580 MANUAL OF PHYSIOLOGY. 



choroid coat. Over this red field are seen a number of blood 

 vessels, which start from the centre of the optic disk, and radi- 

 ating over the fundus send branches to the most anterior parts 

 that can be seen. TheSe are the branches of the vessel which 

 runs in the centre of the nerve. In the very axis of the eye a 

 peculiar depression, free from branches of the blood vessels, can 

 be seen. This central depression (fovea centralis) differs a little 

 in color from the neighboring parts during life, and turns yellow 

 at death, and hence has been called the " yellow spot." The 

 retina is so transparent that we cannot see it with the ophthal- 

 moscope, but the radiating vessels (central arteries and veins of 

 the retina) lie in it and belong to the nervous structure only. 



The ophthalmoscope has proved of inestimable value not only 

 to the ophthalmologist but also to the physician, as a means of 

 arriving at an accurate knowledge of disease. Hence, it has 

 bacome more a pathological than a physiological instrument. 



LIGHT IMPRESSIONS. 



The retina is the part of the eye by which the physical motion, 

 light, is changed into the physiological phenomena known as 

 nerve impulse, by means of which the impression of light is 

 excited in the brain. In reaching the retina the light is not in 

 any way altered from the light with which physicists experiment, 

 but at the retina this physical motion is stopped. The optic 

 nerves no more convey the light waves from the eye to the brain 

 than the tactile nerves carry the objects that stimulate their end- 

 ings, but only send a nerve impulse which the retina, on its expo- 

 sure to the light, excites in the optic nerve. As already stated, 

 any form of stimulation will cause the same kind of impulse to 

 pass to the brain and there set up the same sensation of light. 

 Thus we are told by persons who have had their optic nerves cut 

 that the section was accompanied by the sensation of a flash of 

 light. Any violent injury of the eyeball causes a flash of light 

 to be experienced. This fact has long since been arrived at in a 

 practical manner, for a blow implicating the eyeball is vulgarly 

 said to " make one see stars." Also, without violent injury, if 

 we close the eyes and turn them to the left side, and then press 



