HISTOLOGY OF THE KETINA 



545 



ments. It is supposed that the impulses which effect these movements travel 

 through the nerve fibres already described as descending from the brain to the 

 retinae ; it is for this reason that Engelmann called these fibres ' retinomotor.' 



Other structural changes that are found, by histological investigation, to follow expo- 

 sure of the retina to light, are swelling of the outer limbs of the rods, and the disappear- 

 ance of chromatin granules from the ganglion cells. Both these changes are said to 

 occur more rapidly under the action of rays of short wavelength. 



PHYSICAL CHANGES are also found when the retina is stimulated 

 by light, namely an electrical response somewhat similar to the current 

 A. B. 



H i 



i 



FIG. 275. Sections of the frog's retina. 



A, kept in the dark ; B. after exposure to the light, showing retraction of the 

 cones, and protrusion of the pigmentecl epithelium between the outer limbs of the 

 rods. (ENGELMANN.) 



f action in nerve. Three typical curves and the conditions under 

 which they were obtained are shown in Fig. 276. One point of particular 

 interest should be noted, namely the response to darkness. The complicated 

 nature of these curves has been explained on the supposition that there 

 are three substances present in the retina of different reaction time. It 

 has not however been found possible to identify any of them. The difference 

 in the electric response to light of different colour and intensity has been 

 found to give the following results. With light of any one colour a geometric 

 rise of intensity causes an arithmetic increase in the current. With coloured 

 rays of apparent equal intensity yellow rays are said to give a larger current 

 in the light adapted eye, and green in the dark adapted eye. It is interesting 

 to observe that the current commences after a latent period which 

 is of the same order as that found for the perception of light by the eye. 

 This and other facts mentioned above, would seem to point to the currents 

 observed being the accompaniment of the passage of the nervous impulses 

 to the brain. 



CHEMICAL CHANGES in the retina on exposure to light are of 

 two kinds, firstly a tendency of the retina as a whole to become acid 

 in reaction, as is shown in the change in its behaviour to certain stains, 

 and secondly the bleaching of two pigments, namely, the visual purple and 

 csin. With regard to visual purple (or rhodopsin) a large number of facts 



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