THE SENSE OF SIGHT. 667 



2. The line oj fixation or line oj regard (V C), the line connecting the point 

 viewed with the center of rotation, the latter being situated 6 mm. behind 

 the nodal point of the eye and 9 mm. before the retina. The relation 

 of these lines and certain angles connected with them are shown in Fig. 

 327. The angle included between the line D D (the major axis of the 

 corneal ellipse) and the visual line is the angle alpha, amounting on the 

 average to 5. The angle incuded between the optic axis and the line of 

 fixation or regard is the angle gamma, while the angle between the optic 

 axis and the line of vision is the angle beta. In emmetropia the angle alpha 

 is about 5. In hypermetropia it is greater, amounting to 7 or 8, 

 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 



FIG. 328. DIAGRAM FOR OBSERVING THE SITUATION OF THE BLIND SPOT. 



(HelmhoUz.) 



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 

 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 insensitive 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 existence can be demonstrated by the familiar experiment 



