THE SENSE OF VISION 



619 



Fourthly, there is a contraction of the inner limbs of the cones. 



The Functions of the Retina. The retina is able to receive and 

 transform the ether vibrations into nervous impulses, so that the 

 brain perceives. Not only is light perceived, but at the same time 

 is appreciated the colour and also the form of external bodies. The 

 function of the retina may therefore be said to be to transform light 

 into nervous energy so that luminosity, form, and colour may be 

 perceived by the brain. 



The question arises as to which part of the retina is con- 

 cerned in the reception of the stimulus of the ether vibrations. The 

 effects (described above) which light produces on the layer of rods 

 and cones and pigment layer indicate that the transformation of 

 energy takes place here. This is confirmed by the following con- 

 siderations : 



FIG. 350. COMBINED ACTION or THREE SUBSTANCES, A, B, C, ON DARK EYE. 

 (Einthoven and Jolly.) 



Absc., 1 mm. =0*5 second; ordin., 1 mm. = 10 microvolts. 



1. The bloodvessels supplying the retina are distributed to the 

 anterior portion of the retina in the nerve-fibre and ganglionic layers, 

 the main vessel entering the eyeball at the spot where the optic nerve 

 passes in. Under certain circumstances, these vessels may be seen 

 as a shadow. This being the case, they must be perceived by the 

 parts of the retina lying behind them namely, the layer of rods and 

 cones. 



If a subject turn one eye inwards, and look towards a dark wall, 

 and with a lens a good light is focussed upon the exposed sclerotic, 

 so as to make a small but strongly illuminated area, then on giving 

 the lens a gentle rocking or circular movement, the field will appear 

 to the subject as reddish-yellow, and dark figures will be seen appear- 

 ing in the field, which branch and have the character of the retinal 

 bloodvessels, of which they are really the shadows. In the direct 

 line of vision a small area will be seen free from these branching 

 shadows. This is the yellow spot. So, too, if a white cloud be 

 viewed through a pinhole in a card held close to the eye and the 

 card be given an up-and-down movement, a number of vessels will 



