HEARING. 155 



the two kinds of perception is lost to us, and we speak of the smell 

 of ammonia or of alcohol when it is probably not an olfactory, but 

 a sensory, perception. 



Is the sense of smell very acute ? 



Yes, but not so sharp in man as in many of the lower animals. 

 The distribution of the olfactory nerves is much wider in some of 

 the animals, and the cerebral development is correspondingly in- 

 creased. In man the range of susceptibility is, however, probably 

 greater. The variety of odors and the very minute quantity of 

 stimulant substance required to produce a sensation of smell are 

 quite wonderful. The most delicate analysis may fail to show 

 traces of the substances which can be appreciated by the sense of 

 smell. 



Is the same odor agreeable to all men? 



No. There are some odors pleasant to some which others find al- 

 most intolerable. Musk, for example, is a pleasant perfume to some, 

 while to others it is quite unendurable. In the same way, the 

 acuteness of this sense in some is more marked than in others, and 

 yet this may apply only to certain kinds of odors. 



Is sneezing a reflex from the olfactory nerve ? 



No. It is excited through the fifth. 



Are hallucinations of smell common ? 



They often occur, and in cases of disease of the olfactory centres 

 there is often complaint of a constant bad smell. With normal 

 organs there may be a sensation of an odor which cannot be de- 

 tected by others present. 



HEARING. 

 How does the auditory or eighth cranial nerve originate ? 



The fibres arise from a nucleus of gray matter in the floor of the 

 fourth ventricle, and from this source pass out through the sub- 

 stance of the medulla in a number of small bundles which unite 

 with another root from the cerebellum to form a trunk. This passes 

 with the facial nerve into the internal auditory canal, and terminates 

 in special end-organs in the internal ear. 



Describe, roughly, the auditory apparatus. 



It consists of (1) the external ear; (2) the middle ear; and (3) 

 the internal ear. 



