596 



OV THE MECHANISM OF THE EVK. 



power producing the change. Now, whether 

 we call the lens a muscle or not, it seems de- 

 monstrable, that such a change of figure 

 takes place as can be produced by no exter- 

 nal cause ; and we may at least illustrate it 

 by a comparison with the usualj_ action of 

 muscular fibres. A mnscle never contracts, 

 without at the same time swelling lateraih', 

 and it is of no consequence whicli of the ef- • 

 fects we consider as primary. I was induced, 

 b^' an occasional opacit}', to give the name 

 of membranous tendons to the radiations 

 from the centre of the lens; but on a more 

 accurate examination, nothing really analo- 

 gous to tendon can be discovered. And, if 

 it were supposed that the parts next the axis 

 were throughout of a tendinous, and there- 

 fore unchangeable nature, the contraction 

 must be principally effected by the lateral 

 parts of the fibres ; so that the coats would 

 become thicker towards the margin, by their 

 contraction, while the general alteration of 

 form would require them to be thinner; and 

 there would be a contrariety in the actions of 

 the various parts. But, if we compare the 

 central parts of each surface to the belly of 

 the muscle, it is easy to conceive their thick- 

 ness to be immediately increased, and to 

 produce an immediate elongation of the axis 

 and an increase of the central curvature . 

 while the lateral parts cooperate more or less 

 according to their distance from the centre, 

 and in different individuals in somewhat dif- 

 ferent proportions. On this supposition, we 

 have no longer any difficulty in attributing a 

 power of change to the crystalline of fishes. 

 M. Petit, in a great number of observations, 

 uniformly found the lens of fishes more or 

 less flattened : but, even if it were not, a 

 slight extension of the lateral part of the super- 

 ficial fibres would allow those softer coats to be- 



come thicker at each vertex, and to form the 

 whole lens into a spheroid somewhat oblong ; 

 and here, the lens being the only agent in 

 refraction, a less alteration than in other ani- 

 mals would be sufficient. It is also worthy of 

 inquiry, whether the state of contraction may 

 not immediately add to the refractive pov/er. 

 According to the old experiment, by which 

 Dr. Goddard attempted to show that muscles 

 become more dense as they contract, such an 

 eflcct might naturally be expected. That ex- 

 periment is, however, very indecisive, and the 

 opinion is indeed generally exploded, but 

 perhaps too hastily ; and whoever shall ascer- 

 tain the existence or nonexistence of such a 

 condensation, will render essential service to 

 physiology in general. Some interesting ex- 

 periments, on this subject, have been pro- 

 mised to the public by a very ingenious phy- 

 siologist, who has probably employed a more 

 decisive method of investigation in his re- 

 searches. Swammerdam professes to have 

 found such a condensation in the contrac- 

 tion of a muscle; but it is obvious, that what 

 he has attributed to the heart properly be- 

 longed only to the air which it contained, 

 and one of his experiments, which was free 

 from this source of fallacy, does not ap- 

 pear to have shown any satisfactory result, 

 although conducted with some accuracy, by 

 inclosing a muscle in a bottle filled with 

 water, communicating with a narrow open 

 tube *. 



Dr. Pemberton, in the year 1719, first sys- 

 tematically discussed the opinion of the mus- 

 cularity of the crystalline lens f . He refer- 

 red to Leeuwenhoek's microscopical obser- 

 vations ; but he so overwhelmed his subject 



* Book of Nature, II. 126, 157. 



t De f acultate Oculi qua ad diversas rerum distantiai 

 K accommodat. L. B, 1719. Ap. Hall.Disp. Anat. IV. 30i. 



