196 



PHYSIOLOGY. 



light-grey, ^ ue > ana " several shades and combinations of 

 these. It is on this alone that the colour of the eye de- 

 pends. The colour depends indeed on the refraction of the 

 light, as it falls on the fleecy or velvet-like surface of the 

 membrane, and also upon the degree in which the black 

 paint which covers its back is seen through it ; according as 

 the iris is more or less transparent, will the " colour of the 

 eye" be lighter or darker. 



11. In many of the lower animals, especially birds, the 

 iris assumes a still greater variety of colours. The round 

 hole in the centre of the iris is called the pupil, and it is the 

 dark pigment of the choroid coat, that we see through this 

 when we look into the eye of another. Aitkin says that this 

 name has been given to it, from the fact that when we look 

 into the eye, we see a small image of our own face, like a 

 very minute child or pupil. In albinos, and in animals that 

 see best in the dark, like owls, the dark pigment is wanting 

 not only on the choroid coat, but also on this membrane, and 

 as these parts are very vascular, the blood is seen imparting 

 a red colour to the eye. 



12. The iris has the power of dilating or contracting in 

 order to admit more or less light according as it is needed.. 

 This is effected by muscular fibres.) Of these, there are two 

 kinds. The first set converge from the circumference of the 

 iris to the circular margin of the pupil, and are called the 

 radiated muscles. When these contract they dilate the pupil. 

 The other set is composed of circular fibres which go round 

 the border, and indeed, form the pupil ; these are called 

 the orbicular muscle ; and when they contract diminish the 

 size of the pupil. Now, when more light enters the eye than 

 is wanted, the excited retina immediately gives warning of 

 the danger, and the nerves which are plentifully distributed 

 to the iris, stimulate the orbicular muscle to contract, and 

 the radiated one to relax, by which means the size of the pu- 

 pil is instantly lessened. On the contrary, when in the dark 

 or twilight, more light is needed, to transmit a distinct image 



