7-A8 OF THE ORGANS OF THE SENSES, AND THEIR FUNCTIONS. 



some change in the place or figure of this body is the principal means where- 

 by the ordinary adaptation is effected. The precise changes which occur in 

 the interior of the Eye when it is accommodated for viewing near objects, 

 or during positive accommodation, have been determined by Helmholtz 1 with 

 an instrument that he has termed an Ophthalmometer, the principle of 

 which consists in examining the reflexion from the various ocular media of 

 two flames : alterations in the relative position and figure of these can be 

 readily perceived and measured, furnishing the data for the requisite calcu- 

 lations. By this means it has been ascertained that the essential alteration 

 iS a change effected in the figure of the lens by muscular effort, which in- 

 creases its thickness in the autero-posterior diameter, the convexity of the 

 anterior surface in particular undergoing a considerable augmentation (n, 

 Fig. 260), and by a forward movement approximating the cornea ; whilst 

 the convexity of the posterior surface is but slightly modified, and under- 

 goes scarcely any change of place. The return of the lens to its original 

 figure after the cessation of the effort by which it has been accommo- 

 dated for near objects is probably effected by its own elasticity, which is 

 certainly very considerable. The other and clearly secondary changes 

 which have been observed are a contraction of the circular fibres of the 

 Iris, making the pupil smaller the pupillary edge of the Iris at the same 

 time moving forwards, and the attached or peripheral edge (/>) back- 

 ward. The forward movement of the plane of the iris amounts, accord- 

 ing to Knapp,' 2 to about y^th of an inch. Lastly, according to Becker, 3 

 the points of the ciliary processes retire from the edge of the lens. 

 The object fulfilled by the contraction of the pupil in adaptation of the Eye 

 for near objects, is evidently to exclude the outer rays of the cone or pencil, 

 which, from the large angle of their divergence, would fall so obliquely on 

 the convex surface of the eye as to be much affected by the spherical aberra- 

 tion, and thus to allow the central rays only to enter the eye, so as to pre- 

 serve the clearness of the image ; the principle being exactly the same as 

 that on which the optician applies a stop behind his lenses, which reduces 

 their aperture in proportion to the shortness of their focal distance. The 

 channel through which this action is effected, is evidently the same as that 

 through which the convergence of the eyes is produced, namely, the inferior 

 branch of the Third pair of nerves ; to the action of which, the sensations 

 received through the retina seem to afford the immediate stimulus, in the 

 same manner as they do to the ordinary variation in the diameter of the 

 pupil under the influence of light; but the voluntary determination to fix 

 the vision upon the object, is the original source of the action. That the 

 presence of the Iris is not indispensable for the performance of positive ac- 

 commodation, is shown by the fact that, in a patient from whom Griife re- 

 moved the whole Iris, this faculty remained perfect ; and a similar power 

 has been observed to exist in cases of congenital absence of the Iris. 4 Dur- 

 ing negative accommodation, in other words, in viewing distant objects, the 

 lens (m) becomes flattened, the external margin of the iris is brought for- 



1 A full confirmation of Helmholtz's statement, with a <rood historical rcsumd of 

 the whole subject, will be found in Prof. Allen Thomson's Phenomena and Mechan- 

 ism of the Focal Adjustment of the Eye to Distinct Vision at Different Distances 

 (pamphlet), and a still more complete exposition of all points connected with the 

 Anatomy and Physiology of the Eye in Griife and Siimisch's Handbuch der gesamm- 

 ten Augenheilkunde, 1874-75. 



2 Archiv f. Ophthalmol , Bd. vi, Abtheil. ii, p. 1. 



3 See his interesting paper in Braun, Duchek, and Schlager's Mcdizin. Jahrb., 

 1864, p. 1. 



4 See Dr. Soelberg Wells's instructive paper, On the Paralysis of the Muscles of 

 the Eye, in the Ophthalmic Hospital Reports, vol. ii, 1859-18UO, p. l'J9. 





