correct. The theory that sleep is caused by withdrawal of blood from the brain by contraction of its arterial vessels, is disproved by many considerations. It presupposes that at the time when the cerebro-spinal nervous system is most wearied the organic system is most active; and it assumes that the great volume of blood which circulates through the brain can be cut off without evidence of increased volume of blood and tension of vessel in other parts of the body, a supposition directly negatived by the actual experiment of cutting off the blood from the brain.

There is another potent objection applicable to both theories. When sleep is artificially induced, either by subjecting the brain to pressure of blood or to exhaustion of blood, the sleep is of such a kind that the sleeper cannot be roused until the influence at work to produce the sleep is removed. But in natural sleep the sleeper can always be roused by motion or vibration. We call to a person supposed to be sleeping naturally, or we shake him, and if we cannot rouse him we know there is danger; but how could these simple acts remove pressure from the brain, or relax the contracted vessels feeding the brain?

These two theories set aside, the others I have named need not trouble us; they are mere generalisations, interesting to read, worthless to pursue. Know we then nothing leading towards a solution of the question of the proximate cause of sleep? I cannot say that, for I think we see our way to something which will unravel the phenomenon; but we must work slowly and patiently, and as men assured that in the problem we are endeavouring to solve, we are dealing with a subject of more than ordinary importance. I will try to point out the direction of research.

I find that to induce sleep it is not necessary to produce extreme changes of brain matter. In applying cold, for example, it is not necessary to make the brain substance solid in order to induce stupor, but simply to bring down its temperature ten or twelve degrees. I find also that very slight direct vibrations, concussions, will induce stupor; and I find that in animals of different kinds the profoundness of sleep is greater in proportion as the size of the brain is larger. From these and other facts I infer that the phenomenon of natural sleep is due to a molecular change in the nervous structure itself of the cerebro-spinal system, and that in perfect sleep the whole of the nervous structure is involved in the change-the brain, the cord, the nerves; while in imperfect sleep only parts of this nervous matter are influenced. This is in accord with facts, for I can by cold put to sleep special parts of the nervous mass without putting other parts to sleep. In bad sleep we

have the representation of the same thing in the restlessness of the muscles, the half-conscious wakings, the dreams.

Suppose this idea of the change of nervous matter to be true, is there any clue to the nature of the change itself? I think there is. The change is one very closely resembling that which occurs in the solidification of water surcharged with a saline substance, or in water holding a hydrated colloid, like dialysed silica, in trembling suspension. What is, indeed, the brain and nervous matter? It is a mass of water made sufficiently solid to be reduced into shape and form, by rather less than twenty per cent. of solid matter, consisting of albuminous substance, saline substance, fatty substance. The mechanism for the supply of blood is most delicate, membranous; the mechanism for dialysis or separation of crystalloidal from colloidal substance is perfect, and the conversion of the compound substance of brain from one condition of matter to another is, if we may judge from some changes of water charged with colloidal or fatty substances, extremely simple. I do not now venture on details respecting this peculiarly interesting question, but I venture so far as to express what I feel will one day be the accepted fact, that the matter of the wakeful brain is, on going to sleep, changed, temporarily, into a state of greater solidity; that its molecular parts cease to be moved by external ordinary influences, by chemical influences; that they, in turn, cease to communicate impressions, or, in other words, to stimulate the voluntary muscles; and that then there is sleep which lasts until there is re-solution of structure, whereupon there is wakefulness from renewed motion in brain matter and renewed stimulation of voluntary muscle, through

nerve.

The change of structure of the brain which I assume to be the proximate cause of sleep is possibly the same change as occurs in a more extreme degree when the brain and its subordinate parts actually die. The effects of a concussion of the brain from a blow, the effects of a simple puncture of nervous matter in centres essential to life-as the point in the medulla oblongata which Fluorens has designated the vital point-have never been explained, and admit, I imagine, of no explanation except the change of structure I have now ventured to suggest.

Here, for the moment, my task must end. My object has been to make the scientific reader conversant with what has been said by philosophers upon the subject of sleep and its proximate cause, and to indicate briefly a new line of scientific enquiry. I shall hope on some future occasion to be able to announce further and more fruitful labour.

67

REVIEWS.

THE TRANSFORMATION OF INSECTS.*

AMID the vast host of drawing-room scientific works which the past few

years have introduced into England, the best, it seems to us, is the work of M. Blanchard, edited by Dr. Duncan, which is now before us. Whether we consider the marvellous array of exquisitely beautiful plates, a host of woodcuts, the admirable printing, or last, but not the least, the excellent additions which our English naturalist has made to the text, we must readily give the first place to M. Blanchard's treatise.

Curiously enough, it deals not only with the animals of one class alone, but, besides treating of the insects, it goes comparatively fully into the Arachnida and Crustacea. In this respect it presents, apart from the many features of novelty which we find in other parts of the volume, much that is new to the general reader. For it must be confessed that, while the insecta are comparatively well known in many respects to the world generally, the Arachnids and Crustaceans form—especially the latter-rather new groups for the consideration of the naturalist.

