55 1 



PHYSIOLOGY 



The image seen in the ophthalmoscope is shoun in Fig. 284. 



< )n examining the back ol the eyeball by cither of these methods, the most prominent 

 object is the optic disc or optic nerve-papilla, which marks the point of entrance of the 

 optic nerve. It is seen as a pale oval disc surrounded by a deep red background (Fig. 

 284). From the middle of the papilla the retinal vessels pass into the eyeball, and 

 they are seen diverging from the papilla to ramify over the rest of the retina. The 

 arteries can be distinguished from the veins by their brighter red colour as well as by the 

 stronger reflection of light from their surfaces. The yellow spot is very difficult to see, 

 except in atropinised eyes, since it comes into view only when the observed eye is looking 

 straight into the ophthalmoscope. Under these conditions there is a strong Might 



Fio. 284. Ophthalmoscopic view of fundus of 

 eye, showing the optic disc, or point of entry 

 of the optic nerve, with the retinal vessels branch- 

 ing from its centre. 



FIG. 285. Diagram of the 

 path of the rays of light in 

 the formation of Purkinje'a 

 figures. 

 v represents a retinal vessel. 



When this is illuminated from 



A, a shadow is formed on the 

 hinder layers of the retina at 

 a'. This is projected along a 

 line passing through the opt it- 

 axis, and appears to come from 

 a point (a") on the wall. On 

 moving the light from \ to 



B, the image of the ve< -I 

 appears to move from n" to b. 



reflex,' and the'pupil contracts up to a pin-point, unless paralysed by means of atropine. 

 In order to see the blind-spot, or optic disc, the observed eye must be directed inwards : 

 thus if A is look ing at the right eye of B, B must be told to .look over A's right shoulder. 



By projecting a highly concentrated beam of light on to the side of the eyeball, 

 it is possible to cause sufficient light to pass through the wall that the retina pereeiv* s 

 tin stimulus. When that is the case it is found that the retinal arteries and veins are 

 seen as dark images on a bright ground (Purkinje's figures). By moving the point of 

 illumination and then measuring the apparent shift of the vessels which occurs, it 

 has been found possible to estimate the depth below the vessels at which the re- 

 ceptive surface of the retina is placed, namely -17 to -36 mm. Now the average 

 distance between the vessels and the layer of rods and cones is found to be -2 to -3 

 nun. ; it must therefore be the layer of rods and cones which forms the sensitive 

 layer. The directions taken by the light rays are shown in Fig. 28*5. 



Another method of viewing the vessels in one's own eye is to look through a .small 

 hole in a met il plate at a smooth white surface. < )n oscillating the aperture in relation- 

 ship with the pupil about once a second, the \essels \\ill lie seen as shadows <>n the hue lit 



background. 



