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shrink and be removed, as soon as that object is attained. None of it exists at birth. The arteries of the hyaloid, the central artery of the vitreous humour, and those of the two halves of the .vascular sac, all disappear. The portion of the vascular sac called the capsulo-pupillary membrane disappears before the pupillary membrane; according to Cloquet, the arches formed by the irido-pupillary vessels retreat into the iris to form its little arterial circle; by the end of the eighth month, in general, the whole process of removal is complete, and the dioptric media of the eye left with scarce a shred to disturb the passage of light on to the retina of the child at birth. The temporary structures have been swept away, and the nutrition of the vitreous body and the crystalline is left to those sources which are to supply it through the rest of life. If we wish to see the vessels of the vitreous and crystalline humours, we must inject from the aorta a six or seven months' foetus, and in dissecting the eye, we must bear in mind that sometimes the posterior half of the vascular sac becomes detached from the anterior, the latter being left adhering to the iris, filling the pupil, and answering the common description of the pupillary membrane; while in other cases the lens comes away entirely covered with vessels, the vascular sac having separated in an entire state, and no pupillary membrane, such as is commonly described, being found.

Retina.—Mr. Nunneley occupies thirty-five pages with the retina, which he justly describes as "one of the most complex, perhaps the most complex, structure to be found in the body." (p. 197.) These thirty-five pages are the most laboured of Mr. Nunneley's book, and exhibit on the whole an interesting account of the anatomy of the retina, although it would appear that in attempting to follow up the discoveries which have been made in the structure of some of its parts, he has often been disappointed. -The chief cause of this want of success is unquestionably the extreme dHficulty of the investigation, owing to the minuteness of the organizations and the rapidity with which they become changed after death. Another cause we suspect to be that our author's microscopical examinations of the retina have been made chiefly with artificial light. "The work," he says in his preface, "has been written amidst the constant interruption and distraction of private practice, which has occupied the whole of the day, leaving only the night for scientific investigation." (p. v.) Now, we believe it to be much more difficult to make out the elements of such extremely minute textures as are combined in the retina, and their relations to one another, with artificial than with good daylight. Leisure, too, and the uninterrupted attention which it permits, as well as some share of that mechanical address in the preparing and disposing of objects for the microscope, which practice alone can give, are necessary lor the successful examination of such structur«s as the retina. "Whoever," says Schleiden, "would observe successfully, must observe much, and with strenuous attention, by which he will soon learn that seeing is a difficult art."

The remarks by Mr. Nunneley in his note, p. 199, on the effects of different reagents on the different textures of the retina, are well worthy of the attention of those commencing the microscopical study of this part of the eye. "I believe," says he, "the only way to examine the retina unchanged is to do so immediately after death, and without the addition of any substance whatever.s's'

We shall add a few other hints, which perhaps may prove useful to beginners.

1. Having tied down the eye to a disk of lead about l£ inch in diameter and 1 inch thick, by means of a thread passed through the cornea and through a diametrical canal in the lead, a considerable portion of the sclerotic and choroid is to be dissected away, without removing if possible the pigment membrane, and a thin piece of vitreous humour is to be cut out with the corresponding piece of retina and pigment membrane. The piece is to be placed, pigment uppermost, on a glass slide, and examined. At some point the pigment membrane will probably give way, and become slightly detached, so that the ends of the rods will be seen like a very minute tessellated pavement. All that can be seen in this way is to be examined without covering the object with a thin glass.

2. The same object, or another prepared in the same way, is to be covered with

a thin glass, and examined. As the examination is continued, more of the pigment membrane will probably become detached, so that a more complete view of the stratum bacillosum will be obtained. Some of the rods will be seen overthrown by the pressure of the glass, some perhaps detached altogether. A little pressure over the thin glass with the point of a needle will probably promote these changes.

3. To see the transition of the stratum bacillosum into Jacob's membrane, a drop of water is to be allowed to fall upon the object before covering it with the thin glass, and the observer must continue to watch some time for the change, adding a little more water when necessary, by putting a drop at the edge of the glass, and letting it run under it. Sometimes the transition is well seen without covering the object at all.

4. By pressure and watching, the different layers will come to be seen. The stratum bacillosum in a short time readily becomes detached, and we see the stratum underneath. Then by tearing the piece of retina, and regulating the focus of the microscope, the structures may be discerned in their stratification.

5. By folding a piece of retina and examining the edge of the fold, we get sight of something like a vertical section.

6. After having thus studied a piece of the retina from without inwards, we must next take a piece and examine it from within outwards.

