^{page 231 note *} Vol. x. Part i. p. 43.

^{page 233 note *} See the facts stated in my “Account of the Discovery of the Foramen Centrale of the Retina, in the eyes of certain Reptiles,” published in the Memoirs of the Wernerian Society, vol. 5. p. 1.

^{page 233 note †} I conclude that even when the retina has no longer a tunica vasculosa, as we find to be the case in birds, there still exist two layers, vascularity not being the essential character of either. Dissections by Haller seem to confirm this idea; his words are, “Lamina ergo hic in retina interior fibrosa est (in piscibus), et alia exterior pulposa.” But he allows that it is exceedingly difficult (he might have added impossible) to demonstrate this structure in birds; it must therefore be simply inferred by analogy.

^{page 233 note ‡} The Lacerta superciliosa, scutata, &c.

^{page 234 note *} As I am assured that vascularity is not the essential character of this membrane, I shall prefer calling it by the name of the inner membrane of the retina: the reasons for so doing will be given afterwards.

^{page 236 note *} These internal ciliary processes were called Fibres by Zinn, Lymphatic Vessels by Bertrandi, and Tendinous Fibres by Camper. They are composed chiefly of arteries and veins. Tenon well understood their anatomy, and describes their mode of union with the true ciliary processes, and the important fact of their receiving bloodvessels from the latter organs.

^{page 237 note *} The preparations illustrative of this and numerous other important points in the structure of the eye, amount to forty-seven. A far greater number was destroyed, in order to perfect the researches connected with this and the preceding papers. Of those preserved, thirty-six have been deposited in the Anatomical Museum of the Royal College of Surgeons of Edinburgh; and the remainder, a distinguished ocu-list of this place did me the honour of placing in his own collection. These preparations are intended to illustrate every important fact in the Comparative Anatomy and Physiology of the Eye.

^{page 238 note *} We shall return to this fact more particularly in a future part of the memoir. The descriptions usually given of the structure and formation of the Canal of Petit, by anatomical writers and lecturers, are frequently quite unintelligible.

^{page 239 note *} I have shewn in my former paper, that, in the eyes of certain animals, the inner membrane of the cornea may be traced over the anterior surface of the iris, whilst in others we merely infer its presence by analogy. In fishes, in which the first arrangement is most distinct, the aqueous humor is very small in quantity. I readily confess, that I have not been able satisfactorily to make up my mind as to the source of the aqueous humor; and the assertion, of its not being present in the anterior chamber of the aqueous humor in the fœtus, appears to me extremely doubtful.

In examining the eyes of the fœtus in the mammalia, with a view to the determination of the vascular structure of the pupillary membrane, several appearances have presented themselves to me, which I shall here briefly state, since they may be useful to those who may not have opportunities for making very minute vascular preparations of the eye. It has always appeared to me (and I have sacrificed a great number of very delicate preparations, in order to determine the anatomy of the pupillary membrane), that, by means of this membrane, the anterior and posterior chambers of the aqueous humor, form in the fœtus two distinct shut sacs, each enclosed in a proper capsule. With regard to the anterior of these, it is very evident from several preparations now lying before me, that it is formed by the inner membrane of the cornea, (or at least of a membrane covering the inner surface of the cornea); which is reflected over the whole anterior surface of the iris and pupillary foramen; but I cannot speak so decisively of the formation of the posterior sac, or that situated behind the iris, i. e. I am unable to say, whether it terminates at the equatorial margin of the lens, after investing, though loosely, the floating terminations of the ciliary processes, or whether it invests the anterior surface of the lens. It is very evidently connected with the membrane forming the canal of Petit. Betwixt the portion of membrane extending from the equatorial margin of the lens to the pupillary edge of the iris, (in the fœtus), and which being continued forward, constitutes one layer of the pupillary meipbrane, and the iris itself; there is a triangular space, very distinct in several preparations I have now before me. This space is occasioned by the membrane proceeding from the lens to the iris, in a straight line, like the string of a bow; whereas the iris is arched or concave. It is probable that the diagrammatic figure (Plate X.) may explain, much better than any words could do, the distribution of these membranes in the fœtus or very young animal.

