The Body-Machine and Its Work 353 



stand how the images of the two eyes are blended, and how they 

 enable us to see nature more perfectly. 



Vision, as might be expected, is still very imperfectly under- 

 stood. The retina is a very complex layer of delicate nerve-cells, 

 in which certain parts that are known as "rods" and "cones" seem 

 to be the essential elements. There seems to be chemical action, 

 though whether there are three distinct chemicals for the three pri- 

 mary colours, or one chemical that breaks into separate colours, 

 or what happens, we do not know. It is generally suspected that 

 colour-vision is connected with one or more fine chemicals which 

 may be lacking in "colour-blind" people. However that may be. 

 the nerve-layer closes up at the back of the eye and, as the optic 

 nerve, conveys the images of things in some way to the conscious 

 centre. What precisely travels along the nerve we cannot say, 

 but to imagine that an image or picture is conveyed is to imitate 

 children who think that words travel along a telegraph wire. 



The Sense of Hearing 



The organ of hearing is not less remarkable than the eye. 

 We have already seen that the external ear is, to use the cautious 

 words of Professor Starling, probably of no use whatever. In 

 cases where it has been cut off the sense of hearing was not af- 

 fected at all. But it was useful and mobile in an earlier ancestor 

 of man. From it, in any case, a narrow channel about an inch 

 long, protected against adventurous insects by wax secreted by 

 its glands, conducts the waves of sound to the real ear. 



At the outer end of this passage the sound-waves beat upon 

 a sensitive drum, the tympanum, a membrane of a most ingenious 

 construction. This membrane must not have a period of vibration 

 of its own. It must respond readily and immediately to every 

 sort of wave that impinges on it. It is therefore so constructed 

 that each part of it has a different period of vibration, and it is 

 further "damped" by a little bone pressing against it on the other 

 side. The pressure of air on the outside of the drum, which must 



