it is of the greatest importance to study thor- 

 oughly the activities of fishes. The central nervous apparatus of the 



