CHAPTER XV 



THE OLFACTORY APPARATUS 



THE olfactory part of the brain as a whole is sometimes called 

 the rhinencephalon. In fishes (p. 112 and Figs. 43, 44) almost the 

 whole of the cerebral hemisphere is devoted to this function, and 

 as we pass up the scale of animal life more and more non-ol- 

 factory centers are added to the hemisphere in the corpus stria- 

 turn and cerebral cortex, until in man the non-olfactory part of 

 the hemisphere overshadows the rhinencephalon. The complex 

 form of the human cerebral hemisphere cannot be adequately 

 understood apart from a knowledge of this evolutionary history, 

 which has been studied with great care by comparative neurolo- 

 gists. The metamorphosis of the vertebrate cerebral hemisphere 

 from a simple olfactory reflex apparatus in the lower fishes to the 

 great organ of the higher mental processes upon which all human 

 culture depends is a very dramatic history, into which, unfortu- 

 nately, we cannot here enter. 



Smell is evidently the dominant sense in many of the lower 

 vertebrates. That this is the case in the dogfish is shown by the 

 enormous development of the olfactory centers of the brain, to 

 which reference has just been made. And in most of the labora- 

 tory mammals, such as the rat and the dog, the sense of smell 

 still plays a very much more important part in the behavior 

 complex than in man and other primates, whose olfactory organs 

 are in a reduced condition. 



The nervus terminalis is a slender ganglionated nerve found associated 

 with the olfactory nerve in most classes of vertebrates from fishes to man. 

 Its fibers, which are unmyelinated, reach the mucous membrane of the 

 nose, though the precise method of their ending is unknown. They pass 

 inward in company with those of the olfactory nerve as far as the olfactory 

 bulb. Here they separate from the olfactory fibers and enter the cerebral 

 hemisphere between the attachment of the olfactory bulb and the lamina 

 terminalis (Fig. 43, p. 111). Within the brain they have been followed 

 backward through the entire length of the olfactory area and hypothalamus, 

 but their cerebral connections have never been accurately determined. 

 The function of this nerve is likewise wholly unknown. 



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