578 THE NERVOUS SYSTEM. 



may be seen. If the eye is quickly closed and again opened, this 

 outline may be again seen, and this may be repeated several times. 

 3. A third method of demonstrating the retinal vessels is by con- 

 centrating a strong light on the sclerotic, at a part as distant as 

 possible from the cornea, by means of a lens, as is shown in Fig. 

 356. 4. If a card is perforated with a pin, and then held close 

 to the cornea, and through the pinhole light from a lamp or 

 other source of illumination is allowed to fall on the retina, 

 when the card is moved rapidly up and down or from side to side, 

 but not so much as to prevent the light from entering the pupil, 



FIG. 357 Figure to illustrate the principle of the ophthalmoscope. Rays of 

 light from a point, P, are reflected by a glass plate, M (several plates together in 

 Helmholtz's original form), into the observed eye E'. Their focus would fall, as 

 shown in the figure, at P 1 , a little behind the retina of E'. The portion of the 

 retina A B is therefore illuminated by diffusion circles ; and the rays from a point 

 of it, F will, if E' is emmetropic and unaccommodated, issue parallel from E' and 

 be brought to a focus at F' on the retina of the (emmetropic and unaccommodated) 

 observing eye E. 



a shadow of the blood-vessels of the retina will be seen. These 

 shadows of the retinal blood-vessels are Pur kinje's figures. 



The retinal blood-vessels do not extend beyond the inner 

 nuclear layer, and the fact that these vessels cast a shadow when 

 light is admitted to the eye, as in the experiments just referred to, 

 demonstrates that the sensitive portion of the retina lies behind 

 the blood-vessels, and the distance behind can be calculated by 

 measuring the amount of change of position the shadows undergo 

 when the light is moved about. This has been done, and the 

 distance has been ascertained to be about 0.2 mm. to 0.3 mm. 

 behind the vessels, which corresponds to the layer of rods and 

 cones. 



