HYPOTHESES ACCOUNTING FOR IT. 25 



ner considers that the lens may be drawn forwards as well as perhaps 

 somewhat compressed laterally. Wagner's account of these fibres of the 

 ciliary body and processes is also corroborated by the researches of Mr. 

 Todd and Mr. Bowman,* who describe the fibres as radiating backwards 

 from the junction of the sclerotic and cornea, and spreading over the outer 

 surface of the ciliary body, the more superficial ones being inserted into 

 the posterior part of this body, while the deeper ones seem to dip behind 

 the iris to the more prominent parts of the ciliary processes which approach 

 the lens. These fibres, although they belong to the unstriped variety of 

 muscle yet seem to be analogous to those of the ciliary muscle in birds, 

 which occupy the same position, but are of the striped kind. The contrac- 

 tion of these fibres will, according to Dr. Todd and Mr. Bowman, have the 

 effect of advancing the ciliary processes, and with them the lens to which 

 the processes are attached with considerable firmness, towards the cornea. 

 One difficulty, however, which must ever present itself against the view 

 that the contraction of these various fibres situated about and within the 

 ciliary body belonging, as they are said to do, to the class of involun- 

 tary muscles constitutes the exclusive, or even the principal, condition 

 by which the eye is enabled to accommodate itself to the distinct vision 

 of objects at various degrees of closeness, is the circumstance that this 

 accommodating power can by many persons be effected by a voluntary 

 effort, quite independent of any alteration in the direction of the axes of 

 the eyes. It cannot but be concluded from this circumstance either 

 that there exist some other conditions than the contraction of the above 

 described muscular fibres, by which the eye can adapt itself to distances, 

 or else, which is very improbable, that a voluntary and tolerably rapid 

 movement can be effected by the action of involuntary muscular fibres. 

 The same difficulty occurs also in referring the explanation of the adapting 

 power of the eye to vascular turgescence of the ciliary processes. 



An entirely different explanation of the power of adaptation in the eye 

 has been offered by M. Sturm. f This explanation is founded chiefly upon 

 the result of Chossat's f measurements of the eye of an ox, which shew that 

 none of the refracting media of the eye have a spherical form, but that 

 the anterior surface of the cornea and the two surfaces of the lens repre- 

 sent segments of different ellipsoids. From such a conformation of the 

 refracting bodies it will follow that the several rays of a cone of light pro- 

 ceeding from an object placed before the eye will not be concentrated to a 

 single focal point at a definite distance behind the lens, as is commonly 

 supposed, but will intersect each other at different distances, within cer- 

 tain limits. And in any plane within these limits, although the rays are 

 spread over a minute surface, instead of being collected to a point, yet 



* Op. cit. Part iii. p. 27. t Comptes Rendus, torn. xx. pp. 554 and 761. 



J An. de Chhnie et de Physique, torn. x. 1819. 



