EYE. 



those from the crystalline into the vitreous 

 humour, the indices of refraction of the sepa- 

 rating surface of these humours will be, from 

 the aqueous humour to the outer coat of the 

 crystalline 1.0466, from the aqueous humour 

 to the crystalline, using the mean index, 1.0353, 

 from the vitreous to the outer coat of the cry- 

 stalline 1.0445, from the vitreous to the crystal- 

 line, using the mean index, 1.0332." Dr. 

 Young says, " On the whole it is probable 

 that the refractive power of the centre of the 

 human crystalline, in its living state, is to that 

 of water nearly as 18 to 7; that the water im- 

 bibed after death reduces it to the ratio of 21 to 

 20 ; but that on account of the unequable den- 

 sity, ks effect in the eye is equivalent to a 

 refraction of 14 to 13 for its whole size/* 



Respecting the chemical composition of the 

 lens, Berzelius observes, that " the liquid in 

 its cells is more concentrated than any other 

 in the body. It is completely diaphanous and 

 colourless, holding in solution a particular 

 animal matter belonging evidently to the class 

 of albuminous substances, but differing from 

 fibrine in not coagulating spontaneously, and 

 from albumen, inasmuch as the concentrated 

 solution, instead of becoming a coherent mass 

 on the application of heat, becomes granulated 

 exactly as the colouring matter of the blood 

 when coagulated, from which it only differs in 

 the absence of colour. All those chemical 

 properties are the same as those of the co- 

 louring matter of the blood. The following 

 are the principles of which the lens is com- 

 posed : peculiar coagulable albuminous matter 

 35.9, alcoholic extract with salts 2.4, watery 

 extract with traces of salts 1.3, membrane form- 

 ing the cells 2.4, water 58.0. 



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 Jig. 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 spirit, 

 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. 



Fig. 118. 



B 



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. Leeuwenhoek 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 rnore 

 particularly, the shortest filaments, M K, H N, 

 and O 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 ELI, return 

 back and become the shortest filaments H N. 

 These filaments H N, passing on the other side 



