9 82 THE SENSES 



been suggested, is clearly proved by the fact that atropine, which 

 does not affect the action of these muscles, paralyzes the mechanism 

 of accommodation. To the consideration of that mechanism we 

 now turn. 



The Mechanism of Accommodation. While everybody is agreed 

 that the main factor in accommodation is the alteration in the 

 curvature of the lens, there is by no means the same unanimity 

 as to the manner in which this is brought about. Helmholtz's 

 explanation, which has long been the most popular, is as follows: 

 In the unaccommodated eye the suspensory ligament and the 

 capsule of the lens are tense and taut, the anterior surface of the 

 lens is flattened by their pressure, and parallel rays (or, what is the 

 same thing, rays from a distant object) are focussed on the retina 

 without any sense of effort. In accommodation for a near object, 

 the meridional or antero-posterior fibres of the ciliary muscle by 

 their contraction pull forward the choroid and relax the suspensory 

 ligament. The elasticity of the lens at once causes it to bulge 

 forwards till it is again checked by the tension of the capsule. 



The explanation of Helmholtz, although widely adopted in the text- 

 books, has not escaped question in the archives. Tscherning has put 

 forward the view that when the ciliary muscle contracts, the suspensory 

 ligament is pulled backwards and outwards. Its tension is thus in- 

 creased, and the soft external layers of the lens are in consequence 

 moulded upon the harder nucleus, so as to increase the curvature 

 especially around the anterior pole. And Schoen, reviving a similar 

 theory originated fifty years ago by Mannhardt, believes that the 

 ciliary muscle, in contracting, exerts pressure on the anterior portion 

 of the lens, and so increases its curvature. He likens the process to the 

 bulging of an indiarubber ball when it is held in both hands and com- 

 pressed by the fingers a little behind one of the poles. It will be ob- 

 served that in both of these theories the suspensory ligament is supposed 

 to be stretched during accommodation, not relaxed as Helmholtz sup- 

 posed. While they have certain advantages over the theory of Helm- 

 holtz, particularly in taking account of the presence of radial and circular 

 as well as meridional fibres in the ciliary muscle, they do not agree so 

 well with such experimental tests as have been applied, and therefore 

 Helmholtz's explanation must still be regarded as the best. 



It is supported by the observation of Hess that when the ciliary 

 muscle has been very strongly contracted by eserine the lens can be 

 observed to move about with 'each slight movement of the eye. The 

 suspensory ligament must therefore be slackened by the contraction of 

 the ciliary muscle. When atropine is applied the movability of the 

 lens soon disappears, owing to paralysis of the ciliary muscle. These 

 facts were first established in patients after iridectomy, but have also 

 been demonstrated in the normal eye. Even under the influence of 

 gravity alone, without any movements of the eye, the lens sinks about 

 to mm. in strong accommodation. An additional proof that the 

 suspensory ligament is perfectly slack during accommodation is derived 

 from the result of simultaneous measurements in animals of the pressure 

 in the anterior chamber and in the vitreous. Even in strong accommo- 

 dation no alteration occurs, although even slight contact with the outer 

 surface of the eyeball or contraction of the external eye muscles causes 



