138 



THE SENSES OF ANIMALS 



in the different parts of the picture thrown upon it. But the 

 eye is full of gadgets much more cunning than any in a 

 man-made camera. 



We need not go deeply into the details of the eye's struc- 

 ture but it will be helpful to look briefly at some of its more 

 important features. In all the higher animals, that is those 

 with backbones, the basic plan of the eye is the same. The 

 box is more or less globular and is made of tough, fairly 

 rigid tissue so that it is not easily distorted. It lies in a 

 socket of the skull which gives it protection from accidental 

 damage and it can be rolled round in the socket by six 

 small muscles so that it can point in any direction. 



The front of the eye globe is, naturally, transparent to 

 let in the light, and it is important that this window should 

 be kept clean. There is no problem for the animals that 

 live permanently in water, such as the fishes, for it is being 

 washed all the time. But vertebrates that live on dry land 

 have to do the washing themselves ; a gland beside the globe 



IRIS 

 (Transparent) CORNEA 

 PUPIL 



AQUEOUS 

 HUMOUR 



VITREOUS HUMOUR 

 RETINA 



FOVEA CENTRALIS 

 OPTIC NERVe 



PLIND SPOT 



Fig. 3. A human eye cut in half to show the internal structure. 



pours out a continual drip-flow of tears, and there are 

 movable eyelids that constantly wipe the moistened window 

 to sweep away minute particles of dust. The arrangement 

 is like the windscreen wipers and washers on a motor car, 

 but is even better because there is a small drain pipe at the 

 inner corner of the lower eyelid that carries away the used 

 tears into the nose so that they do not overflow and run down 

 the face. Everyone knows how uncomfortable such an over- 

 flow can be when one has a "cold in the eye" so that the 

 pipe is blocked and in a bad attack the tears may cause 

 inflammation and soreness of the skin. A few mammals, for 



