of cartilages, or gristle, a substance softer than bone, and more elastic and yielding; is most admirable. By this combination of hardness and elasticity, the ribs are fitted to protect the chest against the effects of violence; and even to sustain life, after the muscular power of respiration has become too feeble to continue, without the aid of elasticity.

If the ribs were complete circles, formed entirely of bone from the spine to the breast-bone, there would be greater liability to fracture and danger to life; the rubs and jolts to which the human frame is continually exposed, would be too much for their delicate and brittle texture. This evil is avoided by the interposition of the elastic cartilages. On their fore part, the ribs are eked out and joined to the breast-bone by means of cartilages of a form corresponding to their own; being, as it were, the continuations of the arches of the ribs by substances more adapted to yield, and recoil, in every shock or contusion, than bony hoops. The elasticity of this portion meets and subdues those crushings of the body which would otherwise occasion fracture of the ribs. We lean forward, or to one side, and the ribs accommodate themselves, not by a change of form in the bones, but by the bending or elasticity of the cartilages. A severe blow upon the ribs does not break them, because their extremities recoil and yield to the violence. But it is only in youth, when the human frame is in perfection, that this pliancy and elasticity have full effect. In old age, the cartilages of the ribs become bony; they are firmly attached to the breast-bone, and the extremities of the ribs are fixed, as if the whole arch were formed of bone, unyielding and inelastic. Then a violent blow upon the side will fracture the ribs,—an accident seldom occurring in childhood, or in youth.

There is a purpose still more important to be accomplished by means of the elastic structure of the ribs; that is, in the highly excited respiration which accompanies great efforts of bodily strength. There are two acts of breathing-expiration, or the sending forth of the breath; and inspiration, or drawing in the breath. When the chest is at rest, it is neither in a state of expiration, nor in that of inspiration; it is in the intermediate condition; and the muscular effort by which either the one or the other is produced, is an act opposed to the elasticity of the ribs. The muscles of respiration are excited alternately, to

dilate or to contract the cavity of the chest, and, in doing so, to raise or depress the ribs. Hence it is, that both in inspiration and in expiration, the elasticity of the ribs is called into play; and, were it within our province, it would be easy to show, that after the muscular power had become too weak to continue the breathing unassisted, the action can be carried on, and life preserved, through the aid of the elastic property of the ribs.

From what has been now explained, it will at once be understood that violent exertion is incompatible with the condition of the chest in old age. The elasticity of the cartilages is gone, the circle of the ribs is unyielding, and will not allow that high breathing, that sudden and great dilatation and contraction of the cavity of the chest, which is required for circulating the blood through the lungs, and relieving the heart in the tumultuous flowing of the blood which laborious exertion produces.

Looking to the means of guarding life, nothing can be more important than the condition of the lungs in respect to the quantity of atmospheric air within them. The sensibility, and the rapid contraction of the glottis, near the mouth of the respiratory tube, are for arresting any foreign matter, afloat in the atmosphere, which might be drawn in by the stream of inspired air, and so reach the recesses of the lungs. But were this all, the office would not be half performed. The foreign body would be arrested; but how expelled, if it lodged? In common expiration, the air is never discharged altogether from the lungs; there is enough retained to be propelled against this foreign body, and to eject it. And, but for this, the sensibility of the glottis, and the actions of the expiratory muscles, would be in vain; we should be suffocated by the slightest husk of seed, or subject to deep inflammation from foreign matter drawn into the air-tubes collecting in the lungs.

We may here observe, that the instinctive actions for the protection of the body are calculated, if we may say so, for the natural condition of man. The manufacturer is sometimes removed from that condition; and our invention must be taxed, not only to maintain the purity, in a chemical sense, of the atmosphere in which he works, but to arrest, or convey away, the small portions of material which may be thrown offfor example, by the operations of the flax-dresser in heckling, or of the cutler who grinds the steel after the instrument is

forged, or of the stone-cutter, &c.—and so to prevent those particles from being inhaled. The length of the air-passages which lead to the lungs, the sensibility and muscular apparatus bestowed upon them, and the mucous secretions thrown into them, are the natural means by which foreign matter is arrested and thrown out. But in these artificial conditions of men, insoluble particles are continually floating in the atmosphere which they breathe; these are drawn in and lodge in the lungs, and irritate to disease.

