300 ANNUAL OF SCIENTIFIC DISCOVERY. 



tubules of the dental structure, and conveyed to the pulp cavity, it is 

 highly probable that the sensation is excited .at the latter organ. 



The sense of smell is entirely performed by the olfactory n^rve. 

 This is proved by the corresponding increase of the relative size of the 

 nerve in those animals which are known to possess a particularly acute 

 power of scent, and also by the fact that in paralysis of the trifacial 

 the sense remains unimpaired. The branches of the trifacial, which 

 are distributed to the mucous membrane of the lower and anterior 

 parts of the nasal cavity, endow it with a high degree of common sen- 

 sibility, so as to guard the more delicate part of the organ from injury, 

 by giving warning if we attempt to inhale any irritating vapor. This 

 common sensibility appreciates the pungency of substances in the 

 same way as in the case of any other mucous membrane. Many sub- 

 stances possess pungency beside odor, as ammonia and mustard, for ex- 

 ample. These affect the conjunctiva almost as readily as the nose. 



The organ of smell is affected by substances only when they are in 

 the form of vapor ; hence non-volatile substances have no smell. Va- 

 pors reach the organ in two ways. In the first place, a current of air 

 may be drawn, by a forcible inspiration, so as to be directed by the. 

 external nose to the upper part of its cavity, and impinge upon the 

 filaments of the olfactory nerve. If this air contains particles of any 

 volatile substance, it gives rise to the sensation which we call odor. 

 In the second place, if any volatile substance is taken into the mouth, 

 and carried to the fauces, or swallowed, and a puff of air is allowed to 

 escape from the larynx, it will be directed by the walls of the pharynx, 

 so as to carry the particles of the substance directly to the upper part 

 of the nares, where it produces what we describe as flavor. We un- 

 consciously emit this current of air, immediately after swallowing, and 

 when we are trying to taste anything. Thus we see that " scent and 

 flavor are the same impression on the same nerve at the same part." 



Flavors are connected, in a great majority of instances, with food. 

 This is the reason that the smell of roast meat so strongly excites the 

 appetite of a hungry man. The exercise of the sense of taste is simul- 

 taneous with that of smell in the act of eating, which accounts for the 

 difficulty of distinguishing between them. 



We can only classify these perceptions so far as to say that they are 

 agreeable or disagreeable. Even this distinction cannot always be 

 made ; thus the faint smell of putrid urine closely resembles that of 

 sandal-wood. What is offensive to one person may be pleasant to 

 another. The desire for certain flavors is entirely acquired, and the 

 infant will reject with loathing what may become its favorite food in 

 after-life. An agreeable flavor or odor sometimes becomes disagreea- 

 ble by long continuance. The odors of substances which are similar in 

 other respects are generalty alike, so that we may attempt to classify 

 them according to the sources from which they are derived. The 

 smells of plants are nearly, if not quite, all derived from essential oils. 

 The various ethers have kindred odors. 



Substances differ as to the intensity of their odor without reference 

 to their volatility. Thus the smell of inusk is more intense than that 

 of ether. 



In man, this sense only servos the purpose of giving him pleasure, 

 and guides him to a slight extent in the choice of food ; but with the 



