SMELLING. 117 



CHAPTER IV. , 



OF THE SENSES. 



No animal, of which we have any knowledge, is endowed 

 with more than the five external senses of smelling, tasting, 

 hearing, touch, arid seeing ; and no animal, however imper- 

 fect, is destitute of the whole. Without organs of sensation, 

 in a smaller or greater number, animal or intellectual exist- 

 ence is to us an inconceivable idea. 



Our observations on the different instruments of sensation 

 will proceed in the following order, namely, of the senses of 

 smelling, of tasting, of hearing, of touch, and of seeing. In 

 general, it may be remarked, that all sensation is conveyed to 

 the mind by an unknown influence of the nerves. If the op- 

 tic, olfactory, or any nerve distributed over an organ of sen- 

 sation, be cut, or rendered paralytic, the animal instantly loses 

 that particular sense. This is a fact universally established 

 by experiment. But that the nerves, which are nearly similar 

 in every part of the body, should, when distributed over the 

 eye, the ear, the tongue, the nose, convey to the mind feelings 

 so different, is the most mysterious part of this subject 



Of Smelling. 



In man, and many other animals, the organ by which the 

 sense of smelling is conveyed to the mind, has received the 

 general appellation of nose or nostrils. The more immediate 

 instrument of this sensation is a soft, vascular, porous mem- 

 brane, and is known by the name of the pituitary membrane, 

 or Schneider tan membrane. This membrane is totally covered 

 with infinite ramifications and convolutions of the olfactory 

 nerves. These nerves are almost naked, and exposed to the 

 action of the air which passes through the nose in performing 

 the function of respiration. But Nature, ever attentive to the 

 ease and convenience of her creatures, has furnished the 

 nostrils with a number of glands, or small arteries, which 

 secrete a thick, insipid mucus. By this mucus, the olfactory 

 nerves are defended from the action of the air, and from the 

 painful stimuli of acrid odors. 



The odors perceived by smelling are extremely various. 



