SMELLING. IIU 



vicinity of these two senses forms a double guard in the sd 

 lection of food. Were they placed in distant parts of the 

 body, they could not so readily give mutual aid. 



The air we breathe is perpetually impregnated with an in- 

 finity of different particles, which stimulate the olfactory nerves, 

 and give rise to the sensations of smell. When our senses 

 are not vitiated by unnatural habits, they are not only faithful 

 monitors of danger, but convey to us the most exquisite pleas- 

 ures. Even the sense of smelling is always productive either 

 of pleasure or pain. The fragrance of a rose, and of many 

 other flowers, is not only pleasant, but gives a refreshing and 

 delightful stimulus to the whole system, and may be considered 

 as a species of wholesome nourishment ; while the odors pro- 

 ceeding from hemlock, and from many other noxious vegeta- 

 ble, animal, and mineral substances, are highly offensive to our 

 nostrils. Hence we are naturally compelled to embrace the 

 one class of sensations, and to avoid the other. 



Some animals, as the dog, the fox, the raven, &c., are en- 

 dowed with a most exquisite sense of smelling. A dog scents 

 various kinds of game at a considerable distance ; and, if the 

 fact were not confirmed by daily experience, it could hardly 

 gain credit, that he can trace the odor of his master's foot 

 through all the winding streets of a populous city. If we 

 judge from our own feelings, this extreme sensibility in the 

 nose of a dog is to us perfectly incomprehensible. 



' In animals which do not breathe air, the odoriferous par- 

 ticles are conveyed to the organ of smelling through the me- 

 dium of water. In fishes, the nostrils do not form a tube or 

 canal, as in animals which breathe air, but consist of a simple 

 cavity on each side of the nose, into which the water, impreg- 

 nated with odors, flows, and communicates the sensation of 

 smell. 



' In the invertebral animals, no distinct organ for this sense 

 has ever been discovered, and yet they almost all appear to 

 possess it in no inconsiderable degree. Some insects, as was 

 mentioned in the last chapter, are induced, by the odor of cer- 

 tain fetid plants, to deposit their eggs upon the leaves, be- 

 lieving them to be putrid flesh ; they must, therefore, be pos- 

 sessed of the sense of smelling, for in no other way could 

 they be deceived. A probable conjecture with regard to the 

 organ by which it is exercised, is that which refers it to the 

 air-tubes or tracheae, by which they respire. In the mollusca, 

 it is not improbable that the whole surface of the' skin, which* 

 resembles in its structure the pituitary membrane of othei 



