290 PHYSIOLOGY CHAP. 



called directive lines. The angle formed by the two directive 

 lines is the visual angle. From the visual angle formed by the 

 two directive lines from the two extreme points of an object it 

 is possible to calculate the size of the image projected on to the 

 retinal plane of the dioptric system of the eye. 



IV. In the normal resting eye the posterior focal point of the 

 dioptric system coincides with the most external layer of the 

 retina, in which are the peripheral nerve-cells that are sensitive 

 to light. This is indispensable to the formation of a distinct 

 image. 



EmmetTOpia is the name given to that state of refraction of 

 the resting eye in which the image of an object at infinite 

 distance, the rays from which enter parallel to the eye, is projected 

 directly upon the sensitive layer of the retina. 



Emmetropic refraction is compatible with a varying refractive 

 power of the dioptric system ; it is only essential that there shall 

 be a correct relation between the refractive power and the optical 



FIG. 129. Formation of inverted image of an object on the retina of a reduced eye. 



axis of the eye that is, the distance of the principal point of 

 the reduced eye from the sensory layer of the retina. Thus, e.g., 

 the reduced eye is emmetropic with the following combinations : 



Eefractive power = 60 D. Length of axis = 22-17 mm. 



-63,, =21-11 



=66 =20-15 



Ametropia is the state in which there is an incorrect relation 

 between the refractive power and the length of axis of the eye at 

 rest. It may result from abnormal values of the optic constants, or 

 from abnormal length of the axis. Accordingly ametropia of the 

 axes, of the radii of curvature, and of the refractive indices can be 

 distinguished. By far the most frequent forms of ametropia are 

 those due to excessive or defective length of the axes. The former 

 constitute myopia or liypometropia, the latter liypermetropia. 



In myopia (short sight), parallel rays, that is those coming 

 from an infinite distance, are brought to a focus, not on the 

 sensitive layer of the retina, but in front of it. In hypermetropia 

 (long sight), on the contrary, they come to a focus behind it. 



