708 SPECIAL SENSES. 



in communication with the superior cervical ganglion and is situated near 

 the sublingual nerve. The influence of this centre over the pupil can not be 

 demonstrated by direct stimulation, because it is too near the origin of the 

 fifth, irritation of which affects the iris ; but it is shown by division of its 

 filaments of communication with the iris. 



ACCOMMODATION OF THE EYE FOR VISION AT DIFFERENT DISTANCES. 



Supposing the eye to be adapted to vision at an infinite distance, in which 

 the rays from an object, as they strike the cornea, are practically parallel, it is 

 evident that the foci of the rays, as they form a distinct image upon the reti- 

 na, are all situated at the proper plane. Under these conditions, in a perfectly 

 normal eye, the image, appreciated by the individual or seen by means of the 

 ophthalmoscope, is perfectly clear and distinct. If the foci be situated in 

 front of the retina, the rays, instead of coming to a focus upon a point in the 

 retina, will cross, and from their diffusion or dispersion, will produce indis- 

 tinct vision. Under these conditions a distinct point is not perceived, but 

 every point in the image is surrounded by an indistinct circle. These are 

 called " circles of diffusion." If, now, the eye, adjusted for vision at an infi- 

 nite distance, be brought to bear upon a near object, the rays from which are 

 divergent as they strike the cornea, the image will be no longer distinct, but 

 will be obscured by circles of diffusion. It is the adjustment by which these 

 circles of diffusion are removed, that constitutes accommodation. This fact 

 has been demonstrated by Helmholtz by means of the ophthalmoscope. " If 

 the eye be adjusted to the observation of an object placed at a certain dis- 

 tance, it is found that the image of a flame, placed at the same distance, is 

 produced with perfect distinctness upon the retina, and, at the same time, 

 upon the illuminated plane of the image, the vessels and the other anatomi- 

 cal details of the retina are seen with equal distinctness. But, when the 

 flame is brought considerably nearer, its image becomes confused, while the 

 details of the structure of the retina remain perfectly distinct." 



It is evident that there is a certain condition of the eyes adapted to vision 

 at an infinite distance, and that for the distinct perception of near objects, 

 the transparent media must be so altered in their arrangement or in the cur- 

 vatures of their surfaces, that the refraction will be greater ; for without this, 

 the rays would be brought to a focus beyond the retina. 



The changes in the eye by which accommodation is effected are now 

 known to consist mainly in an increased convexity of the lens for near ob- 

 jects ; and the only points in dispute are a few unimportant details in the 

 mechanism of this action. The simple facts to be borne in mind in study- 

 ing this question are the following : 



When the eye is accommodated to vision at an infinite distance, the parts 

 are passive. 



In the adjustment of the eye for near objects, the convexities of the lens 

 are increased by muscular action. 



In accommodation for near objects, the pupil is contracted ; but this 

 action is merely accessory and is not essential. 



