SENSE OF SIGHT. 607 



ment of the nostrils, a movement which is very marked in many of the 

 lower animals. As the odoriferous vapors arrive in the upper part of 

 the nasal passages, they are probably dissolved in the secretions of the 

 olfactory membrane, and thus brought into relation with its nerves. 

 Inflammatory disorders consequently interfere with the sense of smell, 

 both by altering the secretions of the part, and by producing a tume- 

 faction of the mucous membrane, which prevents the free passage of 

 air through the nasal fossae. 



A distinction is also to be made between the perception of true odors. 

 and the excitement of the general sensibility of the Schneiderian mu- 

 cous membrane by irritating substances. Some of the true odors are 

 similar in their nature to impressions perceived by the sense of taste. 

 Thus we have sweet and sour smells, though none corresponding to the 

 alkaline or the bitter tastes. Most of the odors, however, are of a pe- 

 culiar nature and are difficult to describe ; but they are always distinct 

 from the simply irritating properties which may belong to vapors as 

 well as to liquids. Thus, pure alcohol has little or no odor, and is only 

 stimulating to the mucous membrane ; while the odor of wines, cordials, 

 and perfumes, is communicated to them by other ingredients of a vege- 

 table origin. The vapor of pure acetic acid is simply irritating ; while 

 vinegar has also a peculiar odor, derived from its vegetable constituents. 

 Ammonia is an irritating gas, but contains no proper odoriferous prin- 

 ciple. 



The sensations of smell, like those of taste, remain for a certain time 

 after they have been produced, and modify in this way other less strongly 

 marked odors presented afterward. Asa general rule, the longer the 

 olfactory membrane is exposed to a particular odor, the longer its 

 effect continues ; and in some cases it may be perceived for many hours 

 after the odoriferous substance has been removed. Odors, however, are 

 particularly apt to remain after the removal or destruction of the source 

 from which they were derived, owing to the facility with which the}' are 

 entangled by porous substances, such as plastered walls, carpets, hang- 

 ings, and woollen clothes. 



The sense of smell, which is only moderately developed in the human 

 species, is excessively acute in some of the lower animals. Thus, the 

 clog will not only discover game and follow it by the scent, but will dis- 

 tinguish particular individuals by their odor, or recognize articles of 

 dress belonging to them by the minute quantity of odoriferous vapor 

 adhering to their substance. 



Sense of Sight. 



This is the most remarkable of all the senses, both for the special 

 nature of the impressions which it receives, the complicated structure 

 of its apparatus, and the variety and value of the information which it 

 affords with regard to external objects. It is by this sense that we 

 receive the impressions of light and color, with all their modifications 

 of intensity and combination, and acquire our principal ideas of form, 



