6

where, as Crampton and others had pointed out, the ciliary muscle being distinctly striated, its beginning and end may be tolerably easily made out. It was in the Ophthalmic Review' (for, I think, the year 1865) that, in a paper objecting to Helmholtz's and Donders' notions of the cause of accommodation, I first published my remarks on the relations of the ciliary muscle: the eye of the ostrich having been taken as the example. The subject, however, has not received the attention it deserves from physiologists; and with a view to show that at least in birds, accommodation must be effected in a totally different manner from that proposed by Donders, I am led to bring the question once more under the notice of histologists. I propose from time to time, as I may have leisure, and as the Zoological Society may be able to furnish me with "birds' eyes," to describe the relations of this remarkable muscle in various birds, and first in the green-breasted pheasant, the eyes of which were kindly placed at my disposal by Dr. James Murie.

The description of the ciliary muscle in most of the text-books is, so far as it applies to birds, extremely misleading and inaccurate. It is generally described, both for man and birds, as a ring of muscular or fibrous structure extending all round the eye at the line of union of the sclerotic and cornea. But in birds it is really much more than this.

If we take only the bird which forms the subject of this note, we find that the ciliary muscle is much more than a mere ring. It is in some measure almost a distinct coat. It may be described as a zone or belt of muscular tissue (holding much the same sort of proportion to the orb that the Tropic of Cancer does to the globe), whose fibres run forwards to the line of junction of the cornea and sclerotic, and extend backwards between the sclerotic and choroid for some lines. In section made along the long axis of the eye, the ciliary muscle would have an irregular pear-shape, the head of the pear being at the sclerotico-corneal junction, and the stalk some lines behind and between the sclerotic and cornea. The fibres are broad and somewhat flat, of the usual faintly-yellow colour (caused by steeping structure in chromic acid), and very beautifully striated, and they pass in a regular stream from behind forwards, and from without obliquely inwards, till they end generally in the sclerotico-corneal junction. All the fibres do not pass into this line of junction. The muscle is provided with a very tough and dense sheath of connective tissue, which is so loaded with irregular pigment-cells as to render the definition of the fibres at first rather difficult. But this sheath is more than a mere envelope. It increases in thickness and toughness at the sclerotico-corneal junction, and along the anterior half of its length (along the pear-shaped part) it gives insertion to the anterior extremity of many of the muscular fibres which are tra

velling toward the sclerotico-corneal junction. The insertion then of all the fibres of the ciliary muscle may be said to be the sheath of the muscle at its anterior extremity and the inner lamina of the

cornea.

What is the origin of this muscle? In regard to this, if the muscle of this bird examined in the present instance displays the same relations as that of the ostrich, I have to correct a mis-statement made in my former paper.* The origin of the fibres is not absolutely confined to the inside of the sclerotic. Some few of them may be traced into the choroid coat. These are a few of the most posterior fibres; and my reason for assuming that they have an origin in the choroid is, that when the latter is torn away the fibres are set free, and present the usual sharply broken extremities.

The plan of manipulation I have adopted in making the observations which have led to these conclusions is this:-The eye is first divided into two parts-from above downwards, and in the transverse plane. The posterior half falls away with the vitreous and lens, leaving the cornea, part of the sclerotic, the iris, part of the choroid, and the remainder of the ciliary processes. With a pair of scissors then this hemisphere is cut into halves. Then with a Valentin's knife I take a number of extremely delicate sections, at right angles to the internal surface of the eye, and in the anteroposterior direction. By this means I obtain specimens showing cornea, sclerotic, choroid, iris, and ciliary muscle.

Having placed one of these sections in glycerine, a needle is used-not to "tease out," but merely to prevent the falling together of the several parts. Under a low power, and with Mr. Collins' new dissecting microscope, this can readily be done. It is then seen that the muscle is of the form I have described, and that the choroid is united to the sclerotic at two points. One of these pointsthe anterior one, on a level with the iris-is, as shown in the drawing, merely a feeble union, maintained by a few fine fibres of elastic connective tissue. The other is a muscular union, is more posterior -being, in fact, at the hinder extremity of the ciliary muscle-and is formed, so far as I can see, by the passage of a few muscular fibres into the soft substance of the choroid. At these two points, then, there is connection of the muscle with the choroid. anterior one is merely a loose attachment, and a mere stroke of the needle severs it without injuring the muscle or the choroid; the other is more decided, and when it is broken the two or three muscular fibres involved in it present broken extremities.

The

All through the rest of its course the muscle is distinct; its outer side firmly and inseparably blended with the sclerotic, and its inner one bounded by its special sheath. This is nearly all that I have got to say as to the position of this muscle. My view may *Ophthal. Review.'

be wrong. I don't think it is. But supposing it right, I would ask, What effect can this muscle have on the consistence of the lens, from which it is so distant? How can it advance the lens through the action on choroid, to which its attachment is so far posterior to the lens? and lastly, What can be the effect of the contraction of so important a muscular structure but to bend in the border of the cornea, and thus increase the curvature of the object-glass of the eye? Its origin-the sclerotic-is unyielding; its insertion—the cornea -is. The liquid of the eye resists the inward pressure of the cornea, and driving its central part out, still more increases the curvature. Lastly, in the elastic lamina of the cornea do we not see the antagonist of this powerful muscle. Birds must necessarily possess greater power of focal accommodation than man, but why should the mechanism by which that accommodation is obtained be so different from that of man as the views of Helmholtz would lead us to suppose, if we believe the ciliary muscle of birds to operate as I have suggested?

