EYES 467 



single surface [p) situated in the aqueous humour. Each point 

 of the object may be considered as sending out a pencil of divergent 

 rays which by refraction are made to converge again into a point 

 in an image which is constituted by such points corresponding 

 to those of the object. The diagram shows that the image is 

 inverted — what is the upper part of the object is represented 

 in the lower part of the image and what is on the right-hand 

 side of the object by the left-hand side of the image. The young 

 organism has to learn to associate stimulation of a particular 

 part of the retina with objects that lie in a certain direction in 

 space relative to the body and so is not inconvenienced by this 

 inversion. Without such experience the retinal image could not 

 be interpreted, and there is no question of the image being 

 * re-inverted ' by the brain, as is often supposed. The mammalian 



XiF' 



o 

 c 



■'-> - ^-.. 



V %.,s&^-'^- — 





Fig. 366. — A diagram to show the formation of a retinal image. 



a, b, c, Rays proceeding from the point X ; a', b' , c' , rays proceeding from the point Y ; p, theoretical 

 ' principal surface,' at which the combined refraction caused by the several surfaces of the eye is 

 supposed to take effect. The lens is shaped as in man and the rabbit. 



eye is able to focus objects that stand at varying distances from 

 the eye — from infinity down to ten inches in a healthy adult 

 man, and to even less in a child. This it does by altering the 

 shape, and so the focal length, of the lens. At rest the lens is 

 adjusted to focus distant objects, i.e. it is at its thinnest. It is 

 held thus by the tension of the radial suspensory ligaments 

 which hold it to the cihary body. To focus nearer objects, cihary 

 muscles contract, and in so doing reduce the tension on the 

 ligaments. This allows the elasticity of the capsule which encloses 

 the lens to come into play, and it makes the lens fatter, and so 

 reduces its focal length. This adjustment of the eye to objects 

 at different distances is called accommodation. 



EARS 



Something of the structure of the ear has already been 

 incidentally mentioned (p. 428), and more is shown in Fig. 367. 



