| Anatomy |
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It was in Arabian literature that figures illustrating the anatomy of the eye first made their appearance. Arabic manuscripts still exists in which reference is made in the text to figures, themselves missing, though space from them is provided. The earliest drawing as yet available appears in Hunain ibn Is-hâq's Book of the Ten Treatises on the Eye, recently discovered and edited by Meyerhof (frontispiece).Through lack of illustrations it is difficult to get a clear conception of Greek and Roman knowledge of ocular anatomy, for the descriptions are frequently not only scant, but confused through a multitude of names, which may or may not have had the same meaning.
Pre-Hippocratic anatomy had hardly passed beyond the stage of recognizing a transparent cornea continuous with an opaque sclera, the whole being lined by a layer with a perforation which formed the pupil. These two layers enclosed a fluid substance. This conception of the anatomy of the eye was not based on detailed observation, but on speculation as to the nature of vision. The fluid in the eye was regarded as the principle of vision and a tube leading from the eye to the brain, allowing for the free movement of this visual substance, led Alcamaeon to postulate the pÓroz, poros. This postulated hollow tube is hardly the solid optic nerve of modern anatomy. An advance of these speculations is to be found with Aristotle, who obviously dissected animal eye. (Figure 1). Three layers instead of two are recognized, though knowledge of the retina hardly went beyond the recognition of its existence. Knowledge of the structure of the cavity of the eye was vague. There was no recognition of the anterior chamber; it was held that the three layers of the eye are intimately apposed to each other. The ocular fluid was considered as of uniform consistency, though some differentiation occurred on exposure to air; the lens, as far as it was clearly recognized, was thus regarded as a post-mortem manifestation. The hollow tube of Alcamaeon became three in number, one of which entered the skull and joined with a corresponding structure from the other eye. The recognition of the chiasma and of ocular vessels had therefore been achieved.
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Figure 1. The structure of eye as conceived by Hippocrates & Aristotle |
The Alexandrian school contributed largely to the knowledge of the anatomy of the eye. Herophilus in particular seems to have devoted much attention to the eye; from a reference in Aetius it is clear that he wrote a special treatise on the subject. As no manuscripts of this period have survived one has to rely on Celsus for information (Figure 2), and Celsus' account is by no means clear for the reason, as Hirschberg puts it, that he did not understand the subject. There is a clear recognition of the existence of the lens, a drop-like body named Krustalloidez, crystalloides. Whilst no anterior chamber is indicated -- the second layer is still contiguous with the first, except in the pupillary area, which is a mere perforation -- it is recognized that the retina does not come up to the cornea; it forms a smaller enclosing structure, and comes to surround the ocular fluid including the lens. This arrangement leaves a large empty space -- locus vacuus -- between the two outer layers and the smaller retina. As this locus vacuus is also spoken of as containing "humor", a near approach to the appreciation of the existence of the anterior chamber may have been made. What exactly Celsus knew of the optic nerve is not clear: he does not speak of any hollow canal, nor does he speak of a continuation of the retina into the nerve. The optic nerve probably appeared to him as a continuation of the fused two outer layers of the eye.
Figure 2. The eye as decribed by Celsus |
With Rufus a much clearer conception of ocular structure emerges. The conjunctiva is recognized, though of course not distinct from the capsule of Tenon, which indeed was not described till 1806. Under the name of epidermiz, epidermis, it is regarded as a fourth covering layer extending from the junction of the cornea and sclera to the posterior pole. The corneo-scleral junction stefauh, stephane, is regarded as also indicating the site where the retina branches off to line the posterior aspect (no longer the anterior) of the lens. The lens itself is invested with a lining layer, but whether this is a distinct layer or a decomposition product of the lens (?liquefied cortex) is not clear to Rufus. Of significance is Rufus' conception of the internal structure: as Magnus points out, this approaches the modern view. Two spaces are recognized, one lying between the cornea and iris, and another behind the lens. The first space, a mere chink, was filled with a fluid very much like water, whilst the second contains a substance like the white of a raw egg.Four serious defects mar the description by Rufus. He failed to recognize the existence of the posterior chamber, the greater curvature of the cornea as compared with the sclera, and the inequality in the curvature of the lens surfaces; and his reference to the optic nerve is most scanty. These defects were in a large measure rectified by Galen (Figure 3).
Figure 3. The eye as described by Galen |
Just how much the description given by Galen is the result of his own observations or that of predecessors is not known. But Galen's account is of significance not only because it marked an advance, but even more because no advance was on it till after Vesalius. If pre-Hippocratic anatomy was speculative, and Alexandrian anatomy truly descriptive, anatomy after Galen became a historical exercise on which commentators were busy for well over a thousand years.A fairly clear recognition of the ciliary body seems to have been arrived at. The corneo-scleral junction -- one name for which, incidentally, was iris, a designation that persisted till well into the 18th century -- was also the seat of fusion of the choroid and retina, where in addition a layer lining the anterior surface of the lens also terminated. The posterior chamber was clearly recognized, as was also the fact that it contains the same fluid as the anterior chamber. The greater curvature of the posterior surface of the lens was likewise recognize; the lens itself was held to fuse with the choroid by which it was kept in position.
It should be noted that whilst the recognition of the greater curvature of the cornea over the sclera was obviously the result of observation, the recognition of the existence of the posterior chamber was the result of speculation. Galen's writings are not clear on the subject, and as Magnus points out, he could not possibly find a space between the lens and iris in an eye cut open without the modern methods of preliminary fixation; but his theory of vision which postulated dilatation of the pupil by , pneuma, called for a posterior chamber through which the pneuma could diffuse on to the lens.
