SENSE OP SMELL OR OLFACTION. 115 



action by the contact of substances having a strong savour, they be- 

 come turgid and erect, so as to produce a decided roughness on the 

 surface of the organ. 



There is no special nerve of taste, the sense seems to be divided 

 between the glosso-pharyngeal and the fifth. — The impressions made 

 upon the front of the tongue are conveyed by the fifth ; those upon 

 the back of the organ by the glosso-pharyngeal. The first ministers 

 also to general sensibility ; the latter conveys the impressions that 

 produce nausea. It is also the afferent nerve in the reflex act of 

 swallowing. The ninth pair is also distributed to the tongue, but 

 it is a motor nerve, and not at all concerned in the special sense. 



A necessary condition for the exercise of this sense, is solubility 

 of the substance to be tasted ; if it be insoluble it merely excites the 

 feeling of contact. The sapid substance should also be moved over 

 the surface of the tongue; by this means the taste is very much 

 heightened. In this respect there is a strong analogy between touch 

 and taste. Taste may also be produced by mechanical irritation or 

 chemical excitation of its nerves. A smart blow of the finger, or 

 galvanism, will often excite a taste, sometimes acid, at others alkaline. 

 A large part of the impression made by sapid substances is received 

 through the sense of smell, as may easily be proved by attempting 

 to taste any substance while holding the nose. In inflammation of 

 the Schneiderian membrane too, we lose the power of appreciating 

 the flavour of bodies through the impairment of the sense of smell. 



Taste is an educable sense, as is seen in the case of the spirit tasters, 

 but it is not considered an intellectual one. Its subjective phenomena 

 are not so strongly marked as in some of the other senses, and yet 

 we are constantly experiencing pleasant or unpleasant tastes without 

 any apparent cause. Magendie states that dogs, into whose veins he 

 had injected milk, licked their lips as though they tasted it. 



The sense of taste is designed to guide us in our search for 

 food ; it is therefore placed at the entrance of the digestive apparatus. 

 Impressions of taste remain longer than those of other senses ; but 

 the after taste itself is not always the same as the original. 



Cold air deadens the sense of taste, precisely as it is known to do 

 in the sense of touch. 



SENSE OF SMELL OR OLFACTION. 



This sense is designed to acquaint us with the odorous qualities 

 of particles suspended or dissolved in the atmosphere. It is seated 

 in the mucous membrane of the nose, and at the commencement 

 of the respiratory passages, that it may protect them against the 

 entrance of deleterious /natters. Its principal use, however, is to 

 second the impressions of taste in conveying intelligence of the pro- 

 perties of food. 



The organ of the sense of smell has no capacity of movement in 



