528 PHYSIOLOGY 



Age in years Accommodation in dioptres. 



10 13-8 



15 . 12-6 



20 11-6 



25 10-2 



30 8-9 



35 - 7-3 



40 . ... 5-8 



45 ... 3-7 



:.<> 2-0 



55 1-3 



60 1-1 



This gradual reduction in the amplitude of the accommodation is caused by the 

 hardening of the crystalline lens, which with the advance of years robs it of the elasticity 

 of youth. As a rule the change goes on unheeded till the time comes when the near 

 point of vision has receded so far from the eye that the book has to be held at a distance, 

 in order to be focussed clearly. Then it is found that the print, particularly if it is fine, 

 becomes unreadable, and convex glasses have to be worn. It is at that time that 

 presbyopia may be said to begin, usually at an age of 45 to 50, in emmetropia. In 

 hvpermetropia, presbyopia shows itself earlier than normal, and in myopia (short-sight) 

 later. The reason for these differences will be described later when dealing with these 

 conditions. 



The mechanism of accommodation is affected in a number of pathological con- 

 ditions ; the principal ones will be given. Paralysis of the ciliary muscles frequently 

 occurs after diphtheria and influenza: it is probably due to bacterial toxins 

 circulating in the blood. The paralysis usually disappears during convalescence. Loss 

 of accommodation is frequently one of the first symptoms of glaucoma: in this case tin- 

 cause is the abnormally high intraocular pressure, since that will cause so great an 

 increase in the normal tension of the choroid that the ciliary muscles have not sufficient 

 power to draw the edges of the choroid and ciliary bodies into a circle of smaller radius, 

 and thus to allow the lens to accommodate. Apparent diminution of the amplitude of 

 accommodation combined with apparent myopia (short-sight) occurs in patients who 

 have spasm of the ciliary muscles. This is found in two types of cases, children with 

 eye-strain, and women with hysteria. In both the employment of atropine yields the. 

 normal amplitude and at the same time removes the apparent myopia, thus indicating 

 the cause of the trouble. Occasionally the amplitude of accommodation is different 

 in the two eyes, and is accompanied by unequal pupils. Such a condition may follow 

 an accident or be caused by specific toxins. 



THE EFFECT OF DRUGS on the accommodation has already been alluded to. 

 In ophthalmic practice three are used in order to paralyse the ciliary muscle, namely 

 atropine (one-half per cent, solution), homatropine (2 per cent.) and scopolamine (one- 

 fifth per cent.). One of these drugs is placed within the conjunctival sac, and from here 

 it slowly travels by an at present unknown route to the ciliary and iris muscles, both of 

 which are paralysed. This fact is of the greatest importance, for if this path did not 

 exist the use of these valuable drugs would hardly be possible. Contraction of the 

 ciliary muscles and of the sphincter pupillae is caused by the three drugs eserine, 

 pilocarpine and physostigmine. One of these is often used to counteract the effects 

 of the atropine group. 



