'THE SENSE OF SIGHT 719 



giving to the eye an appearance of divergence. In myopia it is much 

 smaller 2 or in extreme cases may be abolished, the line of vision 

 corresponding with the optic axis or even passing beyond it. The 

 angle gamma is of value in determining the actual deviation of the eye 

 in squint. 



Functions of the Retina. Of all the layers of the retina, the rods and 

 cones appear to be the most essential to vision. It is only this layer that is 

 capable of receiving the light stimulus and of transforming it into some 

 specific form of energy, which in turn arouses in the fibers of the optic nerve 

 the characteristic nerve impulses. A ray of light entering the eye passes 

 entirely through the various layers of the retina, and is arrested only upon 

 reaching the pigmentary epithelium in which the rods and cones are embedded. 

 As to the manner in which the objective stimuli light and color, so called 

 are transformed into nerve impulses, but little is known. It is probable 

 that the ether vibrations are transformed into heat, which excites the rods 

 and cones. These, acting as highly specialized end-organs of the optic 

 nerve, start the impulses on their way to the brain, where the seeing process 

 takes place. As to the relative function of the rods and cones, it has been 

 suggested, from the study of the facts of comparative anatomy, that the rods 

 are impressed only by differences in the intensity of light, while the cones, in 



FIG. 308. DIAGRAM FOR OBSERVING THE SITUATION OF THE BLEND SPOT. 



(Helmholtz) 



addition, are impressed by qualitative differences in color. The nerve- 

 fibers themselves are insensible to the impact of the ether vibrations, and 

 require for their excitation some intermediate form of energy. That this 

 is the case was shown by Bonders, who reflected a beam of light on the optic 

 nerve at its entrance without the individual experiencing any sensation of 

 light. This region, occupied only by the optic-nerve fibers and devoid 

 of any special retinal elements, is therefore an intensive or blind spot. The 

 diameter of this spot is about 1.5 mm., and occupies in the field of vision a 

 space of about 6. It is situated about 3.5 mm. to the nasal side of the 

 visual axis. Its exfstence can be demonstrated by the familiar experiment 

 of Mariotte, which consists in placing before the eye two objects having 

 the relation to each other shown in Fig. 308. With the left eye closed 

 and the right eye directed to the cross, both objects may be visible. But 

 by moving the figure away from or toward the eye, there will be found 

 a distance, about 30 cm., when the circle will be invisible. This occurs 

 when the image falls on the optic nerve at its entrance. The experiment 

 of Purkinje as described in the following paragraph demonstrates also the 

 fact that the sensitive portion of the retina is to be found only in the 

 layer of rods and cones. 



It is well known that the blood-vessels of the retina are situated in its 

 innermost layers a short distance behind the optic nerve-fibers. Owing to 



