756 AN AMERICAN TEXT-BOOK OF PHYSIOLOGY. 



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 Tscherning, 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 222. These theories have an advantage over that 

 offered by Helmholtz, inasmuch as they afford an expla- 

 nation of the presence in the ciliary muscle of circular 

 fibres, which, on the theory of Helmholtz, seem to be su- 

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

 FIG. 222. TO illustrate matic accommodation " comprehensible. This term is 



Schoen's theory of ac- ,. , i , i , in 



commodation. 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. 4 



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 unstriped fibres of the ciliary 

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

 applied to muscular fibres of this sort may be misleading. The voluntary 

 character of the act of accommodation is not affected by the circumstance that 

 the will needs, as a rule, to be assisted by visual sensations. The fact that 

 most persons cannot affect the necessary change in the eye unless they direct 

 their attention to some near or far object is only an instance of the close rela- 

 tion between sensory impressions and motor impulses, which is further exem- 



1 Archives de Physiologic, 1894, p. 40. 2 Archiv fur die gesammte Phys., lix. 427. 



3 See p. 763. 



* Kecent observations by Hess (Archiv f. Ophthalmologie, xJii. 288) tend to confirm the Helm- 

 holtz theory by showing that the suspensory ligament is relaxed and not stretched in accommo- 

 dation for near objects. 



