140 THE SENSES OF ANIMALS 



are nearly always longer and thicker on the upper lid ; they 

 are inconspicuous in most birds although in some, such as the 

 ostrich and the hornbills, they are so luxuriant and sweeping 

 that they might well be the envy of any film star. 



We saw that the globe of the eye is housed in a socket 

 of the skull; in many amphibians, however, the socket is 

 incomplete and has no floor of bone. The eyes are com- 

 paratively large and protuberant, and they protrude not 

 only upwards but downwards so that if we look into a frog's 

 mouth we see two bulges in the roof where the eyeballs lie. 

 As a result of this arrangement frogs and toads use their 

 eyes in a very peculiar way for a purpose that has nothing 

 to do with sight. When a toad eats a large worm it takes 

 one end into its mouth and cleans off the slime and crumbs 

 of earth sticking to it by running the worm through its 

 fingers. At each gulp it shuts its eyes because it pulls them 

 down in their sockets so that they bulge farther than usual 

 into the roof of the mouth and act as ramrods to push the 

 worm down its throat. 



The optical system of the eye is comparatively simple for 

 it consists of a single lens suspended towards the front of 

 the globe. It divides the globe into two parts, or chambers, 

 a small one in front filled with watery fluid, the aqueous 

 humour, and a larger one behind filled with soft jelly, the 

 vitreous humour. The humours in the anterior and posterior 

 chambers help to hold the lens in place so that the sling in 

 which it is suspended does not need to be rigid and im- 

 mobile — indeed it is essential that it should not be. The 

 lens is comparatively hard and gristly, but it is not so hard 

 and fixed in shape as a man-made lens of glass ; it is elastic 

 so that it can be pulled into different shapes. And herein 

 lies the beauty of its mechanism, at least in the higher verte- 

 brates ; when the muscle fibres contract the supporting sling 

 is slackened so that the curvature of the surface of the lens 

 is altered, and with the alteration of curvature the focal 

 length is altered. When the fibres relax the sling tautens 

 and pulls the lens back to its previous shape. The lens, un- 

 like that in a camera, thus has an infinitely variable focus 

 and assumes the right focal length for producing a sharp 

 image of an object at any distance ; it does not have to be 



