14 Mr. T. Smith on the Muscular Structure and Functions 



known to have the power of adjusting the eye to different di- 

 stances, after the lens had been extracted by an operation. 



3rd. All anatomists agree that in young eyes the lens is 

 entirely soft and pulpy; whereas in old eyes, which of course 

 have been often adjusted to near objects, it is firmer, and has 

 a hard nucleus at the centre. In the fcetal calf and lamb I 

 have found the whole lens so fluid that it formed the capsule 

 into the figure of a globule. In very young calves, lambs and 

 rabbits, after birth, the consistence was firmer, but still pulpy 

 throughout; but in old cows, sheep, &c. the central part con- 

 stituted a nucleus hard or firm enough to retain any form 

 given to it. In this respect, therefore, observation also agrees 

 with the theory. 



4th. The position of the hardest part of the crystalline, as- 

 certained by observation on the lenses of oxen, sheep, deer, 

 rabbits, pigs, &c. agrees also in the most correct manner with 

 the theory. I made this observation several years ago with 

 considerable surprise, and long before the slightest idea of its 

 cause had entered into my mind. Wishing to divide the lens 

 into two equal sections in the direction of the axis, without 

 deranging the curvatures of its surfaces, I applied the edges 

 of two sharp scalpels to the opposite faces of the lens, and 

 holding the blades in the same plane pressed them gently 

 together along the axis till they met, which they did of course, 

 in the hardest point. By a memorandum, taken at the time, 

 the distance of this point was 0*28 inch from the anterior, and 

 0*22 inch from the posterior surface. The lens was that of a 

 cow, its diameter being 0*7 inch, its thickness 05 inch. By 

 measurements taken with the utmost care, the radius of curva- 

 ture of the anterior surface was 0*5 inch, the radius of the 

 posterior surface 0*39 inch. By the theory, I have shown that 

 the centre of pressure, and consequently the hardest point of 

 the nucleus, ought to be so situate, that its distance from the 

 anterior surface may be to its distance from the posterior, as 

 the radius of curvature of the former is to that of the latter. 

 But 28 is to 22 as 50 to 39 very nearly. In short, in all ani- 

 mals that have the anterior surface of the lens flatter than the 

 posterior, which I have been able to examine carefully, I have 

 found the hardest part of the lens nearer to the posterior than 

 the anterior surface. But in the roe, in the lens of which 

 I found the anterior surface the most convex, I found the 

 hardest point nearer to it than to the posterior surface. In 

 this respect, therefore, the theory is strongly supported by ob- 

 servation. The lenses of the cod and most other fishes are 

 so nearly spherical, that the relative position of the hardest 



