Paper - The relations of the hyaloid canal in the foetus and in the adult
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Mann IC. The relations of the hyaloid canal in the foetus and in the adult (1928) J Anat. 62(3): 290-296. PubMed 18168748
Mann IC. The developing third nerve nucleus in human embryos (1927) J Anat. 61(4): 424-438. PubMed 17104156
Mann IC. The process of differentiation of the retinal layers in vertebrates (1928) Br J Ophthalmol. 12(9): 449-478. PubMed 18168748
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The Relations of the Hyaloid Canal in the Foetus and in the Adult
By Ida C. Mann, F.R.C.S.
From the Department of Anatomy, St Mary’s Hospital Medical School (1928)
- Under a grant from The Medical Research Council.
- Running forwards through the vitreous body from the entrance of the optic nerve to the posterior surface of the lens, is the hyaloid canal, filled with lymph and lined by a prolongation of the hyaloid membrane. (Gray’s Anatomy, 23rd ed., 1926, p. 1024.)
A statement to this effect (with an accompanying diagram) finds a place in all works on the anatomy of the eye, despite the fact that its verification is difficult and that few attempts are ever made to demonstrate its truth. The existence of the Canal of Cloquet in man has never been seriously questioned, although it cannot normally be seen either in microscopic sections of the adult eye or in the living subject with the ophthalmoscope. The older anatomists demonstrated it occasionally by means of dyes allowed to ﬂow along it, but the main argument for its existence in the adult is the ease with which it can be seen in the foetus. On its development and its early appearance has been built up the standard description of its final condition and relations. The difficulty of demonstrating it in the adult eye is due to two factors. firstly, the vitreous is partially destroyed and so altered in appearance by fixation that reliable microscopic sections of it are very hard to obtain. Secondly, the freshly excised eye is difficult to dissect, and the hyaloid membrane is so injured by even the gentlest of manipulations, that gross observation usually fails to reveal anything conclusive. It was, however, pointed out in a previous paper in this Journal that a new and promising method of examination of the anterior parts of the eye was now available, namely bio-microscopy with the Gullstrand slit-lamp. With this instrument it is possible to examine the surface of the hyaloid membrane lining the fossa patellaris during life. When this was first done it was noticed that no funnel-shaped opening passing back into the vitreous could be seen, and the possibility of the canal of Cloquet normally disappearing completely after birth had to be considered. That this is not the case, however, can be shown by further investigation by the same method, and the present paper is an attempt to demonstrate by correlation of bio-microscopical and embryological findings what may be considered the normal relations of the canal from foetal to adult life. The first embryonic stage which concerns us is that of 48 mm. This is the time, in human embryos, at which the foetal intra-ocular blood system is at its maximum. Every subsequent stage reveals a retrogression of some portion of it. fig. 1 shows the vitreous body and lens of such a stage (48 mm.). The posterior surface of the lens and the terminal branches of the hyaloid artery are enclosed by a circle, the diameter of which represents 1 mm. It is apparent that the artery divides before it reaches the lens and that its branches spread out and form a sort of vascular cone or funnel applied to the lens. The diameter of the mouth of this funnel is at this stage only a little less than that of the lens itself and is roughly 1 mm. The vitreous filling the funnel and surrounding the hyaloid artery is more open in texture than that outside it, and will be referred to as the primary vitreous. The rest of the vitreous body is referred to as the secondary vitreous. It is closer in texture and stains more deeply than the primary vitreous. The primary vitreous containing the vessels does not lie, even at this stage, in the centre of the eye but rather towards the nasal side. Horizontal sections of left eyes are shown in the first three drawings and in each it is apparent that the amount of secondary vitreous is larger on the temporal side of the hyaloid canal than on the nasal. It is to be noticed in fig. 1 that the secondary vitreous only comes into direct contact with the lens at the equator, the rest of the posterior lens surface abutting on primary vitreous.
Figs. 1, 2 and 3. Three stages in the development of the vitreous and Cloquet’s canal, seen at the same magnification. The lens is shown in outline in the upper part of each figure. The secondary vitreous is dotted.