7. Watching, pressure in various degrees, and a little tearing, are the simplest means by which one can get a notion of the structure of the retina.

8. In dissecting the retma, the simple microscope is of very great use in preparing the object to be examined under the compound microscope.

Our author describes the retina as consisting of the following layers, enumerating them from the convex to the concave surface—viz., 1, columnar or bacillar layer, rods, or Jacob's membrane; 2, bulbous or conoidal bodies; 3, granular layers; 4, nucleated vesicular layer; 5, vascular layer; 6, fibrous layer; 7, hyaloid cellular layer. Those who are in the slightest degree acquainted with the anatomy of the retina will see that this enumeration of its structures is very likely to mislead. So also is Mr. Nunneley's vertical section (plate 7, fig. 1), where he represents a vascular layer as placed exteriorly to the fibrous or nervous layer, whereas the vessels do not properly constitute a layer at all, but occupy several of the layers while they chiefly lie close to the concave surface of the retina.

Mr. Nunneley acknowledges, as he proceeds, that the cones or bulbs are placed amongst the rods, so that they are hardly to be considered as a distinct layer, He notices the discrepancies in the descriptions of the cones or bulbs by different microscopists, such as Hannover, Bowman, and Kolliker; and adds that he has searched most carefully for these bodies in the eyes of many animals, but cannot say he has satisfied himself of the existence of any bodies, such as have been described, in the perfectly fresh eyes of any creature except fish.

"There is no difficulty whatever," says he, "when regarding the undisturbed external surface of the retina of either reptiles or mammalia, of recognising the forms figured by Hannover and Bowman, which they consider the cones or bulbs not in focus; bnt I have always failed in detecting the nppearances represented by them in profile, and I am more inclined to think the bodies seen at a deeper level, and out of focus, when the outer ends of the rods are in focus, not as cones or bulbs, but as the outer portions of the granular layer to be presently described, and upon which the ends of the rods rest." (p. 208.)

Under the title "Granular layers," Mr. Nunneley comprises the external granular layer, the intermediate striated layer, and the internal granular layer. The intermediate striated layer is by some microscopists, such as Vintschgau, not noticed at all; but generally, as by Kolliker and Blessig, it is represented as formed by the radial fibres of H. Mflller, running perpendicularly from the rods to the membrana limitans, and intimately connected with the two granular layers. The difficulty of making out the actual arrangement is evidenced by Mr. Nunneley's statement.

"If a section," says lie, "of the dried retina of man, the sheep, or the ox, he examined with water or dilute spirit, there is no difficulty in perceiving this irregular line of very minute granular matter and indistinct fibres, with flattened globules, arranged horizontally in the length of the retina, as described by Bowman; and not vertically in the direction of the thickness, as figured by Kolliker; but it is by no means so easy to detect this separation into two layers in the perfectly fresh retina. I have sometimes fancied it was to be seen in the bullock and sheep, bnt I have so often not been able to find it, that, knowing how greatly every portion of the retina is changed by all fluids, and how little dependence is to be placed in appearances there found, I feel doubtful if there really be two layers." (p. 218.)

Similar difficulties await Mr. Nunneley when he proceeds to examine the nucleated vesicular layer.

"It is very difficult," he says, " to see these brain-cells in situ in the fresh eye, but floating about they ure readily seen; and in a successful examination they may be seen forming a layer in which the fibres of the optic nerve are imbedded and expanded." (p. 214.)

As for the caudate ganglionic cells, detected by Bowman and others, in this layer—

"I have searched most carefully," says onr author, "over and over again, for these long caudate cells in the eyes of man, many mammalia, various birds, reptiles, and fish, and particularly in the almost living eye of the turtle, and must confess, like Hannover, having failed to find them in the perfectly recent eyes. When reagents are employed, when the retina has been dried and moistened with water, or the retina examined is not from an animal just dead, not the same difficulty exists; large, irregular, more or less cuudiform cells are then abundant enough. I am therefore, unwilling as I am not to see what such competent observers speak unhesitatingly of, constrained to doubt if cells such as figured and described, with many long caudate processes, continuous with the nerve-fibres, do really exist in the living eye." (p. 214.)

Mr. Nunneley's fibrous layer consists of the expansion of the optic nerve.

"The nerve-fibres," says he, "expand in every direction, forming a complete layer upon the outer surface of the hyaloid membrane." (p. 216.)

"The fibres," he adds, "lie to the inner side of the granular layer, which separates them from the rods, and they are, as it were, imbedded in the vesicular layer, which, withoutcare, always renders the fibres indistinct." (Ib.)

