68o Journal of Agriculture. [lo Nov., 1908- 



additional pm-pose tliat they act as a circulating agency on the fluids within> 

 the eyeball. Immediately behind the iris is the crystalline lens which 

 is composed of transparent laminae devoid of blood vessels and nerves and 

 arranged like the layers in an onion. The lens is highly elastic and is at- 

 tached firmly along its border to a sheet of tissue called the choroid which 

 sweeps round a great part of the eyeball beneath the sclerotic. The 

 mechanism of focussing for near and far objects in some o.f the lower verte- 

 brates is carried out in much the same way as in a camera, namelv, by 

 moving the lens forwards or backwards. But in mammals this device is 

 supplanted by another, namely, an alteration in the lens itself. When a 

 mammal has its eye focussed for far distance that is, in the resting position, 

 the lens is kept on the stretch by the purely physical character of its- 

 attachments to the choroid. As this stretch is exerted all round, the lens is 

 kept flattened, particularly in its front aspect. When however focus-sing- 

 for near objects is desired, a special muscle called the ciliary muscle, 

 which lies along the root of the iris, pulls on the choroid and draws it for- 

 ward so that the tension on the lens is relieved and the lens, by virtue of 

 its inherent elasticity, bulges, particularly on its front surface, becomes 

 therefore more convex as a whole or, in ordinary parlance, becomes a 

 stronger lens. This explains why focussing for near objects is recognised 

 by us as requiring effort; when focussing for far objects we stop the action 

 of the ciliary muscle and let the choroid, through its greater elastic pull, 

 drag upon the lens margin. The range of focussing in the domestic animals- 

 is not nearly so wide nor is the mechanism as perfect as it is in man and 

 the monkey. Behind the lens is a glairy but transparent fluid, trie 

 VITREOUS HUMOUR which fills up the greater part of the eyeball. In 

 contact with the vitreous humour and sweeping round the inner wall of 

 the eyeball as far as the lens attachment, is the very delicate retina which 

 contains the receptors for light. The greater part of the retina does not 

 lielong at all to the peripheral nervous svstem ; it is in realitv a. protrusion 

 of the brain stem ; the so-called optic "nerve" (second cranial nerve) which 

 can be seen entering the eyeball and spreading out into the retina, being not 

 a nerve trunk but a column of white matter belonging to the central nervous- 

 system. One of the many layers of the retina does however belong to the 

 peripheral nervous system and this is the layer of " rods and cones " which 

 consists of a mosaic of receptors for light attached each to an afferent 

 neuron. At that point in the retina where the optic " nerve " with its 

 attendant blood vessels enters, the layer of light-receptors is absent and so 

 we have the Avell-known blind spot. In many portions of the retina the 

 layer of rods and cones does not present an unbroken surface to the light, 

 tilling traversed by blood vessels and nerve fibres, but in the centre and' 

 aJmost in a line with the pupil and the centre of the lens, the layer of 

 receptors is free from superimposed blood vessels and on it the light can 

 fall without anv interruption. 



The range of ether rhythms (red to violet) to which the retina is respon- 

 Tjive is limited and is very small in comparison with the range of the air 

 rhythms which can be perceived by the ear as sound. The mechanism by 

 which colour is appreciated is not understood. According to one theory 

 there are receptors for the three fundamental colours, red, green and violet, 

 any intermediate colour being perceived by the unequal stimulation of two 

 or all three of these. According to another theory certain colours including 

 white, provoke chemical disintegration within certain receptors whilst other 

 colours including black excite chemical building up in the same receptors.. 



