The mode in which these fibres are laterally united to each other is equally curious. Sir David Brewster says that he ascertained this in looking at a bright light Ihrough a thin lamina of the lens of a cod, when he observed two faint and broad prismatic images, situated in a line exactly perpendicular to that which joined the common coloured images. Their angular distance from the central image was nearly five times greater than that of the first ordinary prismatic images, and no doubt whatsoever could be entertained that they were owing to a number of minute lines perpendicular to the direction of the fibres, and whose distance did not exceed the of an inch. Upon ap plying a good microscope to a well-prepared lamina, the two fibres were found united by a series of teeth exactly like those of rack work, the projecting teeth of one fibre entering into the hollows between the teeth of the adjacent one, as in fig. 124.
I have said that the lens consists of an outer case or capsule totally different from the solid body contained within it. This capsule is strong, elastic, and perfectly transparent. 1 n the paper to which I have alluded in the Me dico-Chirurgical Transactions, I gave the fol lowing detailed description of its nature and properties :— " The real nature of the capsule of the lens has not, I think, been sufficiently attended to its thickness, strength, and elasticity, have cer tainly been noticed, but have not attracted that attention which a fact so interesting, both in a physiological and pathological point of view, deserves. That its structure is cartilaginous, I should conclude, first, from its elasticity, which causes it to assume a peculiar appearance when the lens has been removed, not falling loose into folds as other membranes, but coiled in different directions ; or if the lens be removed by opening the capsule behind, and with drawing it through the vitreous humour, allow ing the water in which the part is immersed to replace the lens, the capsule preserves in a great degree its original form, especially in the eye of the fish; secondly, from the density and firmness of its texture, which may be ascer tained by attempting to wound it by a cataract needle, by cutting it upon a solid body, or compressing it between the teeth; thirdly, from its permanent transparency, which it does not lose except on the application of very strong acid or boiling water, and then only in a slight degree; maceration in water for some months, or immersion in spirit of strength sufficient to preserve anatomical preparations, having little or no effect upon it. If the lens be removed from the eye of a fish dressed for the table, the capsule may be raised by the point of a pin, and be still found almost perfectly transparent, This combination of density and transparency gives the capsule a peculiar sparkling appear ance in water, in consequence of the reflection of light from its surface, resembling a portion of thin glass which had assumed an irregular form while soft; this sparkling I consider very characteristic of this structure. The properties just enumerated appear to me to distinguish it from every other texture but cartilage; still, however, it may be said that cartilage is not transparent, but even the cartilage of the joints is semi-transparent, and, if divided into very thin portions, is sufficiently pellucid to permit the perception of dark objects placed behind it, and we obtain it almost perfectly transparent where it gives form to the globe of the eye, as in the sclerotic of birds and fishes. If the soft consistence, almost approaching to fluidity, of the external part of the lens, be considered, the necessity of a capsule capable itself of pre serving a determinate form is obvious. It the lens were enclosed in a capsule such as that which envelopes the vitreous humour, its sur face could not be expected to present the ne cessary regular and permanent curvature; nor could we expect that if the form of the lens were changed, it could be restored without this provision of an elastic capsule." The capsule is liable to become opaque and constitute cataract, as the body of the lens is. These capsular cataracts are easily distinguished from the lenticular. They never present the stellated appearance frequently observed when the texture of the opaque lens opens in the cap sule as it does when macerated in water, nor the uniform horny or the milky blue appearance of common lenticular cataract. The opacity in capsular cataract exists in the shape of irregular dots or patches, of an opaque paper-white ap pearance, and when touched with the needle are found hard and elastic, like indurated cartilage, the spaces between the specks of opacity fre quently remaining perfectly transparent.
