300 SENSE OF SIGHT. 



greatly in getting the odor. The nerves have peculiar end- 

 ings, and it is not known just how the substances produce 

 their effect. The substances must be in a very finely divided 

 state, probably gaseous. The mucous membrane is supplied 

 with mucus, and the odorous substance, probably, is first dis- 

 solved in the mucus. The lower, or respiratory, passages 

 have a more abundant blood supply, and are redder than the 

 upper. In inflammation, owing to their narrowness, the pas- 

 sages, especially the upper, are often closed by contact of the 

 opposite sides. Substances like ammonia have no odor, but 

 excite the tactile nerves. They are often spoken of as hav- 

 ing a "pungent" odor, but are simply irritants. 



The Sense of Sight. In the fable of the blind man 

 carrying the lame man. whose eyes were good, we have an 

 illustration of the dependence of the various organs on each 

 other. AVe have considered how all our knowledge, both of 

 the condition of our bodies and of the external world, comes 

 through the nervous system. Now, so far as the senses we 

 have studied are concerned, we learn almost nothing of the 

 external world except from actual contact. But sight reveals 

 objects at a distance. Without the eye the body is compara- 

 tively helpless. The lame man that the body carries is a 

 slight burden in comparison with the assistance which he ren- 

 ders. We can 1 well afford to carry with us all the time two of 

 these lame men to keep posted as to the objects beyond our 

 reach. Of course touch is a great aid to our interpretations 

 of what we see. But sight is evidently the main avenue of 

 knowledge, the royal road along which come the messages 

 which bring us the most news, which give us the keenest 

 delight, which make us aware of most that we know of this 

 world, and the only means of knowing that there are other 

 worlds than the one we inhabit. 



In order to understand, in any clearness, the action of the 

 eye, we ueect to examine into its structure. 



