VISION. 



1451 



it does not from thence follow that what is not 

 red is not musculous. The muscular fibres of 

 the guts and stomach have scarce anything of 

 redness in their colour ; and it is also certain 

 that the pupil does contract and dilate itself 

 according as objects are more or less lumi- 

 nous, and yet none of the fibres which perform 

 that action are in the least red. Whence it 

 follows that the fibres of the liganicntum cil'mrc 

 are not to be deprived of a power of con- 

 traction because of a colour different from 

 what generally obtains in other muscles ; nor 

 are we to be surprised that so many accurate 

 anatomists, after a careful examination of this 

 process, have not scrupled to affirm it to be 

 truly muscular." 



MOILS. Pouillet has advanced the hypothesis 

 that, by the peculiar conformation ot the lens, 

 near objects are seen through the medium of 

 the rays passing through its centre, and distant 

 objects by means of the circumferential rays. 

 He describes the crystalline lens as made up of 

 strata, differing in curvature and density, so 

 that its section exhibits a series of concen- 

 trical ellipses having varying excentricities, 

 the internal strata being more curved and 

 lore dense than the external ; whence the 

 lys which pass from the latter converge to a 

 lore distant point than those from the former. 

 According to this theory, the crystalline lens 

 las many different foci, and the effect should be, 

 that when a pencil of rays falls upon it, those 

 ays which are near the axis of the pencil, 

 therefore near the centre of the lens, are 

 )rought to a shorter focus than those which 

 are near the border ; whence near objects 

 would, says M. Pouillet, be seen by means of 

 the central rays, and distant objects by means 

 of those rays which fall near the borders of 

 the crystalline lens. 



It has been observed that De la Hire de- 

 nied that there is an alteration in the focus, 

 whether we look at a near or distant object. 

 He regarded the whole adjustment as a simple 

 enlargement and diminution of the pupil. At 

 first sight this may appear absurd, but there 

 are facts which give some colour to his theory. 

 That a change in the size of the pupil has a 

 considerable influence upon the distinctness 

 of objects at different distances, is known, 

 and if the eye be turned to a near object, as 

 a book, after it has been gazing at a distant 

 ship, the illumination of both being equal, 

 the pupil is observed to contract. Dr. 

 Mackenzie says, it is an error not unfre- 

 quently adopted, that if the rays which pass 

 into the eye from a distant object, and those 

 from a near object, have the same divergence, 

 a circumstance which may depend on a mere 

 change in the size of the pupil, they will be 

 collected on the same point of the retina 

 without any change in the refractive media of 

 the eye. That this cannot be the case, is evi- 

 dent from the fact that the rays from a dis- 

 tant object, and those from a near object, 

 although they may have the same divergence, 

 fall on the cornea at different angles of inci- 

 dence, and must necessarily meet the axis of 

 tiie eye at different points after refraction. 



Now the whole effect of the alteration in the 

 size of the pupil is, as explained elsewhere, 

 to increase the distinctness of objects by 

 cutting off those rays which would cause cir- 

 cles of dispersion on the retina. On the one 

 hand, an enlargement of the pupil gives dis- 

 tinctness to distant objects, by allowing a 

 greater quantity of light to enter the eye, and 

 on the other, its contraction assists in render- 

 ing near objects distinct by cutting off the 

 lateral rays which are not duly refracted, and 

 would cause confusion of the image on the 

 retina. If, however, a proof were required to 

 show that something more than a mere alter- 

 ation in the form of the pupil is necessary 

 to the perfection of vision at all distances, it 

 would be afforded by the change termed prcs- 

 byopia. The eye, in what may be called its 

 state of perfect indolent vision, is adapted 

 only to see distant objects, the adjustment to 

 the near focus requiring an effort : the power 

 to make the effort in question is partially or 

 entirely lost by the presbyopic eye, yet the 

 pupil may act as vigorously as ever. Though 

 it is not unusual to find a degree of sluggish- 

 ness in the actions of the pupil in elderly 

 persons, yet we have repeatedly seen instances 

 in which the iris acted with great vigour, and 

 where a book was obliged to be held at arm's 

 length for the type to be distinctly seen. 



Wagner and Dr. Clay Wallace of New 

 York follow Porterfield in attributing the ad- 

 justment to the action of the corpus ciliare ; 

 the latter considering that, " by the graduat- 

 ing power of the ciliary processes and ciliary 

 muscles, together with the elasticity of the 

 membranes of the vitreous body, the crystalline 

 may be drawn not only backwards and for- 

 wards, but its inclination may be changed so as 

 to throw the image on another part of the re- 

 retina :" and the modus operandi he explains by 

 supposing " the outer ciliary muscle to contract 

 the vessels returning from the ciliary pro- 

 cesses ; the ciiiary processes which are at- 

 tached by the filaments of Ammon to the 

 ciliary -zone and crystalline capsule, to become 

 erect and to draw forward the crystalline 

 body ; and the inner ciliary muscle, aided by 

 the elasticity of the membranes of the vitreous 

 humour, to draw it backwards." 



The opinion, however, which appears most 

 satisfactory, is that advanced by Mr. Bow- 

 man, who has clearly proved the muscular 

 nature of the ciliary body. " It has (says 

 he) the arrangement of a "muscle, very much 

 the structure of a muscle, and is largely sup- 

 plied with nerves, which are in great part 

 derived from a motor source the third 

 pair. This muscle arises, or has its most 

 fixed attachment, at the junction of the scle- 

 rotica and cornea, as much in front of the lens 

 as is possible, consistently with the preserva- 

 tion of the transparency of the cornea. That 

 it may act more freely, a canal, the circular 

 sinux, is interposed between its origin and the 

 portion of the sclerotica which it lies against. 

 Beyond this point it is hardly at all attached 

 to the sclerotica, over which its fibres may be 

 supposed to move in contraction ; but it 



