154 



VETERINARY PHYSIOLOGY 



(6) Of the nature of the changes in the retina when 

 stimulated we know little. But we know that — 



1st. Under the influence of light the cells of the tapetum 

 nigrum expand forward between the rods and cones. 



2nd. A purple pigment, which has been called rhodopsin, 

 exists in the outer segment of the rods, and this is bleached 

 by the action of light. Even although there is no purple in 

 the cones, which alone occupy the sensitive central spot of 

 the eye, this change in colour suggests that a chemical 

 decomposition accompanies stimulation. 



Srd. Electrical changes occur (p. 61). 



(7) Fatigue. — If a bright light be looked at for some 

 time, the parts of the optic mechanism, retina, and nerve 

 centre acted upon is temporarily blinded, and hence, when 



Fig. 74. — The Power of Localising the Source of Illumination on different 

 parts of the retina. The two points, a-b, subtended bj' the small 

 angle, fall close together at a-b near the centre of the retina, and still 

 give rise to a double sensation ; but if two points, c-d, have their 

 images formed on the periphery of the retina, c-d, these images must 

 be far apart to cause a double sensation. 



the eye is directed away from the bright light, a dark spot is 

 seen. This is sometimes called a negative after-image. 

 When coloured lights are used, the phenomena of comple- 

 mental colours are produced (Practical Physiology). 



Sometimes the stimulation of the retina or of the brain 

 neurons connected with it may last after the withdrawal of 

 the stimulus, when a continuance of the sensation — a positive 

 after-image — is seen. This may be observed if, on opening 

 the eyes in the morning, a well-illuminated window is looked 



