From the preceding observations it might reasonably be supposed that the lens is com posed of a homogeneous material, such as al bumen or gelatine, more consolidated in the centre than at the circumference ; but this is not the case ; on the contrary, it exhibits as much of elaborate organization as any other structure in the animal economy. It consists of an outer case or capsule, so totally different from the solid body contained within it, that they must be separately investigated and de scribed. The body of the lens, it has been already stated, consists of certain saline and animal ingredients combined with more than their weight of water, and when perfectly transparent presents the appearance of a tena cious unorganized mass ; but when rendered opaque by disease, loss of vitality, heat, or im mersion in certain fluids, its intimate structure becomes visible. If the lens with the capsule attached to the hyaloid membrane be removed from the eye and placed in water, the following day it is found slightly opaque or opaline, and split into several portions by fissures extending from the centre to the circumference, as seen in fig. 118. This appearance is rendered still more obvious by immersion in spirit, or the addition of a few drops of acid to the water. If a lens thus circumstanced be al lowed to remain some days in water, it con tinues to expand and unfold itself, and if delicately touched and opened by the point of a needle, and carefully transferred to and as it hardens is still more unravelled by dissection, it ultimately presents a remarkable fibrous or tufted appearance, as represented in the figure below, drawn by me some years ago from a preparation of the lens of a fish thus treated (the Lophius piscatorius). The three annexed figures represent the structure of the lens above alluded to: A is the human crystal line in its natural state; B, the same split up into its component plates ; and C, unravelled in the fish.
This very remarkable structure of the body of the lens appears to have been first accu rately described by Leeuwenhoek, subse quently by Dr. Young, and still more recently by Sir David Brewster. Leenwenhoek says, " It may be compared to a small globe or sphere, made up of thin pieces of paper laid one on another, and supposing each paper to be composed of particles or lines placed some what in the position of the meridian lines on a globe, extending from one pole to the other." Again he says, " With regard to the before mentioned scales or coats, I found them so exceedingly thin, that, measuring them by my eye, I must say that there were more than two thousand of them lying one upon another." " And, lastly, I saw that each of these coats or scales was formed of filaments or threads placed in regular order, side by side, each coat being the thickness of one such filament." The peculiar arrangement of these fibres he describes as follows : " Hence we may collect how ex cessively thin these filaments are; and we shall be struck with admiration in viewing the won derful manner they take their course, not in a regular circle round the ball of the crystalline humour, as I first thought, but by three dif ferent circuits proceeding from the point L, which point I will call their axis or centre.
They do not on the other side of the sphere approach each other in a centre like this at L, but return in a short or sudden turn or bend, where they are the shortest, so that the filaments of which each coat is composed have not in reality any termination or end. To explain this more particularly, the shortest filaments, M K, II N, and 0 F, which fill the space on the other side of the sphere, constitute a kind of axis or centre, similar to this at L, so that the fila ments M K, having gone their extent, and filled up the space on the other side, in like manner as is here shewn by the lines E L I, return back and become the shortest filaments H N. These filaments H N, passing on the other side of the sphere, again form another axis or centre, and return in the direction 0 F, and the fila ments 0 I', again on the other side of the sphere, collect round a third centre, and thence return in the direction li M ; so that the fila ments which are on this side of the sphere collect round a third centre, and thence return in the direction IC M; so that the filaments which are on this side the shortest, on the other side are the longest, and those which there are the shortest are here the longest." Annexed is Leeuwenhoek's representation (fig. 119).
Sir David Brewster says that the direction of the fibres is different in different animals; the simplest arrangement being that of birds, and the cod, haddock, and several other fishes. In it the fibres, like the meridians of a globe, con verge to two opposite points of a spheroidal or lenticular solid, as in the annexed figure.
Dr. Young differs from Leeuwenhoek as to the arrangement of the fibres and other parti culars, and in his last paper corrects the de scription given by himself in a former one ; he says, " The number of radiations (of the fibres) is of little consequence, but I find that in the human crystalline there are ten on each side, not three, as I once from a hasty observation Concluded." " In quadrupeds the fibres at their angular meeting are certainly not conti nued as Leeuwenhoek imagined." Beneath is Dr. Young's last view of the arrangement of the fibres, which Dr. Brewster has shown to be incorrect, but the introduction of which is jus tified by the source from which it is derived.
The second or next simplest structure lie detected in the salmon, shark, trout, and other fishes ; as well as in the hare, rabbit, and por poise among the mammalia; and in the alli gator, gecko, and others among reptiles. Such lenses have two septa at each pole, as in the annexed figure.
The third or more complex structure exists in mammalia in general, " in which three septa diverge from each pole of the lens, at angles of 1200, the septa of the posterior surface bisect ing the angles formed by the septa of the ante rior surface, as in the annexed figure (fig.123).