130 ESSENTIALS OF PHYSIOLOGY. 



3. Drugs. Morphia taken internally, and eserine or pilocarpine, 

 either taken internally or applied directly to the eye, cause the pupil to 

 contract. Contraction also takes place in the third stage of chloroform 

 or ether anaesthesia. 



Dilatation of the pupil occurs : 



(1) On the removal of the light stimulus from the eye. 



(2) As a result of stimulation of sensory nerves. 



(3) In emotional conditions, such as fear. 



(4) From the action of drugs, for example, from the internal admini- 

 stration or local application of atropine, or from the local application 

 or intravenous injection of adrenalin. The pupil is also dilated in the 

 early stages of chloroform or ether anaesthesia. 



THE RETINAL IMAGE. 



When light falls upon the eye it excites chemical, histological, and 

 electrical changes in the retina. 



The Chemical Changes. We have seen that the outer segments of 

 the rods contain a substance called rhodopsin or visual purple. 

 Rhodopsin can be dissolved out of the retina by a solution of bile salts, 

 and it is rapidly decolorised on exposure to light. If an animal be 

 kept in the dark for some time and then killed and its retina examined, 

 the latter will be of a deep red colour, which soon fades on exposure to 

 light. If, on the other hand, the eyes have been exposed to bright light 

 the retina is pale. The effect of light in bleaching rhodopsin is in pro- 

 portion to its intensity, so that if a rabbit is kept in the dark for a 

 time and then its eye is exposed opposite a window, a picture of the 

 window, called an optogram, is formed on the retina ; in the optogram 

 the window pane areas are bleached, while the rhodopsin is not 

 decolorised in the shaded parts corresponding with the bars. As 

 visual purple is not present in the cones, the fovea does riot show this 

 chemical change. 



The Histological Changes. In an eye which has been exposed to light 

 fine processes of the pigment cells are found to extend between the 

 rods and cones, the processes themselves being laden with pigment 

 granules, whereas if the animal has been kept in darkness before the 

 examination of its eye, the cells of the pigment layer are flat, the pro- 

 cesses being retracted. Further, in some animals, for example the frog, 

 the cones are retracted on exposure to- light and extended when the 

 animal is kept in darkness (fig. 41). The cells of the pigment layer 

 have the power of restoring the visual purple, for when a retina, which 

 has been bleached by exposure to light, is laid upon the pigment layer, 

 rhodopsin again appears in the rods. 