This part of our subject suggests the consideration of that law of fluids which appears, at first, so contradictory as to be called the "hydrostatic paradox." Suppose a machine formed of two boards of equal diameter, and joined together by leather nailed to their margins, like a pair of bellows; a hole is made in the upper board, into which is inserted a tube. If a person mount upon this apparatus when it is filled with water, and blow into the pipe, he can raise the upper board, carrying himself upwards by the force of his own breath-indeed, by the power of his cheeks alone. It is on the same principle that, when a forcing-pump is let into a closed reservoir of water, it produces surprising effects. The piston of the hydraulic press being loaded with a weight of one pound, the same degree of pressure will be transmitted to every part of the surface of the reservoir that is given to the bottom of the tube, and the power of raising the upper lid will be multiplied in the proportion that its surface is larger than the diameter of the tube. Or, to state it conversely: suppose we had to raise the column of water in the tube by compressing the reservoir; it would require the weight of a pound on every portion of the superficies of the reservoir equal in extent to the base of the piston, 'before the water could be raised in the tube. Were the apparatus which we have described full of air instead of water, we should witness a similar effect; for all fluids, whether elastic or not, press equally in all directions; and this is the law on which the phenomenon depends. If we blow into the nozzle of a common pair of bellows, it is surprising what a weight of books we can heave up if laid upon its board.

Understanding, then, that the power of the hydraulic press, in raising the lid, depends on the size of the reservoir, and its relation to the tube; and again, that in pressing the fluid up

through the tube, the pressure upon the sides of the reservoir must be the greater the larger the cavity, we can conceive how a glass-blower propels the air into his blow-pipe with great ease, if he blow by means of the contraction of the cheeks, the smaller cavity; but that it will be with an exhausting effort, if he blow by the compression of the larger cavity, the chest. Dr Young made a calculation that, in propelling the air through a tube of the same calibre, a weight of four pounds, operating upon a cavity of the size of the mouth, would be equal to the weight of seventy pounds, pressing upon a cavity of the dimensions of the chest.

Let us see how beautifully this hydraulic principle is introduced to give strength in the common actions of the body. We have remarked that the extension of the superficies of the thorax is necessary to the powerful action of the muscles which lie upon it; and these are the muscles of the arms. In preparation for a great effort, we draw the breath and expand the chest. The start of surprise, and of readiness for exertion, in man and animals, is this instinctive act. But unless there were other means of preserving the lungs distended, the action of those muscles which should be thrown upon the arms would be wasted in keeping the chest expanded. It is here, then, that the principle which we have noticed is brought into play. The chink of the glottis, which the reader has already understood to be the top of that tube which descends into the lungs, is closed by a muscle not weighing a thousandth part of the muscles which clothe the chest; yet this little muscle controls them all! A sailor leaning his breast over a yard-arm, and exerting every muscle on the rigging, gives a direction to the whole muscular system, and applies the muscles of respiration to the motions of the trunk and arm, through the influence of this small muscle, that is not capable of raising a thousandth part of the weight of his body; because this little muscle operates upon the chink of the glottis, and is capable of opposing the whole combined power of all the muscles of expiration. It closes the tube just in the same way that the man standing on the hydraulic bellows can with his lips support his whole weight. Thus it is that the muscles which would else be engaged in dilating the chest, are permitted to give their power to the motions of the arms.

Some cruel experiments have been made, which, for whatever intended, illustrate the necessity of closing the top of the windpipe during exertion. The wind-pipe of a dog was opened, which produced no defect until the animal was solicited by his master to leap across a ditch, when it fell into the water in the act of leaping; it failed in its leap, because the muscles which should have given force to the fore-legs lost their power by the sudden sinking of the chest. This experiment is sufficiently repugnant to our feelings; and I need not offend the reader by giving instances in further illustration, from what sometimes takes place in man.]

RELATION BETWEEN THE SKELETON OF THE BIRD, AND ITS MODE OF PRODUCING ITS OFFSPRING.-Having, in the earlier part of the volume, noticed some of the more remarkable peculiarities of the skeleton of the bird, we may take this opportunity of observing the relation between its general form, and one of its principal functions. Putting out of the question, for the present, digestion and respiration, functions necessary for preserving the life of the individual, the continuation of the species is the next in importance. If a bird is to be buoyant and capable of flying, it cannot be viviparous. We have seen that a full stomach impeded the flight of a carnivorous bird; now, from that it is evident that it could not have carried its young within it. Is it not curiously provided, then, that the bird shall produce its offspring by a succession of small eggs; and that these shall accumulate in the nest, instead of growing in the body? In short, it requires no argument to prove that the hollow bones of the skeleton, the extension of the breast-bone, the air-cells, the quill-feathers, the bill, and the laying of eggs, are all in necessary relation to each other.

OF THE KANGAROO.-Since we have spoken of the adaptation of the skeleton of the bird to its mode of producing its young, we may, for the same object, advert to the subject in a quadruped. In the mammalia, there is no deviation from the general form of the skeleton more extraordinary than that in the kangaroo ; and there is, at the same time, a remarkable peculiarity in the manner in which it produces its offspring. Instead of remaining within the mother for the usual period of gestation, the young, by a singular process, not perfectly understood, is excluded, and found attached to the teat there, covered by an

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