The hyaloid cells which are found forming a layer between the retina and the vitreous humour, Mr. Nunneley inclines to consider as more intimately connected with the retina than with the hyaloid membrane.

Although the author enumerates a vascular layer, he tells us that

"At first the larger branches [of the central artery] are on the inner side of the fibrous layer, but as they pass forwards they gradually jwnetrate this, some of the branches running parallel with the nerve fibres for a considerable distance; but the smaller branches and capillaries, which form beautiful loops with each other, appear to be exclusively distributed in the vesicular .and granular layers, on the outer surface of the fibrous; none, so far as can be detected, passing into the bacillar layer." (p. 219.)

The very thin membrana limitans, described as structureless, hyaline, and elastic, which bounds the retina on its concave surface, is not acknowledged by our author.

As to the foramen centrale of Soemmerring (which it might be better to call a macula, as it is certainly not a hole), the author propounds a theory which we consider as altogether unfounded:

"On the whole," says he, "I incline to regard this much-debated spot as a vestigiform remains of the spot where a large bloodvessel has passed through the retina in the development of the eye, and carried with it some of the choroidal colouring matter." (p. 227.)

No such bloodvessel has been seen in the eye of the foetus. Perhaps, bethinking himself of this objection, Mr. Nunneley hazards another conjecture as to the central spot, but no sooner does he venture to express it than he adds its refutation. "It may be," says he, "an undeveloped analogue of the marsupium in birds, but if so, why it should not be generally distributed it would be difficult to ?ay.» (p. 227.)

It is generally acknowledged that the central spot and its yellow border exist in the eyes of the quadrumana; they were found in the eyes of this class of animals by Home, Blumenbach, Cuvier, and D. W. Soemmerring; and Mr. Bowman describes* their appearance in one specimen which he minutely examined; but Mr. Nunneley is unsuccessful in his researches on this point, as on many others: "I have dissected the eyes of two species of baboons and three monkeys without finding it." (p. 220.)

Mr. Nunneley makes so slight reference to the fibres, first distinctly described by H. Mtlller, which pass in the direction of the radius of the eye, through the layers of the retina, between the rods and the membrana limitans, and which constitute what is termed the radial system of fibres, in contradistinction to the horizontal system formed by the fibres of the optic nerve, that we presume they have not come under his observation. Of the nature of the radial fibres, the most opposite views have been adopted; for while Kolliker speaks of the rods, cones, and radial fibres as true nervous elements directly engaged in the functions of visual sensation, Remak regards the radial fibres as a structure merely of the connective kind, for binding the different elements of t he retina together. H. Mtlller did not at first incline to attribute a nervous function to the radial fibres, although afterwards he adopted Kolliker's views on this point. Hannover excludes the layer of cones and rods from the nervous tissues of the retina. Brttcko regards it as a catoptric reflecting apparatus. Almost as much diversity, in fact, exists in the views entertained by microscopical physiologists, as to the functions of the tissues forming the retina, as is to be found in the various anatomical descriptions of these tissues.'

Eye of the Cuttle-fish.—Passing on to Chapter V., which treats of the Comparative Anatomy of the Eye, we find Mr. Nunneley, at page 278, introducing a woodcut of the eye of the cuttle-fish {Sepia officinalis), taken from John Hunter's figuref (as usual, without acknowledgment), and in explaining it, mooting the old difficulty first started by Cuvier, how the animal can see, con-dering that between the retina and the vitreous humour a "dark choroid coat is interposed, which being placed anterior to the nervous expansion, thus cuts off all the rays of light." To remove this difficulty, Mr. Nunneley quotes the statement, which is perfectly correct so far as it goes, of Prof. Owen:

"That there is a nervous layer anterior to the choroid, in the same situation the retina occupies in quadrupeds, and that although it may be difficult to discover the connexion between the anterior aud posterior layers, yet he has no doubt the anterior is composed of the fibres of the optic norve, and that it constitutes a true pre-pigmental retina." (p. 279.)

The connexion which Professor Owen had not "discovered, had, previously to the Professor's article Cephalopoda,' in the 'Cyclopaedia of Anatomy and Physiology,' been made known by Mr. Wharton Jones, and illustrated by an engraving in the 'Philosophical Magazine' for January, 1836. The reader will there find the structure of the retina, both of the sepia and the octopus, fully explained. The optic fibrils, which penetrate singly the thin cartilaginous lamina, which corresponds to a sclerotic, expand into a layer of a light reddish-brown tinge, constituting the first layer of the retina. The second layer of the retina is also of a reddish-brown colour. Betwixt the two layers there intervenes a pretty thick and dark layer of pigment, through apertures in which the nervous substance penetrates from the first layer to the second. Examined with the microscope, the second layer is observed to be composed of short fibres perpendicular to its surfaces; and these fibres, towards the inner surface, end in a delicate, pulpy, nervous substance, also tinged of a reddish-brown colour, and presenting a corrugated or papillary appearance.