The great share the hyaloid membrane of the vitreous humor has in the formation of these membranes, as well by its intimate connection with the capsule of the lens, as by its most evident and distinct connection with the posterior layer of the pupillary membrane, is very remarkable, when contrasted with the extreme delicacy of the vascular membrane of the retina, which scarcely contributes any thing towards the formation of this admirable and very singular structure. Viewing the interior of the eye-ball philosophically, we might say, that it is composed originally of a series, or suite, of colourless capsules, forming generally shut sacs, in which are deposited perfectly transparent substances, having various refractive powers. Posteriorly we have the hyaloid sac, and the contained vitreous body : 2dly, The crystalline capsule and lens: Sdty, The posterior chamber of the aqueous humour, formed in the fœtus by its own capsule : lastly, The anterior chamber, like the rest, a shut sac. The membranes forming these various capsules appear in the fœtus to be continuous.

In the fœtus, or very young animal, the quantity of fluid in either of the aqueous chambers of the eye, must be very trifling, if it actually exist; for the pupillary membrane is in contact with the anterior surface of the capsule of the lens. On the other hand, the crystalline humor in the fœtus is very large, and has the posterior segment of its capsule entirely covered with bloodvessels. The branch of the central artery of the retina which passes through the vitreous humor in a young animal, apparently quite disproportionate to the other branches of the arteria centralis, is for the purpose, undoubtedly, of furnishing the lens with the means of rapid growth. In the adult animal it is altogether obliterated, seemingly because no longer required, the lens having, at an early period of life, acquired its full growth, and being, like the enamel of the teeth, subject neither to decay nor renovation.

The vessels of the pupillary membrane come to it chiefly from those of the iris: it may receive a few from the terminating branches of the artery distributed to the capsule of the lens, but assuredly these must be very few. They were demonstrated by Dr W. Hunter, who also describes, very accurately, the distribution of the posterior layer of the pupillary membrane: “Where the membrana pupillæ exists, there is a fine vascular membrane all around, which passes in the posterior aqueous chamber, from near the edge of the lens to the edge of the pupilla.” Now, it is extremely easy to demonstrate, that the membrana pupillaris consists of two layers; the posterior of which is a continuation of the one just described by Dr W. Hunter. From the peculiar form of this membrane, (I mean the portion proceeding from the margin of the lens and hyaloid membranes to the pupillary margin of the iris), I should imagine it to be immediately ruptured and destroyed by the contraction of the iris, on the first admission of light through the pupil.

^{page 241 note *} Medical Commentaries, p. 63.

^{page 243 note *} It seems unnecessary to remark, that the secretion must ultimately be effected by means of vessels carrying colourless fluids, proceeding from those of a larger caliber, and distributed on the parietes of the canal of Petit. Vessels carrying colourless fluids no doubt penetrate into every part of the vitreous humor. Physiologists are as yet but little acquainted with the nature of the vessels carrying colourless fluids only.

^{page 244 note *} I am not sure if I rightly understand a passage in M. De Blainville's work entitled, “Principes de l'Anatomie Comparée.” He there states, that he has seen a bloodvessel entering the marsupium in the eye of the casuary; but several of the preparations exhibited to the Society, shew that sixteen or seventeen distinct vascular trunks may be counted in the marsupium of the common domestic fowl.

^{page 245 note *} It would seem that in some fishes the vitreous humor is supplied by vessels in both ways†, i. e. a branch of the central artery of the retina passes directly into the centre of the vitreous humor, whilst the remaining branches pass into the same, but more anteriorly through the medium of the marsupium. They anastomose about the centre of the vitreous humor. We owe this fact to Haller. It seems to me quite conclusive as to the real functions of the marsupium in fishes; yet this great anatomist calls the marsupium the sustentaculum of the leas, and says that in birds, it carries the blood to the crystalline humor.

