492 THE SPECIAL SENSES. 



case of all other continuous and unvarying impressions. Our at- 

 tention is only called to it when we meet suddenly with another 

 and familiar odor. This second odor, we find, does not produce its 

 usual impression, because it is mingled with and modified by the 

 other, which is more persistent and powerful. Thus we are again 

 made aware of the former one, to which we had become insensible 

 by reason of its constant presence. 



The sense of smell is comparatively feeble in the human species, 

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

 dog will not only distinguish different kinds of game in the forest 

 by this sense, and follow them by their tracks, but will readily dis- 

 tinguish particular individuals by their odor, and will recognize 

 articles of dress belonging to them by the minute quantity of odor- 

 iferous vapors adhering to their substance. 



SIGHT. The sight undoubtedly occupies the first rank in the 

 list of special nervous endowments. It is the most peculiar in its 

 operation, and the most immaterial in its nature, of all the senses, 

 and it is through it that we receive the most varied and valuable 

 impressions. The physical agent, also, to which the organ of sight 

 is adapted, and by which its sensibility is excited, is more subtle 

 and peculiar than any of those which act upon our other senses. 

 For the senses of touch, taste, and smell require, for their exercise, 

 the actual contact of a foreign body, either in a solid, liquid, or 

 aeriform condition; and even the hearing depends upon the me- 

 chanical vibrations of the atmosphere, or some other sonorous 

 medium. But the eye does not need to be in contact with the 

 luminous body. It will receive the impressions of light with per- 

 fect distinctness, even when they are transmitted from an immea- 

 surable distance, as in the case of the fixed stars ; ancj the light 

 itself is not only immaterial in its nature, so far as we can ascertain, 

 but is also capable of being transmitted through space without the 

 intervention of any material conducting medium, yet discoverable. 



Finally, the apparatus of vision is more complicated in its struc- 

 ture than that of any other of the special senses. This apparatus 

 consists, first, of the retina, as a special sensitive nervous membrane ; 

 and secondly, of the vitreous body, crystalline lens, choroid, scle- 

 rotic, iris and cornea, together with the muscles moving the eye- 

 ball and eyelids, lachrymal gland, &c., as accessory organs. The 

 arrangement of the parts, constituting the globe of the eye, is shown 

 in the following figure. (Fig. 155.) 



