208 



THE HUMAN BODY 



fibers would be stimulated and the result would be the recognition 

 of two separate red objects. In our eyes there are certain refract- 

 r 



FIG. 78. Diagram illustrating the indistinctness of vision with a retina alone. 

 K, a surface on which are two spots, A and B; r r, the retina. The diverging lines 

 represent rays of light spread uniformly over the retina from each spot. 



ing media which lie in front of the retina and take the place of the 

 lens L in Fig. 79. That portion of physiology which treats of the 



FIG. 79. Illustrating the use of a lens in giving definite retinal images. A, B, K, 

 r r, as in Fig. 78. L, a biconvex lens so placed that it brings to a focus on the 

 points a and 6 of the retina, rays of light diverging from A and B respectively. 



physical action of these media or, in other words, of the eye as an 

 optical instrument, is known as the dioptrics of the eye. 



The Appendages of the Eye. The eyeball itself consists of the 

 retina and refracting media, together with supporting and nutri- 

 tive structures and other accessory apparatuses, as, for example, 

 some controlling the light-converging power of the media, and 

 others regulating the size of the aperture (pupil) by which light 

 enters. Outside the ball lie muscles which bring about its move- 

 ments, and other parts serving to protect it. 



Each orbit is a pyramidal cavity occupied by connective tissue, 

 muscles, blood-vessels, and nerves, and in great part by fat, which 

 forms a soft cushion on which the back of the eyeball lies and 

 rolls during its movements. The contents of the orbit being for 

 the most part incompressible, the eye cannot be drawn into its 