^{page 245 note †} The coloration of the iris, in itself a curious subject, is rendered more so by several facts, which do not very readily admit of explanation; such as the iris being differently coloured in the two eyes of the same individual, an appearance I have seen not unfrequently. Quintus Curtius. relates, that the eyes of Alexander the Great presented this appearance. Tenon gives a very good engraving of an iris on which there may be seen the letter T very, distinctly.

^{page 245 note ‡} Element. Physiol. t. v. p. 391.

^{page 246 note *} I do not mean that there are no branches proceeding to the retina in birds, but merely that they are so small as entirely to escape notice, and no longer constitute a network on the inner surface of the retina, as in the mammalia.

^{page 247 note *} Marked h on the accompanying figure.

^{page 247 note †} I should think Mr Jacobson in error, with regard to the openings he supposed he discovered in the canal of Petit. The membrane passing betwixt the internal ciliary processes, or its outer paries, is excessively delicate, and may possibly have been ruptured. On this subject Haller expresses himself very positively, and with much accuracy and brevity: “Flatu enim immisso adparet circulum Petit solum inflari, neque aërem aut in lentem subire, aut in vitreum corpus, eum ergo circulum versus lentem capsula claudi. Deinde soda lends capsula facile inflatur, neque aër aut in circulum, aut in vitreum transit. Denique etiam vitreum corpus aërem recipit, qui neque in lentem transit, neque in Petit annulum.”

Haller's description of the canal of Petit is obviously incorrect; and Mr Cloquet, by copying this description, but superadding to it the well ascertained fact, that the layer of the hyaloid membrane covering the anterior aapect of the vitreous humor, that, viz. upon which the lens with its capsule reposes, does not Adhere to the capsule, has become thereby quite unintelligible. I here quote both passages.

“Lamina posterior vitreæ membranae discedit ad originem processuum ciliorium, et ad lentem introrsum etiam, sed paulo posterius advenit, recta protensa, et eam porro postquam attigit, tenaciter satis conjuncta, posterius includit.

“Inter has duas teneras laminas flatus potest immitti, qui circularem canalem, frenulis subinde adstrictum, efficit,” &c. But I have shewn, that if a delicate membrane be actually stretched over the posterior surface of the capsule of the lens neither this lamina, nor that placed immediately behind it, inclosing the vitreous humor, have any thing to do with the formation of the canal of Petit.

The passage alluded to in Mr Cloquet's work is as follows: “Au niveau des procés ciliaires, vers le contour du crystallin, cette membrane (hyaloide) se divise en deux Tames; Tune passe devant la capsule de ce corps, et l'autre tapisse la concavité qui le reçoit en arriere. Il result de leur ecartement un espace de la forme d'un prisme circurhairre à trois pans, complete par le circonference du crystallin. C'est cet espace vide qu'on appele Canal Godronne ou Goudronna, ou Canal de Petit.”

Haller says that the canal of Petit is present in all quadrupeds; an observation which agrees with the almost innumerable dissections I have made of that organ. He thinks it totally wanting in birds: I have proved that the only important part entering into its composition, viz. its vascular part, is wanting in these animals for this very obvious reason, that its place is supplied by the marsupium; but it seems to me that there still exists, as it were, a rudiment of the part, though by no means distinet.

I am still inclined to think, that a very delicate membrane is detached from the anterior termination of the canal of Petit, at the point where its parietes reunite, and incloses the whole of the posterior surface of the capsule of the lens, closely adhering to this capsule: it is very obvious that this membrane must be quite transparent, as it is in the immediate line of the pupil. It would be extremely interesting to know, whether it is this membrane, or the capsule itself of the lens, which, in certain diseased states of the organ, becomes vascular, thickened, and opaque.

