214 



ELEMENTARY PHYSIOLOGY 



White light is composed of a multitude of colours the 

 colours of the rainbow as may be proved by allowing a beam of 

 such light to pass through a prism of glass. If we look steadily 

 for some time on a bright red spot on a sheet of white paper, and 

 then turn the eyes aside to a clear portion of the paper, or to the 

 ceiling, we see a green spot. The sensibility of that portion of the 

 retina on which the red rays fell has been exhausted so far as 

 those rays are concerned, and it remains sensitive only to the 

 other colours which compose white light. Consequently, when 

 the white light from the paper falls on the exhausted portion of 

 the retina, the red rays produce no effect, and we see the comple- 

 mentary colour to red ; that is, the colour produced by the com- 

 bined effect of all the other rays of the white light. Conversely, 

 if we exhaust a portion of the retina with green rays, we produce 

 the complementary red image. 



Some persons are colour-blind, that is, they cannot dis- 

 tinguish between two or more colours, different rays affecting the 

 retina in exactly the same manner. This is probably due to a 

 defect in the retina. 



SFMMAKY. 



, Eyeball. 

 I Muscles. 

 I Blood-vessels. 

 1 Nerves. 

 I Fatty tissue. 



* Lachrymal or tear apparatus. 

 Consist of folds of the skin. 

 Lined with a mucous membrane (the con- 

 junctiva\ which is also reflected over 

 the front of the ball. 



Closed by the circular orbicularis muscle. 

 Upper lid raised by a special muscle. 



Superior turns the eyeball 



upwards. 

 Inferior turns the eyeball 



downwards. 

 External turns the eyeball 



outwards. 



Internal turns the eyeball 

 inwards. 



(Superior turns the eyeball 

 outwards and downwards. 

 Inferior turns the eyeball 

 outwards and upwards. 

 Lachrymal gland in the upper and outer 



part of the orbit. 

 Several small ducts. 



i Constant. 

 Secretion . j Moistens the eyeball. 



* Carried off by the nasal duct. 



Contents of the Orbit 



r Lids 



THE EYE 



Muscles 



Lachrymal 

 apparatus 



4 Becti. 



V 2 Oblique 





