24 ADAPTATION OF THE EYE TO DISTANCES. 



cular turgescence of the ciliary processes ; and in this Tourtual agrees with 

 him. Mr. White Cooper* and Mr. Alfred Smee,| who have lately written 

 on this subject, also advocate the same view. In Mr. Smee's opinion tur- 

 gescence of the ciliary processes will produce pressure outwards against the 

 cornea, and inwards towards the vitreous body, and the result of this will be 

 that the lens is carried directly forwards ; its subsidence to its former posi- 

 tion will ensue passively on the cessation of the turgescence. But the most 

 probable explanation, and one which has received the most support, is that 

 the forward movement of the lens is effected by a contractile, probably 

 muscular, tissue, situated in the parts surrounding the margin of the lens. 

 According to many of the older observers, such a contractile tissue was 

 supposed to exist in the lens itself; and this opinion was formerly enter- 

 tained by Volkmann, who of late, however, has seen reason to abandon it. 

 Hueck, who has especially occupied himself with the consideration of this 

 subject, states that the contractile tissue by which the lens is acted upon is 

 situated along the anterior and inner portion of the ciliary body, and con- 

 sists of transversely arranged, firm, probably muscular fibres, connected 

 together in a kind of network. Briicke's J account corresponds very closely 

 with this, though he considers the whole ciliary body to be composed of 

 muscular fibres, which pass backwards to be inserted into the choroid coat. 

 In birds, and many amphibia, he describes these fibres as being of the 

 striped variety ; but in man and mammalia they are unstriped. Huschke 

 is of opinion that on the contraction of these fibres, which ensues during the 

 vision of near objects, the fluid contents of the canal of Petit are com- 

 pressed against the margin of the lens, whereby the lens itself is lessened 

 in diameter, and becomes more curved forwards on its anterior surface. 

 Briicke, on the other hand, considers that the action of these fibres will be 

 to draw the choroid, and with it the retina closer around the vitreous body 

 so as to compress it [and thus probably push the lens forwards, so as to 

 assist in the vision of near objects]. On relaxation of these fibres the 

 choroid regains its former position by the recoil of a layer of elastic 

 fibres, which, according to Briicke, are situated between the ciliary body 

 and the choroid. Dr. Clay Wallace, who was among the first to describe 

 these muscular fibres of the ciliary body, considers that they act in adapt- 

 ing the eye to near vision by compressing the ciliary veins, and so pro- 

 ducing the turgescence of the ciliary processes, which he, as well as the 

 other observers already mentioned, recognizes as the cause of the accom- 

 modating power of the eye. According to Wagner, || the ciliary processes 

 as well as the ciliary body, contain contractile fibres, which have all the 

 characters of those of organic muscle : by the action of these fibres, Wag- 



* On Near and Distant Vision, 1847, p. 1 95. 



t Vision in Health and Disease, &c. 1847. $ Miiller's Archiv. 1846, p. 370. 



London Medical Gazette, 1842. Dec. 16 and 23. 



|| Lehrbuch der spezielle Physiologic, 1843. Second edition, p. 405. 



