ABNORMALITIES IN THE REFRACTION OF THE EYE 855 



excitation of the retina by strong pulsations of the retinal blood-vessels, 

 increased intraocular pressure, and venous stagnation caused, for ex- 

 ample, by the acts of coughing and sneezing. 



Inconstant Optical Defects of the Eye. — It is the purpose of the 

 normal eye to bring rays of light to a sharp intersecting point upon 

 the retina. An eye which accomplishes this end, is said to be em- 

 metropic. This condition of normal refraction is designated as 

 emmetropia. Conversely, any eye which is not capable of producing 

 a precise focus, is said to be ametropic. This condition of abnormal 

 refraction is known as ametropia. The causes underlying the latter 

 may be arranged in the following order: 



(a) Imperfect curvature of the cornea, astigmatism. 

 (6) Diminished elasticity of the lens, presbyopia. 

 (c) Imperfect shape of the eyeball. 



(1) Myopia, the eyeball is too long. 



(2) Hypermetropia, the eyeball is too short. 



The condition of presbyopia has been fully discussed in one of the pre- 

 ceding chapters and need not be considered again at this time. Fur- 

 thermore, while astigmatism is ascribed in this outline to a faulty 

 curvature of the cornea, we should not lose sight of the fact that this 

 condition may also be caused by an imperfect curvature of the lens; 

 in fact, even a so-called normal eye is not entirely free from astigmatism, 

 due very largely to an improper centration of the constituents of the 

 lens. Regarding the exciting causes of ametropia no perfectly definite 

 statements can be made. The shape of the eyeball is inherited together 

 with other biological characteristics; hence, all these conditions may 

 be entirely beyond our power of preventing them. This is also true 

 of those defects which arise later on in life in consequence of retro- 

 gressive changes, such as infiltrations, alterations in the, intraocular 

 pressure, and senile weaknesses of the coats of the eyeball. In the 

 latter case, the eyeball becomes more pliable and adjusts itself more 

 completely to the shape of the orbital cavity. In spite of this impor- 

 tant element of inheritance, however, it cannot be denied that these 

 defects may also be acquired in consequence of an improper mode of 

 living, and erroneous methods in the use of the eyes. Thus, the 

 inhabitants in cities are constantly subjected to near work; their hori- 

 zon being limited in many cases by the walls of the houses on the 

 opposite side of the street. Besides, their daily work requires strong 

 convergence of the visual axes which in itself heightens the intraocular 

 pressure. The contrary picture is presented by the inhabitant of the 

 open country whose visual impressions are in large part received 

 from distances greater than 50 m., i.e., from distances which require 

 no accommodation at all. Civilization imposes upon us many condi- 

 tions which can only be met by carefully following the most fundamen- 

 tal rules regarding physiological optics. 



Astigmatism. — In accordance with perfect refraction, the cornea 

 should form a section of a true sphere, but this is not always the case, 



