94 OBSERVATIONS ON THE STRUCTURE OF THE VITREOUS HUMOR. (Reprinted from THE DUBLIN QUARTERLY JOURNAL OF MEDICAL SCIENCE of August 1848.) DURING the last few years considerable attention has been given in Germany to the anatomical structure of the vitreous body. This part has been hitherto loosely held to consist of an extremely delicate filamentous tissue, so interlaced as to inclose cellular spaces, in which the water was supposed to be retained, which slowly escaped when the hyaloid investment was punctured. Not that any anatomist had seen these filaments or cells, but their existence was deduced from the fragmentary form taken by the contained water when frozen, particularly as it afforded a plausible explanation of the more obvious properties of the structure, by a fancied analogy with common areolar tissue. When, however, more perfect means of investigation came to be applied without success to the detection of the filamentary substratum, there was room for new researches of a different kind, with a view to the discovery of some other structural cause of the curious and very peculiar physical properties exhibited by the fresh and healthy vitreous body. Pappenheim appears to have been the first to call attention to the fact that evidence may be obtained of an internal artificial arrangement of parts. He announced that the vitreous body, treated with a solution of carbonate of potass, exhibited a succession of concentric layers, something like those of an onion.* Brücke, in the following year,† pursued this hint, imagining that * Specielle Geweblehre des Auges, Breslau, 1842, s. 182. STRUCTURE OF THE VITREOUS HUMOR. 95 there might exist in the substance of the vitreous body a series of membranes capable of anatomical demonstration; and he thought it probable that, by steeping the humor in a solution which would furnish a precipitate as it permeated the vitreous substance, these membranes might arrest the precipitate, and thus become apparent to the eye. He accordingly exposed the surface of the vitreous in a sheep's eye, by removing the sclerotic, choroid, and retina, about as far forward as the ora serrata, and placed it in a concentrated solution of acetate of lead. The surface became immediately covered with a white crust, and when, after some hours, he cut a small slice from the hinder region, he found the cut surface marked with fine milk-white lines, running parallel to the original surface, and presenting throughout the appearance of a finely striped agate. He soon satisfied himself that these stripes proceeded from milk-white layers traversing the vitreous substance in suchwise that the outermost was almost parallel to the retina or hyaloid, and the innermost to the back of the crystalline lens; the intervals being consequently greatest in the axis of the eye, and least towards the zone of Zinn (suspensory ligament). Here the outer layers were closely approximated, and terminated by uniting with that portion of the hyaloid which lies against the zone; but as respects the middle and inner layers, he was unable to satisfy himself how they ended. Proceeding onwards, he examined the texture of these layers. With the naked eye or an ordinary lens they appeared simply to consist of a milky, transparent membrane, but with a higher magnifying power a fine granular precipitate (probably chloride of lead) became visible in the position of the white lines, and in their intervals either a perfectly transparent space, or else a smaller quantity of a more delicate and similar granular deposit. Brücke further observed that the vitreous body thus prepared tore most easily in the direction of these layers; and he noticed that the transparent spaces between the white layers were occupied by an apparently gelatinous mass, similar in constitution to the rest, and were not free spaces containing fluid. He offered no explanation why the aqueous fluid of the vitreous escapes so readily on a puncture, which, he rightly remarks, never happens from a true jelly. In a subsequent communication* the same anatomist observes, that the frozen vitreous body, far from affording ground for the idea of a * Müller's Archiv, 1845, s. 130. cellular constitution, in reality accords with, and even favours that which he had previously advocated-viz. that the vitreous body is made up of concentric membranes, enclosed one with another. He states that if a well-frozen eye be brought into a warm room, so as to thaw the tunics investing the vitreous, and if these be then carefully removed, the frozen vitreous body appears as a coherent mass of ice, from the surface of which minute flakes may be detached with the point of a scalpel, as the external warmth gradually acts upon it. These flakes he has traced to near the lens, and as they seem to him to have the same direction as those exhibited by the aid of the metallic salt, he concludes that they are the result of the same membranous stratification which he had before demonstrated. Another distinguished physiologist has also devoted a paper to this interesting subject. He made his observations on the eyes of mammalia, after immersing them for at least six months in a solution of chromic acid. He describes somewhat more particularly the concentric layers which had been noticed by the previous authors, and states that if the eye be divided by a median transverse section the appearance is that of an onion similarly cut; and this he finds in the cat, dog, ox, and sheep. But in man he says that a different structure obtains, rudely comparable to that of an orange, there being segments, of which the convexities are turned outwards, while the angles converge