71 
on the Mechanism of the Eye. 
From this investigation of the change of the figure of the 
lens, it appears that the action which I formerly attributed to 
the external coats, cannot afford an explanation of the pheno- 
menon. The necessary effect of such an action would be, to 
produce a figure approaching to that of an oblate spheroid; 
and, to say nothing of the inconvenience attending a dimi- 
nution of the diameter of the lens, the lateral refraction would 
be much more increased than the central; nor would the 
slight change of density, at an equal distance from the axis, 
be at all equivalent to the increase of curvature: we must 
therefore suppose some different mode of action in the power 
producing the change. Now, whether we call the lens a 
muscle or not, it seems demonstrable, that such a change of 
figure takes place as can be produced by no external cause ; 
and we may at least illustrate it by a comparison with the 
usual action of muscular fibres. A muscle never contracts, 
without at the same time swelling laterally, and it is of no 
consequence which of the effects we consider as primary. I 
was induced, by an occasional opacity, to give the name of 
membranous tendons to the radiations from the centre of the 
lens ; but, on a more accurate examination, nothing really ana- 
logous to tendon can be discovered. And, if it were supposed 
that the parts next the axis were throughout of a tendinous, and 
therefore unchangeable nature, the contraction must be princi- 
pally 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, there is no difficulty in 