It appears to be generally assumed by writers on anatomy that a watery fluid is interposed between the body of the lens and its capsule, from an incidental observation of Morgagni when discussing the difference in density be tween the surface and centre of the lens; hence it has been called the aqua Morgagni. The observation of this celebrated anatomist, in his Adversaria Anatomico, which has led to the universal adoption of this notion, is, however, merely that upon opening the capsule he had frequently found a fluid to escape. " Deinde eadem tunicft in vitulis etiam, bobusque sive recens, sive non ita recens occisis perforata, pluries animadverti, illico humorem quendam aqueum prodire : quod et in homine observare visus sum, atque adeo credidi, hujus humoris secretione prohibit5, crystallinum siccum, et opacum fieri fere ut in extracto exsiccatoque crystallino contingit." Ile does not, however, subsequently dwell upon or insist upon the point. I do not believe that any such fluid exists in a natural state, but that its accumula tion is a consequence of loss of vitality; the water combined with the solid parts of the lens escaping to the surface and being detained by the capsule, as occurs in the pericardium and other parts of the body. In the eyes of sheep and oxen, when examined a few hours after death, not a trace of any such fluid can be detected, but after about twenty-four hours it is found in considerable quantity. In the human eye a fluid sometimes accumulates in the capsule, constituting a particular form of cataract, which presses against the iris, and almost touches the cornea; but such eyes are, I believe, always unsound. From this erro neous notion of an interposed fluid between the lens and its capsule has arisen the adop tion of an unsustained and improbable conclu sion, that the lens has no vital connexion with its capsule, and consequently must be produced and preserved by some process analogous to secretion. Respecting this matter I have ob
served, in the paper above alluded to, " The lens has been considered by some as having no connexion with its capsule, and consequently that its formation and growth is accomplished without the assistance of vessels; such a notion is so completely at variance with the known laws of the animal economy, that we are justi fied in rejecting it, unless supported by un questionable proof. The only reasons which have been advanced in support of this conclu sion are, the failure of attempts to inject its vessels, and the ease with which it may be separated from its capsule when that mem brane is opened. These reasons are far from being satisfactory; it does not necessarily follow that parts do not contain vessels, be cause we cannot inject them ; we frequently fail when thete can be no doubt of their exist ence, especially where they do not carry red blood. I have not myself succeeded in in jecting the vessels of the lens, but I have not repeated the trial so often as to make me despair of accomplishing it, more especially as Albinus, an anatomist whose accuracy is universally acknowledged, asserts, that after a successful injection of the capsule of the lens, he could see a vessel passing into the centre of the lens itself. Lobe, who was his pupil, bears testimony to this. The assertion that the lens is not connected with its capsule, I think I can show to be incorrect; it has been made from want of care in pursuing the inves tigation, and from a notion that a fluid exists throughout between the lens and its capsule. When the capsule is opened, its elasticity causes it to separate from the lens; especially if the eye be examined some days after death, or has been kept in water, as then the lens swells, and often even bursts the capsule and protrudes through the opening, by which the connexion is destroyed. I have however satis fied myself that the lens is connected with its capsule (and that connexion by no means slight) by the following method. I remove the cornea and iris from an eye, within a few hours after death, and place it in water, then with a pair of sharp-pointed scissors I divide the capsule all round at the circumference of the lens, taking care that the division is made behind the anterior convexity, so that the lens cannot be retained by any portion of the cap sule supporting it in front. I next invert the eye, holding it by the optic nerve, when I find that the lens cannot be displaced by agitation, if the eye be sufficiently fresh. In the eye of a young man about six hours dead, I found that, on pushing a cataract needle into the lens, after the anterior part of the capsule had been removed, I could raise the eye from the bottom of the vessel, and even half way out of the water, by the connexion between the lens and its capsule. It afterwards required consider able force to separate them, by passing the needle beneath the lens, and raising it from its situation. I believe those who have been in the habit of performing the operation of ex traction, have occasionally encountered consi derable difficulty itt detaching the lens from its situation after the capsule had been freely opened, this difficulty I consider fairly refer able to the natural connexion just noticed." When the lens enclosed in its capsule is de tached from the hyaloid membrane, the con nexion between it and the capsule is destroyed by the handling, and, in consequence, it moves freely within that covering, affording to those who believe that there is no union between the two surfaces fallacious evidence in support of that opinion, which, if not sustained by better proof, should be abandoned. Dr. Young in sists upon the existence of the natural con nexion by vessels and even by nerves between the lens and its capsule ; he says, " The cap sule adheres to the ciliary substance, and the lens to the capsule, principally in two or three points; but I confess 1 have not been able to observe that these points are exactly opposite to the trunks of nerves; so that probably the adhesion is chiefly caused by those vessels which are sometimes seen passing to the cap sule in injected eyes. We may, however, dis cover ramifications from some of these points upon and within the substance of the lens, generally following a direction near to that of the fibres, and sometimes proceeding from a point opposite to one of the radiating lines of the same surface. But the principal vessels of the lens appear to be derived from the central artery, by two or three branches at some little distance from the posterior vortex, which I conceive to be the cause of the frequent adhe sion of a portion of a cataract to the capsule about this point ; they follow nearly the course of the radiations and then of the fibres ; but there is often a superficial subdivision of one of the radii at the spot where one of them enters." The great size of the vessels distri buted on the back of the capsule in the foetus strengthens the conclusion that the lens is fur nished with vessels as the rest of the body. When the eye of a foetus of seven or eight months is finely injected, a branch from the central artery of the retina is filled and may be traced through the centre of the vitreous hu mour to the back of the capsule, where it ramifies in a remarkably beautiful manner, assuming, according to Sommerring, a stellated or radiating arrangement. Zinn declares that he found branches from this vessel penetrating the lens : " Optime autem placet observatio arteriolre lentis, in oculo infantis, cujus vasa cera optime erant repleta, summit voluptate mihi visa', quam prope marginem ad convexi tatem posteriorem dilatam, duobus ramulis perfomta capsula in ipsam substantiam lentis profunde se immergentem cortissime con spexi." lie also quotes the authority of Ituysch, Moeller, Albinos, and Winslow, as favouring the same view. Against such au thority I find that of the French systematic writer Bichat advanced; but on such a point his opinion is of little value. Annexed is Zinn's representa tion of the distribu tion of the branch of the central artery on the back of the capsule, from a preparation in Lieberkithri's mu seum. Similar fi gures have been given by Albinos, Sommerring, and Sir Charles Bell.