Speculation also entered into the description of the optic nerve. Whilst Galen recognized its solid structure he had to maintain a central hollow canal, in the sense of Alcmaeon. At the chiasma fusion of the hollow canals of both nerves took place. That Galen drew on animal dissection is clearly seen from his description of extraocular muscles, of which there are seven -- the six of present-day human anatomy with an additional massive ensheathing muscle which arises from where the optic nerve enters the orbit -- obviously the retractor bulbi of comparative anatomy. Furthermore, in describing the lacrimal apparatus he speaks of two glands, one in the upper and one in the lower lid. Galen recognized another source of tears - glands in the conjunctiva of the lids. The conjunctiva itself he held to be derived from the pericranium.
Arabian anatomy was the anatomy of Galen modified not by the evidence of dissection but by conclusions drawn from speculation. Depression of cataract extensively practised; and as the prevailing view was that a corrupted humour in front of the lens was displaced in the process, it was necessary to conceive the lens as being situated further back than in Galen's scheme. This view as to the seat of the lens persisted till the beginning of the 17th century.
With the coming of Vesalius, anatomy turned once more from speculation and commentaries to dispassionate observation. But to ocular anatomy Vesalius contributed nothing (Figure 4). His teaching is distinctly inferior to that of Galen and even of Arabian ophthalmology. The recognition of the greater curvature of the cornea over the sclera, and of the posterior surface of the lens over the anterior, is lost. A central position of the lens is once more in evidence. Even more astounding is Vesalius' acceptance of Galen's retractor bulbi.
Figure 4. Illustration from Vesalius (1514-1565) |
Modern anatomy of the eye did not emerge till the physicists had demolished the old conceptions of the nature of vision. It began when it was realized that the lens is not the seat of vision, but part of a refractive system. With Fabricius as a precursor in showing the true position of the lens (A.D. 1600), a host of observers rapidly built up the basis of the anatomical scheme as we know it today. Fallopius rediscovered the greater curvature of the cornea and stressed the difference in structure as between the cornea and sclera. A clearer view of the capsule of the lens and a description of the hyaloid membrane likewise came from him. He differed from Vesalius in regarding the ciliary body as a membrane, and held it to be a ligament binding the lens to the choroid. Incidentally, he also disproved the existence of the retractor bulbi in man. Ruysch, who studies the vascular structure of the choroid, is also responsible for showing the existence of circular muscle fibres in the iris. Briggs, who is remembered for his demonstration of the existence of the optic papilla (regarded by him as a projection, as its name implies), showed that the retina extended up to the ciliary "ligament." What the 16th century began falteringly was well done in the 17th. A comparison of two reproduction showing the state of anatomical knowledge towards the beginning and the end of the 17th century is of interest (Figure 5 and 6).
Figure 5. Illustration from Scheiner (1575-1650)
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The finer methods of anatomical study were first used in that century; Ruysch employed injected preparations for the study of the vascular system of the eye; Malpighi used the hand-lens and Leeuwenhoek made the first observations with the microscope; but it was left for the succeeding century to introduce the study of the frozen eye, an innovation due to Petit. The combination of these methods led to the rise of a detailed anatomy, for the bold outlines were by now firmly established. Petit was the first to attempt measurements of the components of the eye. Priority in the description of Descemet's membrane was the subject of a word dispute between Demours and Descemet, but its first indication is really to be found in Duddell.In studying the constitution of the lens, Morgagni found fluid between the capsule and the lens fibres. This fluid was held to nourish the lens - a mistaken notion but one which, at any rate, was an advance on the belief that the lens and cornea contained vasa serosa, which possessed the property of impermeability to red blood cells. To the anatomy of this period belongs the description of the spaces of Fontana, as also the discovery by Demours of the canal of Petit, so named by him, the Zonula of Zinn commemorates the name of an observer who also contributed studies on the blood-vessels around the entry of the optic nerve (circulus arteriosus of Zinn) and on the action of the ciliary body.
The presence of muscle fibres in the ciliary body was a matter of much discussion; some held with Morgagni that they existed and affected accommodation, others with Zinn, that they were non-existent. Similarly contraction and dilatation of the pupil were explained on the conflicting view that different degrees of congestion of the vessels of the iris produced changes in the size of the pupil.
It is noteworthy that even at this late stage some gross points were still unsettled. Though Petit in 1728 had clearly demonstrated the posterior chamber, its existence was being questioned down to 1855 and it was not until the work of Helmholtz, Henle and Arlt that this question was finally settled.
Whilst by the end of the 18th century the uveal tract had been fairly well described, the retina was barely recognized, for the day of cellular anatomy had not yet come. At the turn of the century Buzzi, Sömmering and Reil described the macula lutea. The additions to our knowledge of the anatomy of the eye during the 19th century are largely the history of the consequences of the introduction of the compound microscope and the rise of the cellular theory.
The advances recorded during the earlier part of the 19th century, before the introduction of the microscope, are typified by the description of Jacob's membrane. Jacob described a serious layer in the eye, lying between the retina and the choroid; this ultimately came to be regarded as a constituent part of the retina, which was held to consist of three layers, a limiting layer, a nervous layer -- the retina proper -- and Jacob's membrane. Jacob's membrane is indeed nothing else than the rods and cones of modern histology. To this period belongs also the discovery of the canal of Schlemm.
The compound microscope opened a new realm of observation, and the realization of the significance of the new facts which were rapidly gathered, culminated in Schwann's theory that all living matter consists of cells. As early as 1722 Leeuwenhoek had noted the rods and cones of the retina, but their existence had to be rediscovered in 1834 by Treviranus. And just as the retina was gradually being recognized, so other tissues were studies by the new microscopic methods. In a few brilliant years of intense work