234 



not obscured by what would otherwise be equivalent to a 

 flaw in the lens. Moreover, we find the lens gradually in- 

 creasing in density from the anterior surface to the nucleus, 

 and again, of course, decreasing in density from the nucleus 

 to the posterior surface, determining a curvilinear course to 

 the refracted ray, which we can but imperfectly imitate in 

 our most scientific combinations of refracting media. It must 

 be apparent from all this that a very gentle lateral pressure, 

 as evenly applied as the plan of construction is calculated to 

 permit, will produce such an alteration, both in the figure 

 and density of the lens, as materially to affect the focal range. 

 In reference to the calculation of Albers Dr. Kirkes remarks 

 that " the change in the distance of the retina from the lens 

 required for vision at all distances, supposing the cornea and 

 lens to maintain the same form, would not be more than about 

 one line ;" and Dr. Young " estimated the necessary change 

 at one sixth of the length of the axis of the eye ;" but the 

 former deduction is, perhaps, the more reliable. 



Without multiplying authorities, it appears clear, however, 

 that a very trifling alteration in the conditions of figure and 

 density in the refracting media, and in their position with 

 respect to the percipient surface, will suffice for all focal 

 adaptation. 



Though the remarks made in connection with the foregoing 

 anatomical details have anticipated the conclusion to some 

 extent, the view here taken of the mechanism of adaptation 

 may be set forth as follows, taking for illustration a 

 diagramatic section of the fore part of the eye of a pig 

 (Plate XVI, fig. 5). 



The pillars of the iris {g) being the common tendons, so to 

 speak, of the radiating fibres and of the ciliary muscle (o), 

 assuming these structures to be muscular, it stands to reason 

 that their contraction can only produce on the one hand 

 dilatation of the pupil, and on the other draw the ciliary 

 processes [i) forwards and a little inwards, so as to stretch 

 the ligament of the lens and draw that body forwards. 

 Hand in hand with this change, the lenticular pit in the 

 fore part of the hyaloid (w) would become shallower by pro- 

 portionate pressure on the vitreous humour, while the 

 pressure would be removed from the fluid in the canal of 

 Petit (m), and thereby from the free margin of the lens (/). 

 Now, as the sphincter surrounding the pupil antagonises the 

 radiating fibres of the iris, so the annular muscular band {h) 

 on the inner wall of the circular sinus, acting in concert with 

 the sphincter pupillae, would antagonise the radiating fibres 

 of the ciliary muscle (o), and also draw backwards and 