From the 4-8 mm. stage onwards the increase in size of the vitreous body is brought about by increase of the secondary vitreous only. At 65 mm. (fig. 2) the branches of the hyaloid artery still arise from the main trunk (which has itself increased in length) a little less than a millimetre behind the lens, and form a vascular cone of a similar size to that seen in the previous stage, although the size of the vitreous as a whole has increased considerably. This is apparent on comparing the portions within the small circles in figs. 1 and 2, the magnification of which is the same. The displacement of the canal towards the nasal side of the eye is even more marked than in the previous stage. There is a condensation, which was foreshadowed at 48 mm., on the surface of the secondary vitreous, separating it from the primary vitreous. This condensation is the so-called hyaloid membrane, and is continuous with the wall of Cloquet’s canal. It is to be noted that the hyaloid membrane is adherent to the lens capsule just behind the equator only. The wall of Cloquet’s canal is separated from the lens capsule by the primary vitreous which fills the fossa patellaris.
After the 65 mm. stage the most marked change in the appearance of the vitreous is due to the atrophy of the intra-ocular vascular system. The secondary vitreous increases enormously in size and is quite avascular. The main trunk of the hyaloid artery increases in length but becomes thinner. The cone of branches on the back of the lens remains of the same size (roughly 1 mm. in diameter at its base) as in previous stages but the branches are fewer and become, by the seventh month, extremely atrophied. The original area of the posterior capsule in contact with the base of the Vascular cone can still be recognised as a small circular region (situated slightly below and to the nasal side of the exact posterior pole of the lens) which sometimes presents a slightly stippled appearance and sometimes appears to be enclosed by a narrow line. The area is formed by the cone of vessels, not by the mouth of Cloquet’s canal, which is simply continuous with the hyaloid membrane and is not adherent to the lens capsule. The hyaloid membrane itself is adherent to the lens capsule, slightly behind the equator only, as it was in previous stages. fig. 3 shows diagrammatically the relations of the vessels, the wall of Cloquet’s canal, and the hyaloid membrane, to the lens capsule of a seven months human foetus. The magnification and the size of the small circle enclosing the bifurcating artery are the same as in figs. 1 and 2. The similarity of the region enclosed by the circle in all three diagrams is obvious. It will be seen that, apart from the existence of actual vessels at this stage, the condition is similar to that figured for the adult in most text-books.
fig. 4. Slit-lamp appearance of Cloquet’s canal in infancy.
fig. 5. Slit-lamp appearance of Cloquet’s canal in a child.
fig. 6. Slit-lamp appearance of the retro-lental space and the hyaloid remnant in a normal adult.
The subsequent changes which occur are the result of continued atrophy of the vessels. During the eighth month the main trunk of the hyaloid artery becomes impervious to blood in its central portion, and here gradually atrophies and disappears. It is well known that atrophying vessels in the vitreous (e.g. the vasa hyaloidea propria (Versari)) as they lose connection with their trunk of supply, tend to curl up and assume corkscrew forms. This is no doubt due to the presence of a certain amount of elastic tissue in their walls. The main trunk of the hyaloid is no exception to this rule. It begins to curl during
fig. 7. A. The relations of Cloquet’s canal at birth.
fig. 7. B. The relations of Cloquet’s canal in the adult with the eye at rest.
fig. 7, C. Momentary relations of Cloquet’s canal in the adult eye after sudden movement upwards.
the latter part of foetal life, and at birth looks like a fine wavy white thread extending straight backwards from just below and to the nasal side of the posterior pole of the lens. The walls of Cloquet’s canal, now at some distance from the atrophic vascular remnant, extend horizontally backwards towards the disc. These appearances can be seen with the slit-lamp both in fresh eyes of stillborn foetuses and, more easily, in the living eyes of children of variable ages up to four years and often older. Fig. 4 shows the slit-lamp aspect. The faint curved line enclosing a faintly stippled area within which the hyaloid artery breaks up is known to clinical workers as the “arc line” (Bogenlinie (Vog't)). It can be measured in the living eye at all ages and always has a diameter of roughly 1 mm. Thus no actual growth can occur in the terminal branches of the hyaloid artery after the 48 mm. stage.
