branched tubes. On the other hand, the respiratory apparatus of the grub of this insect, as indeed of most others, is exclusively tubular; and these tubes have a very different distribution from that which they present in the perfect insect. The same is the case with the caterpillar of the silkworm; but even in the perfect animal the respiratory apparatus, in this instance, is tubular alone. Of these tubes one large one runs along each side of the body, and gives off, opposite to each of the numerous openings upon the surface, two sets of branches, one to the lower part of the body, and the other to the upper, in such a manner that the former branches go chiefly to the muscles moving the feet, and the latter to the dorsal blood-vessel and to the several entrails, which in insects are always situated near their back. The stigmata, or orifices of the respiratory tubes, in the caterpillar of this insect, are furnished with a kind of lips, which open or close them at pleasure; and it is probably, by a similar apparatus, that all terrestrial insects regulate the ingress and egress of air employed in respiration. But some terrestrial insects are capable of respiring even under water, and the means by which they do this are extremely curious. In general they carry down with them a considerable portion of air in the interstices of the hairs with which their bodies are covered, and which continually exuding an oily fluid prevents the water from coming in contact with it; they breathe, therefore, under these circumstances, in a kind of natural diving-bell. In some insects however, such as the water scorpion, the air tubes, instead of this contrivance, are provided with long processes extending from the posterior part of the body, the extremities of which, being always above the water, furnish them with a constant supply of fresh air. They are, in fact, a kind of water serpent, or cetaceous animal, in this respect; the bulk of their bodies being under water, while their spiracles, or the holes through which they breathe, are above it. In all insects which fly, it seems to have been the object of nature to carry rather the air to the blood than the blood to the air, and how excellently adapted to this purpose is the tubular and ramified structure of their respiratory apparatus must be sufficiently evident.

In the Myriapoda, such as the multipedes and the centipedes, the air is taken into the body through a series of minute pores or spiracles, placed on each side along the entire length of the animal.

In the Annelida, or red-blooded worms, of which the leech is an example, a series of membranous pouches is provided for respiration, into which narrow ducts open, by which aërated water enters.

In the inferior tribes traces of respiratory organs are still discoverable, though these are so various and so obscure as to render it impossible to comprehend within our limits any particulars of these most rudimentary forms of the function.

The Kidney. The urinary organs do not occur in the non-vertebrated animals. They appear for the first time in fishes.

The kidneys are voluminous in fishes. They are composed of microscopic tubun, which terminate in the larger uriniferous tubes, termed ureters.

In some reptiles the kidneys are lobed, and in the higher species have much the same structure as in birds.

In birds the kidney consists of several distinct lobes, connected by the branches of the ureters; and, in many respects, approach nearer and nearer the character of the kidneys in mammals.

The structure of the kidneys in mammals is somewhat complex: the blood from which the secretion is derived is arterial blood. The kidney consists of two substances; the cortical, or outer surface, and the internal, or medullary. Of these the

former is the most vascular, and the latter chiefly composed of uriniferous tubes. The blood-vessels undergo a peculiar mode of convolution into minute masses, which are termed "Malpighian bodies." The tubules do not communicate directly with the bloodvessels. The secretion, however, first takes place in these tubules, which gradually unite, and finally, under different names, convey the secretion into the canals by which it is transmitted to the bladder.

VERTICAL SECTION OF KIDNEY

IN MAN.

The urinary secretion is plainly of vast importance in the animal economy. When this secretion is interrupted death speedily takes place; and the kind of death which occurs is regarded as a species of poisoning, owing to the blood becoming contaminated with noxious chemical products, which should have been thrown off by the

secretion.

Urea and uric acid are the two most remarkable substances known to exist in the urine. Between four and five hundred grains of urea are thrown off by the kidney Its medullary substance, termi- from the living system in the adult male. The quantity nating in uriniferous tubes, is considerably less in females, in children, and in old passing their secretion into the people. The quantity of uric acid thrown off in twentyfour hours is much less, being hardly more than onethirtieth part of the quantity of urea. Both these substances, as before stated, contain a large proportion of nitrogen. They are products of the disintegration of the solids of the body. The saline matters contained in the urine for the most part have a similar source.

ureter.

Urine contains about seven per cent. of solid matter to ninety-three per cent. of water. Of this solid matter about three per cent. are urea, one-tenth per cent. uric acid, half per cent. phosphates. As the phosphates exist in the solid parts, one source, at least, of these in the urine is the disintegration of the solids of the body. After very violent and long-continued exercise, the urine is observed to contain an unusual abundance of phosphates. There can be no doubt of the general truth of the proposition that the great purpose of the urinary secretion is to convey out of the system certain chemical products, arising from the disintegration of the living parts, though the precise series of chemical changes which take place be not yet fully determined. A general view, then, of this subject is all that is compatible with the plan of this treatise.

