506 DIOPTRIC IMPERFECTIONS. [BOOK HI. 



of the third ventricle, and which is especially connected with the 

 most anterior bundles of the roots of the third nerve. This centre 

 is under the command of our will : when we wish to accommodate 

 for near objects we throw it into action, and it, when in action, calls 

 also into action by ' association ' the centre for the contraction of 

 the pupil ; when the action of the accommodation centre ceases and 

 the eye falls back to the condition of rest, in which it is accommo- 

 dated for far objects, the action of the pupil-contracting centre 

 ceases also, and the pupil therefore widens. 



The mechanism of accommodation may also be affected in a 

 local manner. And the drugs which have a special action on the 

 pupil, such as atropin and calabar bean, also affect the mechanism 

 of accommodation. Atropin paralyses it, so that the eye remains 

 adjusted' for far objects ; and physostigmin throws the eye into a 

 condition of forced accommodation for near objects. This double 

 action has been explained by the supposition that while atropin 

 paralyses, physostigmin throws into tonic or tetanic contraction, 

 on the one hand the circular muscles of the iris and on the other 

 the ciliary muscles; but the phenomena, on inquiry, appear too 

 complicated to be explained in so simple a manner. 



We can accommodate at will ; but few persons can effect the 

 necessary change in the eye unless they direct their attention to 

 some near or far object, as the case may be, and thus assist their 

 will by visual sensations. By practice, however, the aid of external 

 objects may be dispensed with; and it is when this is achieved 

 that the pupil may seem to be made to dilate or contract at 

 pleasure, accommodation being effected without the eye being 

 turned to any particular object. 



Imperfections in the Dioptric Apparatus. 



The emmetropic eye may be taken as the normal eye. The 

 myopic and hypermetropic eyes may be considered as imperfect 

 eyes, though the former possesses certain advantages over the 

 normal eye. An eye might be myopic from too great a convexity 

 of the cornea, or of the anterior surface of the lens, or from per- 

 manent spasm of the accommodation-mechanism, or from too great 

 a length of the long axis of the eyeball. The last appears to 

 be the usual cause. Similarly, most hypermetropic eyes possess 

 too short a bulb. Moreover in the strongly marked myopic eye 

 there is frequently hypertrophy of the longitudinal (meridional) 

 fibres of the ciliary muscle, often spoken of exclusively as the 

 ciliary muscle, and atrophy or absence of the circular fibres ; in the 

 hypermetropic eye on the other hand the circular fibres are well 

 developed and the meridional fibres scanty. The presbyopic eye 