The first chapters, of course, deal with questions of physiology, such as those of metamorphosis, and the general character of the animals described, and the succeeding ones with the metamorphoses of the various groups of insects; then in the same manner, though in briefer fashion, are subsequently detailed the classes Myriapoda, Arachnida, and Crustacea. In the earlier chapters those, in fact, which deal with the general anatomy and physiology of each group—we fancy that Dr. Duncan has had the heaviest work to perform. Not merely as a translator, of course, but in bringing the French edition up to the state of our recent knowledge. We have only seen the French book momentarily, and of course we may be wrong; but we fancy that a great deal of new matter meets the eye in this part of the work, and we heartily thank Dr. Duncan for the trouble he has taken; for, if we do not

"The Transformation, or Metamorphoses of Insects" (Insecta Myriapoda, Arachnida, Crustacea). Being an Adaptation, for English Readers, of M. Émile Blanchard's" Métamorphoses, mœurs et instincts des Insectes," and a Compilation from the Works of Newport, Charles Darwin, Spence Bate, Fritz Müller, Packard, Lubbock, Stainton, and others. By P. Martin Duncan, F.R.S., Professor of Geology in King's College, London. London; Cassell, Petter, & Co. 1871.

mistake, he has been at pains to make a work, that was intended merely as a popular one, do service of a good kind to both naturalists and the public generally. We believe that it is he who introduced much that relates to the nervous system of insects, for we find in this work a great deal of matter that is absolutely new to popular treatises. Especially so is the account of the development of the nervous system in the larva of Vanessa urtica, which is taken from that wondrous worker among insects, our English Newport, and which is briefly as follows:-"Two hours after the larva of Vanessa urtica has suspended itself to undergo its transformation, and in which state it remains from six, eight, ten, or even twenty-four hours-according to the strength of the individual, and other circumstances— before it throws off its last larva-skin, a considerable alteration has already taken place in the body of the larva. The ganglions in the head are still distinct from each other, but are a little in form, although not yet enlarged. The sub-oesophageal ganglion is enlarged to nearly twice its original size, and the cords which join it to the brain are shortened, and so are those that connect the second, third, fourth, and fifth ganglions. The last two are separated only by a short interval. The fifth, sixth, and seventh are drawn closer together, the ends between them are disposed in a zig-zag manner, and the longitudinal direction of the nervous chain is in consequence altered. The ganglions, from the seventh to the terminal one, remain as in the active larva. A little while before the old skin is thrown off there is great excitement throughout the body of the insect. About half-an-hour before this occurs there is a considerable enlargement of the brain, the subœsophageal, and the second, third, fourth, and fifth ganglions. The cords that extend between them diverge very much, and those between the fifth, sixth, and seventh are disposed in a zigzag direction. Immediately after the insect has entered the pupa state all the ganglions are brought closer together, in consequence of the ends being disposed more irregularly than at any other period, which has been occasioned by the shortening that has taken place in every segment, by which the cords are rendered too long to lie in a direct line. Seven hours afterwards there is a greater enlargement of the brain, optic nerves, and ganglions of the thoracic segments, which begin to approach each other. At twelve hours the thoracic ganglions have united, and at eighteen the nerves of the wings have increased in size, and the nervous chain in the abdomen has become straight again. At thirty-six hours the optic nerves have grown nearly as large as the brain, and the gullet is completely surrounded by an extension of the ganglion under it, and the brain above. The sixth ganglion has disappeared; and at the end of forty-eight hours the seventh is no longer seen, but the thoracic centres increase in size gradually. At fifty-eight hours the middle part of the chest has greatly increased in size, and the great nervous centres and nerve twigs, which will supply the wings eventually with energy, occupy it. The optic and antennal nerves have nearly attained their full development, and the arrangement of the whole nervous system is now nearly as it exists in the perfect insect. The whole of these important changes are thus seen to take place within the first three days after the insect has undergone its metamorphoses, and they precede those of the digestive system." We have given this rather long quotation for various reasons: in the first place, it illustrates

a wonderful phase in normal development; again, it shows the marvellous insight of Newport, who must have spent many and many a weary hour ere he could lay down tersely the information above given. Lastly, it shows how carefully the editor (for we credit him with the quotation) has brought the work to its proper development in thus giving the general reader some small idea of the amount of work done by a labourer who is known but to anatomists, but who deserves to be remembered when many of the present race of naturalists are no more.

But the general reader, uninterested in metamorphoses, may think the above of very little significance, and may naturally be more interested in the general habits or other conditions of insects. If he be, he cannot fail to find abundance to interest him in this work. For example, let us take some of the curious facts relating to parthenogenesis, which are recorded, and for which we think again we are indebted to Dr. Duncan. Von Siebold, he says, "collected a great number of the cases, or sacs, as he calls them, of Solenobia lichenella and S. triquetrella, and to his great astonishment none but female individuals came out of them, and only a single locality furnished him with a couple of males. IIe kept these females carefully in little vessels closed with glass lids, and found that they clung to their cases, resting upon the outside of them. These virgin females laid eggs and filled their sacs with them, and did not wait for a fertilising male, for they commenced egg-laying very soon after they escaped from the pupa-case or the chrysalis condition. When the Solenobia were removed from their sacs, they had such a violent impulse to lay, that they pushed their laying tube about in search of the surface of the sac, and at last let their eggs fall openly. He writes, "If I had wondered at the zeal for oviposition in these husbandless Solenobia, how was I astonished when all the eggs of these females, of whose virgin state I was most positively convinced, gave birth to young caterpillars which looked about with the greatest assiduity, in search of materials for the manufacture of little sacs." This is curious enough, but is just a mite from the immense store of information which the book contains; and we only wish that our space permitted us a more extensive quotation. But we have said, we hope, sufficient to show generally the great value and importance of the work, which is an admirable, tersely written, English work, and whose engravings are beyond comparison as they are almost beyond number.

THE

ELEMENTS OF MECHANISM.*

HE present work is one of a series which Messrs. Longmans are issuing, as we suppose, in opposition to the many works of a somewhat similar kind which are being circulated about at present. It is one of a series which the editor tells us is intended for the members of a large class which

"The Elements of Mechanism." Designed for Students of Applied Mechanics. By T. M. Goodeve, R.A., Lecturer on Applied Mechanics at the Royal School of Mines. London: Longmans. 1870.