NEW BOOKS, WITH SHORT NOTICES.

Recherches sur l'embryogénie des Crustacés.

I. Observations sur le

développement de l'Asellus aquaticus. Par Edouard Van Beneden, 1869.-Although the valuable memoir which M. Van Beneden has been good enough to send us is not a book in the publishers' sense of the word, it is a work of so much importance that it justifies our noticing it under the head of Reviews. It is the reprint of a communication recently made to the Royal Academy of Belgium on an interesting species of fresh-water crustacean, and it expresses results of the highest value, especially in relation to the curious researches of Fritz Müller and others in reference to those Nauplius forms which have so singular a bearing on Mr. Darwin's views. M. Van Beneden first sketches briefly the labours of those who have preceded him in the field, and he does so in that appreciative and kindly manner which is so characteristic of the genuine lover of scientific research.

Fritz Müller, in his 'Für Darwin,' a book some time since noticed in these columns, made known, under the name of larval membrane, larvenhaut, a structureless membrane, which in the Isopoda, and especially in Ligia, is formed round the embryo in the first stages of its development. This cuticular layer has the shape of an elongated sac, without lateral processes in the form of appendages, and should be considered, says the author, not as a dependent membrane of the ovum, but as the residue of the first embryonic moult. Now, M. Dohrn has recognized that there is found round the embryo of the young Asellus just such a membrane as that which Fritz Müller has described in Ligia and others. M. Sars has also observed this, and has seen its relations to the antennæ, which had escaped M. Dohrn, and which is a morphological fact of great import. MM. Dohrn and Sars consider that the egg, at the moment it passes into the incubating pouch, is surrounded by two membranes, the outer of which represents the chorion, and the inner of which is a vitelline membrane. M. Dohrn has not tried to determine its significance, and he has simply termed the inner egg-membrane innere Eihaut.

But the author has satisfied himself that at the moment of the egg's passing into the incubating pouch it is surrounded by a single membrane, which is directly applied to the vitellus. Soon, however, this separates, and leaves between it and the vitellus a transparent liquid. The single envelope on the recently-deposited ovum is what is generally styled the chorion. The other or inner membrane forms itself in the ordinary course of development after the egg has remained for some hours in the incubating pouch. What is the value and import of this envelope? Is it part of the ovum, or is it rather an embryonic formation and the remnant of an embryonic moult which precedes the Nauplius moult?

A number of questions remain to be decided:-(1) Is the ovum of Asellus really covered at the moment of deposition by two envelopes, as MM. Dohrn and Sars allege? (2) What is the morphological significance attaching to the inner membrane, which according to M. Sars is a vitelline membrane? (3) Is M. Dohrn correct in comparing the membrane which he calls Larvenhaut in Asellus with the larval membrane of Ligia? It is to the consideration of these three problems that M. Van Beneden devotes his attention in the memoir before us. He goes into the details of the embryology of Asellus which bear on the point; traces out the whole early development of the species; and illustrates it by four plates containing several well-drawn representations of the egg and larva in different phases; and finally concludes by laying down the following propositions :-(1) The ovarian egg, at the time of deposition, consists of only one membrane, the chorion. (2) This membrane remains for some considerable period the sole membrane of the egg. (3) The membrane, which both MM. Sars and Dohrn associate with the ovum, is really an embryonic membrane. (4) The study of the inferior crustacea, and especially of Anchorella and Lerneopodæ, demonstrates that in these Lernes the embryo undergoes three moults even while in the egg: a Blastodermic moult, a Nauplian moult, and a Cyclopean or Zoëan moult. That each of these moults consists in the loss of a cuticular membrane, having the form which the embryo has at the time the membrane is formed. The first embryonic membrane of Asellus is a Blastodermic cuticle, and subsequently there is formed a Nauplian cuticle which corresponds to the larval membrane of Ligia. M. Van Beneden's memoir is one which must be carefully studied by every student of Embryology.

PROGRESS OF MICROSCOPICAL SCIENCE.

The Development of the Chotopoda.-Under this title a very valuable memoir has been written jointly by Professor Claparède and Mecznikow, in the last number of Siebold and Kölliker's Zeitschrift. It is illustrated by a great number (upwards of a hundred) beautifullydrawn coloured figures, and traces the stages of development in the typical members of the family of this group. Each species selected is dealt with separately and completely, and the whole memoir is one of the fullest published on the subject.

The Anatomy of the Bed-bug.-The structure of Cimex lactularius is very fully described in another paper in Siebold's Zeitschrift by Herr Professor Landois. This is a continuation of a paper published some time since in this Journal by the same author. In this part Dr. Landois treats of the respiratory, reproductive, and muscular systems.

VOL. II.