798 SPECIAL SENSES. 



about | of a second later than the direct action, and the consensual dilatation, about \ 

 of a second later. (Bonders.) 



Budge and Waller have shown that the filaments of the sympathetic which produce 

 dilatation of the pupil take their origin from the spinal cord. In the spinal cord, between 

 the sixth cervical and the second thoracic nerves, is situated the inferior cilio-spinal 

 centre. When the spinal cord is stimulated in this situation, both pupils become dilated. 

 If the cord be divided longitudinally and the two halves be separated from each other by 

 a glass plate, stimulation of the right half produces dilatation of the right pupil, and mce 

 versa. This does not occur when the sympathetic in the neck has been divided. In 

 addition to the inferior cilio-spinal centre, there is a superior centre, which is in com- 

 munication with the superior cervical ganglion and is situated near the sublingual nerve. 

 The influence of this centre over the pupil cannot be demonstrated by direct stimulation, 

 because it is too near the origin of the fifth, irritation of which has an influence over the 

 iris; but it is shown by division of its filaments of communication with the iris. 



Section and galvanization of the different nerves which regulate the movements of the 

 iris have a certain influence upon its vascularity ; and, indeed, it has been thought that 

 contraction is in a measure due to congestion of its vessels, and dilatation, to an opposite 

 condition. This view is adopted by some of those who deny the existence of the radi- 

 ating muscular fibres of the iris. Assuming that the size of the pupil is, to a certain 

 extent, affected by the condition of the vessels, it is evident that the more extensive move- 

 ments of the iris are due mainly to muscular action. It has been also shown that the 

 changes in the iris produced by injection of its vessels are not to be compared in their 

 extent with its physiological movements. The changes in vascularity produced by divid- 

 ing or galvanizing the sympathetic do not differ from the phenomena noted in experi- 

 ments upon other portions of the sympathetic system. 



Accommodation of the Eye to Vision at Different Distances. 



The mechanism by which the eye is adjusted for distinct vision at different distances 

 is one of the most interesting and important points connected with the physiology of the 

 sight. At the present day, this point may be regarded as definitely settled, particularly 

 since the variations in the thickness and the curvatures of the crystalline lens have been 

 so accurately measured by Helmholtz. We shall have little to say with regard to the 

 various theories of accommodation advanced by the older physiologists, except to indicate, 

 in a very general way, the most plausible views that have been adopted from time to time 

 by physiological writers. In the first place, we shall note certain physical laws and their 

 application to the eye, which show the necessity for accommodation. 



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 retina, 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 circumstances, 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 infinite distance, be brought to bear upon & 

 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 distance, it is found that the 

 image of a flame, placed at the same distance, is produced with perfect distinctness upon 



