266 SENSE OF SIGHT. 



to bodies at different distances, he expected, if the figure of the eye 

 were modified, that the spot, caused by the pressure, would be altered 

 in shape and dimensions; but no such effect occurred; the power of 

 accommodation was as extensive as ever, and there was no perceptible 

 change either in the size or figure of the oval spot. Again, Sir Everard 

 Home asserts, that all the ingenuity of the distinguished mechanician, 

 Ramsden, was unable to decide, whether, in the adjustment of the eye, 

 there be any alteration produced in the curvature of the cornea. These 

 facts would alone induce a doubt of the existence of this kind of adjust- 

 ment, even if we had not the additional evidence, that many animals 

 are incapable of altering the shape of the eyeball, by the muscles at 

 least. The c%tacea, the ray amongst fishes, and the lizard amongst 

 reptiles, have the sclerotica so inflexible as to render any variation in it 

 impossible. 



With regard to many of the particular views that have been men- 

 tioned, they are mere "cobwebs of the brain," and unworthy of serious 

 argument. In the action of the orbicularis palpebrarum, as suggested 

 by Dr. Monro, there is, however, something so plausible, that many 

 persons have been misled by it. He made a set of experiments to show, 

 that this muscle, by compressing the eyeball, causes the cornea to pro- 

 trude, and thus enables the eye to see near objects more distinctly. 

 When he opened his eyelids wide, and endeavoured to read letters, 

 which were so near the eye as to be indistinct, he failed; but when he 

 kept the head in the same relation to the book, brought the edges of 

 the eyelids within a quarter of an inch of each other, and made an ex- 

 ertion to read, he found he could see the letters distinctly. But Sir 

 Charles Bell 1 properly remarks on this experiment, that if the eyelids 

 have any effect upon the eyeball by their approximation, it must be to 

 flatten the cornea; and that the improvement in near vision produced 

 by such approximation, is owing to the most divergent rays being shut 

 off, as in the experiment of the pinhole through paper. 



2. The second hypothesis, which attributes the adaptation to a change 

 of figure in the crystalline itself, has been embraced by all who regard 

 that body to be muscular, and therefore by Leeuwenhoek, 2 Des Cartes, 3 

 and Dr. Young. 4 These muscular fibres, however, could never be ex- 

 cited by Dr. Young to contraction so as to change the focal power; and 

 their very existence is more than doubtful. The increasing density of 

 the lens towards its centre indicates rather a cellular structure, the cells 

 being filled with transparent matter of various degrees of concentration ; 

 and an examination into its intimate physical constitution affords no 

 evidence of muscularity. 



Professor Forbes, 5 of Edinburgh, embraced the view, that the 

 adaptation is owing to a change of figure in the crystalline ; but his 

 explanation of its mode of production varies from that given by pre- 



1 Anat. and Physiology, Arner. edit, by Dr. Godman, ii. 227, New York, 1827. 



2 Boerhaav. Prselect., 527, torn. iv. p. 92; and Haller, Element. PhysioL lib. xvi. sect. 2. 



3 Op. cit. 4 Op. citat. 



5 Proceedings of the Royal Society of Edinb., No. 25, cited in the Amer. Journ. of the Med. 

 Sciences, Oct., 1845, p. 504. 



