PHYSIOLOGICAL 243 



bility, in quickness of adjustment, in economy, if not in optical 

 precision, the eye far surpasses any camera. 



The surface of the eye, where it looks out upon the world, is 

 protected by a delicate membrane called the conjunctiva; behind this 

 is the cornea, a transparent window in the thick, opaque white outer 

 wall of the eyeball. Behind the cornea is a small space filled with a 

 clear fluid, the aqueous humour, and behind this is the lens, a flat- 

 tened sphere of glass-clear protein. But in front of the lens, as 

 everybody knows, lies the iris, a thin ring of opaque tissue which 

 appears blue unless it happens to contain brown pigment. The iris 

 can contract or expand, so that the aperture it surrounds, the pupil, 

 may be small or large ; in this way the amount of light entering the 

 eye proper can be controlled. Behind the iris and the lens is a fairly 

 large space, comparable to the box or bellows of a camera, but 

 filled with a clear jelly, the vitreous humour. The wall of this chamber 

 is hned by the sensitive membrane, the retina, which receives the 

 light-rays and translates them into nervous impulses which pass to 

 the brain by the optic nerves. 



The eyeball is mounted upon a ball-and-socket joint, elaborately 

 cushioned and sprung, and its movements are controlled by six 

 muscles. Four of these run from the wall of the orbit to the equator 

 of the eyeball, and, when they contract, act like reins, pulHng the 

 eyeball round in one direction or another ; the other two are shorter 

 and oblique, and serve to balance and restrict the action of the 

 "reins". As everybody knows, there is the closest correlation between 

 the eye-muscles on the two sides of the body; the eyes move 

 together, with the most delicate adjustment, to ensure that their 

 axes shall converge upon the object looked at, be it near or distant. 

 The external muscles of the eyeball are excellent objects for the 

 study of co-ordination in muscular movement, brought to its highest 

 pitch. 



The iris is also muscular, so that the dimensions of the pupil may 

 be varied. When a bright light shines into the eye and stimulates 

 the retina, a reflex comes into action and causes the circular muscle 

 of the iris to contract, thus diminishing the size of the pupil. When 

 the Hght stimulus is removed the pupil dilates again, and in 

 absolute darkness remains dilated; but if any light remains, the 

 pupil gradually contracts as the eye becomes used to the dim light. 

 In this reflex, too, there is great correlation between the two eyes; 

 a light shone into one eye causes contraction of both pupils. 



The eye differs from a simple camera in its optical system, for it 

 has two lenses — the "lens" proper, behind the iris, and the "cornea" 

 in front, and the latter is the more important of the two in refracting 

 light rays. But in fishes, or any other vertebrate which may happen 

 to find itself under water, the whole work is done by the lens ; the 

 cornea can only refract light rays when they come to it through the 