This view of the matter has been followed up by H. Mttller, who describes the

* Lectures on the Parte concerned in the Operations on the Eye, p. 91. London, 1849.

t Descriptive and Illustrated Catalogue of the Physiological Series, &c., vol. ill. plate 42, fig. 2. London, ISM.

innermost layer of the retina in the cephalopoda, as consisting of elongated, slender, transparent cylinders, similar in many respects to the rods of the vertebrata, and like them densely crowded together, and disposed in the direction of the radius of the eye. The layer of pigment he describes as penetrated by fusiform filamentary prolongations of each cylinder, by which means a connexion is effected with the outer layers of the retina, the outermost of which is the horizontal expansion of the fibres of the optic nerve. He regards the arrangement of the elements, then, as one pretty nearly the opposite of that which exists in the vertebrata.

The conclusion which H. Mtiller draws from these facts is, that it is hardly conceivable that the light should act directly upon the fibres of the optic nerve, which lie behind the pigment, but that the perception must, in the first instance, proceed from the radially-disposed elements which alone are opposed to the light.

Choroid Gland.—Figure 127, page 282, is said to show the posterior part of the sclerotic, cut open and reflected, to expose the choroid gland surrounding the optic nerve. It will be impossible for any one to form the slightest conception of the form of the choroid gland from this woodcut. Mr. Nunneley states that the so-called gland is situated between the layers of the choroid, is peculiar to osseous fishes, of a red colour, of a horse-shoe shape, and made up of convoluted bloodvessels. He describes the choroid of fishes as double—the outer layer of a shining metallic lustre, and the inner dense and black.

Usually, the choroid in fishes is described as dividing itself into four layers ; the outer, or lamina argentea, such as Mr. Nunneley describes it; a second, the tunica vasculosa Halleri, in which is the so-called gland; a third, the proper choroid, or tunica Ruyschiana; and a fourth, a pigment layer. The silvery membrane which lines the sclerotic, does not appear to us to belong to the choroid. When we remove it, along with the sclerotic, to which it adheres, we expose a thin fascia closely adherent to the outer surface of the choroid gland and vascular choroid. Removing the vascular choroid and gland, we expose the dense pigment-layer, differing from the membrane of the pigment of mammals only in being more consistent, m consequence of the prolongation of the cells; and under it we meet with a soft grey layer, which exhibits under the microscope the small rods and large bigeminal cones of Jacob's membrane. In our opinion, the tunica vasculosa is the proper choroid, and must include the tunica Ruyschiana or capillary network.

That the choroid gland is made up of convoluted bloodvessels, as is stated by our author, is true; but the addition of a few lines would have put the reader in possession of the fact, presumed by Albers, but first, through injection and the microscope, demonstrated by Mr. Wharton Jones, that the structure in question is a rete mirabile, or rather the centre of a series of retia mirabilia. The artery which supplies the whole of the gland and vascular choroid, having entered the back of the eye, divides into two branches, which run along the inner or concave edge of the gland, giving off twigs, which by subdivision and anastomosis form a network. The outer or convex edge of the gland is also surrounded by a network of vessels large enough to be visible to the naked eye. On examining the gland itself with the microscope, it is seen to consist in an innumerable quantity of minute straight vessels, which passing across, establish a connexion between the inner and outer networks. The large vessels of the inner network divide into the minute straight vessels which form the body of the gland, and these minute vessels coalesce in their turn to form the large vessels of the outer network; thus exhibiting the well-known character of a rete mirabile. Many of the vessels, however, derived from the outer network, as they proceed forward in the tunica vasculosa, subdivide into small branches, which again unite, thus forming secondary and more simple retia mirabilia. Lastly, the arteries of the tunica vasculosa terminate in a star-like manner, and the radiating twigs of these stars inosculate together.*

The sixth Chapter, on the Eyes of Extinct Animals, is short and amusing. The seventh, on the Physiology of Vision, occupies seventy-three pages, and discusses

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* Wharton Jodm on the Choroid Gland; Medical Gaiette, vol. ill pp. 850, 864.s London, 1888.

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