Usually at or before the age of four the horizontal hyaloid remnant of infancy begins to sag under the inﬂuence of gravity and with it the walls of Cloquet’s canal, which become extremely lax. fig .5 shows the appearance in the eye of a child of twelve in whom the process of sagging was not yet complete, the hyaloid remnant making an angle of about 45° with the posterior surface of the lens. The coincident sagging of the wall of Cloquet’s canal can be seen. The upper wall now passes downwards and backwards practically parallel with the direction of the hyaloid remnant. The lower wall can just be seen. It has almost dropped below the lower border of the pupil. The hyaloid remnant has become more curly.
This process of sagging continues and is usually complete before puberty. fig. 6 shows the normal condition in the adult as seen with the slit-lamp. The hyaloid remnant hangs down like a corkscrew behind the lens. The walls of Cloquet’s canal have sagged so much that the opening cannot be seen through the pupil. The hyaloid membrane (front face of the (secondary) vitreous) can, however, be seen to slope downwards and backwards behind the lens, and would, if it could be followed, lead to the open mouth of the canal. This can be made to ﬂoat up into view if the subject be asked to look downwards and then quickly upwards again. If the coincident movement of the vitreous be watched with the slit-lamp the canal will be seen to ﬂoat up and to assume for a second or so its foetal position. As it settles down again it passes through the various developmental positions described, and its appearance at successive seconds as it falls is similar to that seen in figs, 4, 5 and 6. The space between the lens and the hyaloid membrane seen in fig. 6 is known to slit-lamp workers as the “retro-lental space.” It is obviously the fossa patellaris or hyaloid fossa of anatomists and from an embryological point of view contains primary vitreous continuous with that surrounding the vessels in Cloquet’s canal.
These appearances can be seen, by anyone conversant with the use of the slit-lamp, in the great majority of normal adult eyes. Individual differences in the length of the hyaloid remnant, the shape of the arc line and the density of the hyaloid membrane are extremely common, but the presence of the backward slope of the vitreous face and some indication of the position of the hyaloid remnant are practically invariable, even in advanced life. One can therefore conclude that the minimal persistence of the central end of the hyaloid artery and the displacement of Cloquet’s canal into the lower part of the eye are normal. fig. 7 shows diagrammatically the relations of Cloquet’s canal at birth and in the adult.
In conclusion we may consider that the presence of Cloquet’s canal as an anatomical structure throughout life is vindicated by the most certain of methods, namely, intra-vitam microscopy. The usual median horizontal position assigned to it in text-books is inaccurate if the body be supposed to be in the anatomical attitude. Further its situation may vary on movement or under the inﬂuence of gravity. Another anatomical fact, namely, that minimal persistence of the lental end of the hyaloid artery is normal in man, has been brought to light by the same method.
- The figures, which are drawn to scale, are diagrammatic and give no indication of the appearance of the tissue, which is actually fibrillar, not granular.
- Druault's nomenclature for the regions of the developing vitreous is made use of for convenienee. The question of the accuracy of his theories regarding it is not under discussion.
- In studying slit-lamp diagrams it is to be remembered that the appearance obtained with the instrument is that of a fairly thick section, seen stereoscopically, of the transparent parts of the eye. The lens appears greyish, the primary vitreous almost black, the secondary vitreous very dark grey and fibrillar, and the vascular remnants ‘and the hyaloid membranes as white threads and wrinkled diaphanous sheets. It is impossible to represent the exact appearances in a black and white drawing.
MANN, IDA 0. “Notes on the Anatomy of the living Eye as revealed by the Gullstrand Slit-lamp.” Journ. of Anat. vol. Lrx, pt. 2, Jan. 1925.
VERSARI, R. “Le fasi d.i sviluppo e di regresso della tunica vasculosa lentis e la morfogenesi dei vasi sanguiferi nei processi ciliari e nell’ iride dell’ uomo.” Richerche di Morfologia, vol. III, Fasc. 2 e 3, Roma, 1923.
Voot, A. Slit-lamp Atlas of the Living Eye. Berlin, 1921.
Cite this page: Hill, M.A. (2021, April 20) Embryology Paper - The relations of the hyaloid canal in the foetus and in the adult. Retrieved from https://embryology.med.unsw.edu.au/embryology/index.php/Paper_-_The_relations_of_the_hyaloid_canal_in_the_foetus_and_in_the_adult
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