THE SENSES 789 



ciliary mr^cle 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 increased, 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 forty 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 compressed by the fingers a little behind one 

 of the poles. It will be observed that in both of these theories the 

 suspensory ligament is supposed to be stretched during accommo- 

 dation, not relaxed as Helmholtz supposed. While they have certain 

 advantages over the theory of Helmholtz, 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. 



Changes in the Pupil during Accommodation. It has been 

 already mentioned that along with the alteration in the 

 curvature of the lens a change in the diameter of the pupil 

 takes place in accommodation. When a distant object is 

 looked at, the pupil becomes larger; when a near object is 

 looked at, it becomes smaller. Narrowing of the pupil is 

 thus associated with contraction of the ciliary muscle, and 

 widening of the pupil with its relaxation. 



This physiological correlation has its anatomical counterpart ; for 

 the third nerve supplies both the iris and the ciliary muscle. Stimu- 

 lation of the nerve within the cranium causes contraction of the pupil, 

 while stimulation of certain portions of its nucleus in the floor of the 

 third ventricle and the Sylvian aqueduct or of the short ciliary nerves 

 coming off from the ophthalmic ganglion (Fig. 293), which receives 

 branches from the third nerve, or of the ganglion itself, is followed by 

 that change in the anterior surface of the lens which constitutes 

 accommodation (Hensen and Voelckers). This can be observed 

 either through a window in the sclerotic in a dog or by following the 

 movements of a needle thrust into the eyeball. By carefully local- 

 ized stimulation near the junction of the aqueduct with the third 

 ventricle, it is possible to bring about the forward bulging of the lens 



