228 ELEMENTS OF BIOLOGY 



MATiSM. Near-sightedness may be the effect of an abnormally 

 shaped eyeball, the lens being thus advanced too far forward. Or, 

 as is frequently the case, the tension on the lens is not sufficient to 

 reduce the curvature for far vision. Presbyopia, or old-, or far- 

 sightedness is said to be the effect of decreased elasticity of the lens 

 so that no matter how vigorously the ciliary muscles may contract 

 the lens does not assume a correspondingly greater curvature. This 

 disorder commonly occurs in Man after the age of forty, and as old 

 age comes on the whole mechanism of accommodation may fail, 

 although acuity of vision may not be seriously impaired. Astig- 

 matism is caused by imperfections in the curvature of the lens so 

 that all the rays of light are not converged on the retina. The result 

 is that the image is blurred in some regions, the regions correspond- 

 ing to the defective regions of the lens. For near-sightedness the 

 correction is made by the use of suitable concave lenses; for far- 

 sightedness convex lenses are employed; for astigmatism lenses are 

 so ground that they compensate for the imperfections in the lens of 

 the eye. 



A complex mechanism, consisting of two sets of muscles, opens 

 and closes the aperture of entrance of light, thus regulating the 

 amount of light that reaches the lens. This iris diaphragm is ex- 

 tremely sensitive to changes in the intensity of light and is con- 

 stantly responding to the changing shadows and lights of the 

 environment. The opening in the iris diaphragm (Fig. 154), the 

 PUPIL, has different shapes in different mammals. In the cat it forms 

 a perpendicular slit; in cattle it is a transverse oval; in Man it is 

 circular. 



The mammalian eye is moved about in its socket by six muscles, 

 attached to the heavy sclerotic coat of the eyeball and anchored to 

 the bony walls of the socket (Fig. 155). They are called the recti 

 muscles; one is above, one below, one internal, one external, one 

 obliquely set above, and one lower oblique. The perfect correlation 

 between these muscles accounts for the fact that the normal eyes 



