THE REFRACTION OF THE EYE 537 



that there is less accommodation remaining for the focussing of near objects, and there- 

 fore an inability to see distinctly at relatively short distances from the eye. Thus the 

 use of the term long-sight. 



Hypermetropia ha adults is therefore more an abnormality than a disease ; it causes 

 a disposition however to three more serious conditions, namely glaucoma, internal strabis- 

 mus and eye-strain. Glaucoma has already been described (see page 517 ); it is due to 

 an abnormal rise in the intraocular pressure, which occurs owing to the free escape of 

 the aqueous humour at the filtration angle being checked. Now in hypermetropia we 

 have seen that the eyeball is too small for its optical apparatus, and therefore the lens 

 occupies too much of the space in the small anterior and posterior chambers. This 

 causes the ciliary bodies and roots of the iris to be squeezed and greatly reduces 

 the space at the filtration angle. An attack of glaucoma is therefore more liable to 

 occur in the hypermetrope than in a person with normal refraction. 



Internal strabismus is caused in hypermetropia by the accommodative effort that 

 is made in order to focus an image on the retina, because, as we have seen above (page 

 497), convergence and accommodation are associated actions. When therefore the long 

 sight has been corrected by means of spectacles, and the accommodation is no longer 

 called into play for seeing at a distance, the associated convergence no longer occurs 

 and the strabismus disappears. 



Eye-strain is caused in hypermetropia by the continual call for accommodation. 

 Further, this must occur without convergence, for otherwise diplopia (seeing double) 

 and strabismus develop as just described. A special strain is therefore placed not only 

 on the ciliary muscles but also on the external eye muscles. This state of affairs very 

 rapidly causes fatigue, headaches are therefore common. 



The treatment of long-sight consists in prescribing suitable convex spectacles. It^ 

 should be noted that the amount of long-sight actually present is shown only when the 

 accommodation has been paralysed by atropine, because the patient has grown so accus- 

 tomed to use his accommodation in ordinary vision that he is unable voluntarily to relax it. 

 There is a certain amount of spasm of the accommodation. Because of this the glasses pre- 

 scribed should be less strong at first than the full correction shown to be necessary. 

 These may be substituted by more powerful ones later. 



MYOPIA OR SHORT-SIGHT. In this condition parallel rays, that is those coming 

 from distant objects, come to a focus so far in front of the retina that the image appears 

 blurred (see Fig. 268). Myopia may be caused in two ways, which are similar to, 

 but opposite in action to those that cause hypermetropia ; the first type is caused 

 by the eyeball being too long, and the second by the refraction of the lens being too 

 high. The former, which is the more common, usually develops in youth, particularly 

 at the school age when the growth of the body makes special demands on the system, 

 and at the same time feeding is usually bad. The constant use of the eye for near work 

 causes them considerable strain which they are unable to withstand owing to their 

 being unable to compete for nourishment with the rest of the body. The choroid and 

 solera therefore become thin, are no longer able to stand the tension set up by the intra- 

 ocular pressure, and therefore expand, causing the eyeball to become larger than normal, 

 and taking the retina beyond the focus of the optical system. The treatment 

 of myopia is therefore not only the wearing of spectacles, but the absolute prohibition 

 of near work or close study, the administration of extra-nourishing food and an 

 open-air life for a year or more. If these steps are taken at once, the myopia may 

 get no worse, and may in fact get better. But if neglected the condition will almost 

 certainly get worse. As myopia is a disease, particularly liable to occur at the school 

 age, schoolmasters and others associating with children should be on the look out for 

 conditions likely to cause it, such as bad light, bad food and poor ventilation, and for 

 its presence in any of the children. Glasses should always be prescribed and care taken 

 that the child wears them constantly, because it is found that beside assisting good 



B definition and relieving eye-strain, they actually tend to check the further development 

 of the trouble. 



