578 MANUAL OF PHYSIOLOGY. 



dark outside, we can see every object clearly. So if we illuminate 

 the inside of the eye by any means we shall be able to see the de- 

 tails of the inside of the eye-chamber. 



This means is supplied by the ophthalmoscope, which reflects the 

 light from a lamp into the chamber of the eye so as to illuminate 

 it completely, and when the surroundings are not too bright, the 

 fundus of the eye can be clearly seen and investigated. A lens 

 is usually interposed between the eyes of the observer and the ob- 

 served, in order not only to illuminate but also to magnify the 

 fundus and enable the observer to see all the details of the parts. 

 With this instrument a round whitish part is seen a little to the 

 nasal side of the axis of the eye, where the nerve pierces the dark 

 choroid coat. This is called the optic disk. The fundus now, 

 when lighted up, does not look black, but is of a lurid red color, 

 owing to the great vascularity of the choroid coat. Over this red 

 field are seen a number of bloodvessels, which start from the cen- 

 tre of the optic disk, and radiating 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 bloodvessels, can be seen. This central depression (fovea 

 centralis) differs a little in color from the neighboring pa"rts 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 ophthalmoscope, but the radiating vessels (central arte- 

 ries and veins of the retina) lie in it and belong to the nervous 

 structure only. 



The opthalmoscope 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 be- 

 come 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 



