310 



AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



Fig. 134— To illustrate 



Scln ii-n's theory of ac- 

 commodation. 



point of union of the cornea and sclerotic, extend radially outward in every 

 direction and are attached to the front part of the choroid. The contrac- 

 tion of the ciliary muscle, drawing forward the membranes of the eye, will 

 relax the tension of the suspensory ligament and allow the lens to take 

 the form determined by its own elastic structure. According to another 

 theory of accommodation proposed by Tschcrning, 1 the suspensory liga- 

 ment is stretched and not relaxed by the contraction of the ciliary muscle. 

 In consequence of the pressure thus produced upon the 

 lens, the soft external portions are moulded upon the 

 harder nuclear portion in such a way as to give to the 

 anterior (and to some extent to the posterior) surface a 

 hyperboloid instead of a spherical form. A similar theory 

 has been recently brought forward by Schoen, 2 who com- 

 pares the action of the ciliary muscle upon the lens to that 

 of the fingers compressing a rubber ball, as shown in Fig- 

 ure 134. These theories have an advantage over that 

 offered by Helmholtz, inasmuch as they afford a better 

 explanation of the presence of circular fibres in the ciliary 

 muscle. They also make the fact of so-called " astig- 

 matic accommodation " comprehensible. This term is 

 applied to the power said to be sometimes gradually 

 acquired by persons with astigmatic 3 eyes of correcting 

 this defect of vision by accommodating the eye more 

 strongly in one meridian than another. The theory of Tscherning is sup- 

 ported by Crzellitzer ' as the result of investigations into the hyperboloid 

 form of the lens in accommodation. On the other hand, it is maintained by 

 Priestley Smith 5 that this form of the lens is not inconsistent with the Helm- 

 holtz theory. Moreover, it has been shown by Hess 6 and Heine 7 that in 

 extreme accommodation the lens drops slightly toward the lower part of the 

 eye, a movement which seems to indicate a relaxation of the suspensory liga- 

 ment. The weight of evidence seems, therefore, on the whole, to be on the 

 side of the theory of Helmholtz. 



Whatever views may be entertained as to the exact mechanism by which its 

 change of shape is brought about, there can be no doubt that the lens is the 

 portion of the eye chiefly or wholly concerned in accommodation, and it is 

 accordingly found that the removal of the lens in the operation for cataract 

 destroys the power of accommodation, and the patient is compelled to use 

 convex lenses for distant and still stronger ones for near objects. 



It is interesting to notice that the act of accommodation, though distinctly 

 voluntary, is performed by the agency of the wastriped fibres of the ciliary 

 muscles. It is evident, therefore, that the term " involuntary " sometimes 



1 Archives fie Physiologie, 1894, p. 40. 2 Archiv fur die gesammie Physiologie, lix. 427. 



' flee |>. 317. * Archiv j'iir Ophthalmologic, xlii. (4) S. 36. 



5 Ophthalmic Review, xvii. p. 341. 



6 Archiv J'iir Ophthalmologic, xlii. S. 288, and xliii. S. 477. 



7 Ibid., xliv. (2) S. 299, and xlvii. (2) S. 662. 



