242 THE HUMAN MECHANISM 



excite no sensation of light. And the same thing is true of 

 other sensations. Conversely, after the amputation of a limb 

 it often happens that sensations are felt, as if they came 

 from the lost member. In this case the stump of the cut 

 nerve is stimulated in some way, and the impulses thus 

 sent to the brain excite the same sensations as if they came 

 from the usual endings of the nerve. When one hits his 

 " funny " or " crazy " bone (that is, directly stimulates the 

 ulnar nerve) the sensations developed in the brain may be 

 referred to the fingers in which the nerve originates. 



In the development of every sensation, therefore, we have 

 to distinguish between (1) what takes place in the sense 

 organ or end organ, (2) the passage of a nervous impulse 

 from this organ to the central nervous system, and (3) the 

 events which the arrival of the nervous impulse excites in 

 the brain. It is only the last (3) that, strictly speaking, we 

 can call sensation. The sense organs and their afferent fibers 

 are merely tributary mechanisms which serve to excite the 

 sensations in the brain. We are not aware that it is the 

 brain which is thus active, for we refer the sensation either 

 to the organ or to some external object. 



5. The sense of sight ; the eye. Sight is one of the most 

 highly specialized of the senses. The eye is the only organ 

 in which originate sensations of light or color, and it is a 

 wonderfully constructed apparatus, the function of which is 

 to stimulate the optic nerve by rays of light. It is essen- 

 tially a living camera in which, by means of a lens, an 

 image of things around us is formed upon the retina; just 

 as in the photographer's camera the lens forms an image on 

 the ground glass or on the sensitive plate or film. 



6. Structure of the eye. The eyeball consists of three 

 concentric coats surrounding and inclosing transparent sub- 

 stances through which rays of light pass to the retina. The 

 outer, or sclerotic, coat (the white of the eye) is composed of 

 very tough, dense connective tissue, and forms the protecting 