towards, but do not reach, the axis of the eye, where the hyaloid canal exists in infants. In two specimens Hannover was able to count 180 rays, and he therefore concludes upon that number of segments: he was unable to ascertain whether each segment had its proper containing membrane, or whether a single membrane was common to two contiguous segments. Examined with the microscope, he finds the walls of the segments to present the aspect of transparent membrane without structure, covered with numberless granules, which he thinks are probably the result of a precipitation. He concludes with an account of the exact arrangement of the parts about the zone of Zinn, into which it is unnecessary here to follow him. As I am not aware that these researches have been at all prosecuted as yet by any anatomist in this country, and as it seems probable from my own investigations that much difference of opinion is likely to exist with regard to the true interpretation of the appearances * Hannover, Entdeckung des Baues des Glaskörper's, Müller's Archiv, 1845. THE VITREOUS BODY IN CHROMIC ACID. 97 which have been above briefly described, I am desirous to communicate in a simple form the result of my own observations on the vitreous humor of man and other mammalia, and of birds and fishes. In doing so, I am very sensible how much is still wanted to render them complete. Action of chromic acid on the human vitreous body. Having more than a year ago placed several human eyes, as fresh as possible (i. e. within twenty-four hours after death), and which had been removed from the orbits with special care, to avoid compression of the humors, in dilute solution of chromic acid (the strength indicated by a light straw colour), I soon found them to have become distended and tense, and the outer coat hardened, and I recently made sections of them in various directions, with a very sharp knife, taking care in making the sections that the globes were not squeezed or cut unevenly. The best marked examples are delineated in figs. 1 to 7.* * Sections of the human vitreous body, made after being immersed for upwards of a year in weak solution of chromic acid. Immersion commenced within twenty-four hours after death, and every care being taken to avoid injury. FIG. 1.-Horizontal section of the globe. The dark lines in the figure indicate the situation of the white lines in the preparation; in the centre is an irregular cavity. FIG. 2.-Vertical antero-posterior section of the globe; a, white lines running up to the hyaloid at the ora serrata. FIG. 3.-Vertical transverse section of the globe, anterior half seen from be H The vitreous body in all was rendered semi-opaque throughout, but the opacity was more obvious and decided in some directions. Most of the sections dividing the eye into an anterior and posterior half (figs. 3, 4) exhibited faint lines of greater opacity, parallel to the retina, running partly or altogether round the section. These lines were more opaque when the observer looked at them in certain directions, and with a little care it was easy to see that they were the edges of opaque lamellæ which followed the curvature of the retina. In no case did these circular lines extend more than about one-third of the way from the retina to the centre of the vitreous, and when they ceased they were replaced by others of a straight or slightly waved character, hind. Within the hyaloid, concentric light and dark lines are seen; within these, at a, are several dark dots, which are tubular spaces cut across, which dip inwards and approach the central irregular cavity in a curved course. The radiating lines, b, are dark in the preparation, and the substance is not interrupted where they occur, as far as I could ascertain. FIG. 4. Similar section from another eye; posterior half seen from the front. In this figure the shaded parts indicate the darker parts of the preparation; in the centre is an irregular cavity; from this or its vicinity, radiate many lines, most of which are tubular, as I proved by subsequently carrying a second section, á, through a a. The cause of what a geologist would call the unconformable arrangement of the lines at b, I cannot account for the knife evidently has not produced them. At c is a dark line, the cut edge of a layer of the vitreous which has not been whitened like the rest, and which is concentric with the retina; tracing this towards d we come to a number of tubes cut across, dipping inwards in a continuation of the same plane. : FIG. 5.-Vertical transverse section of the vitreous humor of a nine months' human fœtus, after immersion for a year in a dilute solution of chromic acid. The hyaloid canal is seen near the centre; and a very obvious radiation of somewhat curved lines from the wall of that canal: the texture was apt to tear in the direction of these lines. FIG. 6.-Horizontal section of the retina, vitreous humor, and lens of a nine months' fœtal eye, similarly treated. The section is carried through the optic nerve, spot of Soemmerring, hyaloid canal, and lens: the vitreous humor exhibited an uniform opacity in this section. The spot of Soemmerring is represented at a, where the retina is seen to recede from the vitreous body, and form a follicular pouch with a narrow neck, over which the hyaloid passes without entering; the hyaloid canal has been cut open in its hinder half, while the anterior portion remains tubular on the very surface of the preparation. No stem from the extremity of the optic nerve entering the canal can be distinguished in the actual specimen. FIG. 7.-Fragment of the vitreous substance of the same eye, seen under a power of 300 diameters. |