^{page 249 note *} Haller saw portions of this membrane, which he describes as a sort of inorganic mucus. His words are: “Ut magnas maculas nigras sæpe retinæ tunicæ ad hærentes viderim, in homine, ave, quadrupede. Eæ maculœ in pisce in membranœ speciem confluunt totamque retinam tegunt.” It is very evident, however, from several passages in the Elementa, that, with many others, he confounded, in most instances, the membrane of Jacob with the pigmentum nigrum.

^{page 250 note *} Such was also the opinion of Haller: “In animalibus quadrupedíbus ab ea parte abest, in qua tapetum illud lucidum conspicitur,”

^{page 250 note †} In cuniculo albo pupillis rubris, niger iste mucus desideretur, eaque ipsa est ratio, quare per corneam membranam vasa retinae et choroideae percipias.”

^{page 250 note ‡} The hypothesis that the iris is composed entirely of bloodvessels, chiefly arteries, is founded on the supposed correctness of some preparations said to be in the possession of Prochaska: those I have made, with very delicate injections, negatively disprove the hypothesis.

The mode usually adopted to demonstrate the circular and radiating fibres of the iris merits particular notice. If we hold the iris of man betwixt us and the light, there may be seen, near to its pupillary margin, a portion of its texture of a definite breadth, much more opaque than any other portion of the moveable iris.

Now, this opaque circular stripe, which may be seen in the iris of man, of the quadrumana, and many other mammiferous animals, (very remarkably in the otter), and in almost all birds, is pompously called the Sphincter Muscle of the iris. But to demonstrate the radiating fibres, a little artifice is resorted to, violating at the same time, all the rules of philosophic anatomy. The iris of the ox, or of some other large quadruped, is selected, in which the parallel membranous striæ, so abundant on the choroidean portion of the iris, are remarkably striking, and these are called the Radiating Muscles of the Iris. These striae are not distinct in man, and in several other animals, in whom the iris is extremely irritable; its choroidean portion becoming proportionally delicate†.

^{page 251 note *} Walter says, that, in the human eye, the retina does not receive any arteries from the central artery of the retina: “Retinam nullas accipere arterias a centrale arteria.” He proves, that, by filling the ophthalmic artery with coloured injection, nearly the whole veins of the eye-ball are also filled; and it was by comparing such vascular preparations with those in which the veins only were injected, that he finally adopted the above opinion. Succeeding anatomists do not seem to me to agree with him relative to the distribution of the central artery of the retina.

I have made relative to these matters very considerable researches, but not on the human subject. I find that when the eye of a quadruped, as the ox, sheep, deer, &c. is removed from the head, and injected by the branch of the ophthalmic artery, proceeding to the eye-ball, the injection very readily fills, not only the arterial system, but also the whole of the vasa vorticosa, the vessels of the capsule of Petit, and a great portion of those supplying the iris; but to succeed perfectly, the injection must be thrown in with considerable force, and some of the vessels are generally ruptured. I have not succeeded so well as I could have wished in those preparations intended to exhibit the veins only. Hence I am not able to speak positively as to the relative development of the veins, compared with the arteries, in the capsule of Petit; it is not improbable, however, that they observe the same proportion as in the other parts of the eye. I have not remarked in any adult quadrupeds, whose eyes were prepared by me for dissection, any vessel, or branch of a vessel, passing through the centre of the vitreous humor; the whole of the arteria centrcdis retina being apparently distributed, in the first instance, to the inner membrane of the retina. In birds, as I have observed, it is quite the reverse.

It is in the eye of the cat that most of the facts may be best observed; for, in the eyes of oxen, sheep, &c. the vessels are very apt to give way, and the injected fluid becomes effused into the canal of Petit and betwixt the membranes. The principal vascular union betwixt the two sorts of ciliary processes is close to the edge of the lens.

^{page 251 note †} See Memoir read before the Royal Society of Edinburgh, entitled, “On the Comparative Anatomy of the Eye.” Page 43. et seq. of this volume.