We speak currently of the effete matter of the living system being continually thrown off, and that such a separation is essential to the well-being of the body. It is not remarked, however, that merely slightly altered or exhausted portions of the various tissues, such as bone, cartilage, muscle, tendon, nerve, and blood-vessel, are thrown off in those processes by which effete matter is got rid of. It has long been noticed that the fluid contained in those vessels, which have been supposed to perform what is termed interstitial absorption, is homogeneous, and that it never shows signs of having been derived from the disintegration of any such solids as those above enumerated. Hence it was always concluded, in former times, that these absorbent vessels had not only the property of taking up the effete matter at the points where it formed, but that they had also the property of decomposing such effete matters, and of converting them into such a homogeneous fluid as is found in the absorbent vessels. It

scems now, however, very doubtful if these absorbent vessels take up anything else but liquids; since it is far more probable that the process by which solid parts are absorbed, consists, first, of a chemical decomposition of the solid, under the influence of the oxygen, conveyed to all parts of the system by the arterial blood, by which it is reduced to a soluble form, and then of its transmission through the coats of the minute veins into the blood. Thus the saline matters, such as the phosphates contained in the portion of solid living substance disintegrated, becomes at once mingled with the blood; while the organic tissues, as consisting of oxygen, hydrogen, carbon, and nitrogen, are converted into water, carbonic acid, urea, and uric acid.

We may judge of the extent to which this conversion goes on from the continual renewal of the blood and the solids of the body, which, while the same weight is retained, cannot take place without a corresponding removal of such parts as, by the progress of development to maturity, have reached the stage of decay and disintegration. The component parts of the living solids plainly undergo changes analogous to the growth, the maturity, the decay and death of the whole body. The new portions of nutriment supplied by the blood, in its successive circulations, correspond, at first, to the embryonic development of a young individual. By degrees these parts advance to maturity, and begin, after a short period of efficient service, to lose the energy of their vitality; so that they are now ready to become the prey of the ordinary chemical affinities of their component elements, and, under the action of the oxygen conveyed by the arterial blood, they become again reduced to matter but one degree removed from the inertness of the dust of the earth.

Of the Sap of Vegetables.-The proper sap of plants undoubtedly corresponds to the blood of animals. By proper sap, however, we are to understand not the ascending but the descending sap-that which, after its ascent from the roots, has undergone an elaboration in the leaves, so as to be prepared to afford to the several tissues a new supply of their proper substance.

For

The crude or ascending sap is totally different from the elaborated sap. example, the crude sap of a plant, when flowing upwards in abundance, may afford a refreshing drink, though, after elaboration in the leaves, it may become of a poisonous nature. The Euphorbia canariensis is the plant which affords the resin euphorbium of the shops, formerly employed as a blistering substance. This plant the inhabitants of the Canary Islands are said to tap, and draw off the ascending current for the purpose of refreshment, notwithstanding the acrid character of the sap after elaboration.

The descending or elaborated sap abounds in globules, and often, after being withdrawn from the plant, undergoes a species of coagulation. This sap-the proper juice or blood of the plant-plainly contains the materials of the solid parts which compose the structure of the plant, as well as those which enter into its various secretions and excretions.

What, then, is the foundation of the difference between the elaborated, or descending sap, and the ascending or crude sap? In the first place it is evident that crude sap does not contain all the materials which, by a certain transformation, may be converted into the constituents of the perfect sap. Whence, then, are those new materials obtained, which, being added to those of the crude sap, explains the development of the perfect sap? It is plainly the office of the leaves to add those new materials.

It has been a prevalent idea that the leaves of plants correspond to the lungs of animals, and that their use is merely to ventilate or purify the crude sap, as the lungs do the venous blood of animals. A more exact scrutiny of the office of the

leaves shows that they are the channel by which a most important part of the food is conveyed into the substance of the plant.

Although carbonic acid is continually given off by certain parts of plants, it is proved, beyond all doubt, that this generation of carbonic acid amounts to but a very small deduction to be made from the far more extensive decomposition of that gas which takes place in the leaves of plants under the influence of light. The carbon derived from this decomposition of carbonic acid becomes fixed in the plant, while free oxygen is given off. Thus the leaves in reality correspond to the digestive organs of animals, since, though nourishment is derived from other sources, yet a most important part enters by this channel; and it even appears that some other parts of the food of plants enters by the leaves, besides the carbon.

The food of plants consists of water, carbonic acid gas, ammonia, and some saline matters; and these several articles of food enter partly by the spongioles of the radicles, and partly by the leaves. The crude or ascending sap is derived from the spongioles of the radicles, and doubtless contains all the saline and earthy matters which enter into the constitution of the plants; it appears, also, to contain portions of the other aliments, particularly the watery part and the ammonia. By the additions made to this sap derived from the spongioles, the sap becomes matured, and prepared for the general nutrition of the tissues, and the supply of the secretions. It comes now to contain fecula, gum, sugar, lignin, and also the proteine compounds, albumen, fibrine, caseine, &c.; or substances readily convertible into these, by which the annual additions to the stem is made, the fruit is developed, and the several peculiar secretions, such as oil, fixed or volatile, resin, gum-resin, balsam, camphor, and the like, are supplied. And after the sap has served these uses, there is a surplus of nutritive matter left, which is laid up for the supply of the wants of the vegetable economy in the subsequent year; for, obscure as this subject still is, it seems certain that the sap which first rises from the soil in spring becomes mingled with organic products formed in the previous year, by the aid of which, before the leaves have commenced their office, various important effects in the vegetable economy are accomplished; and this in accordance with a rule of organic nature already referred to, namely, that for materials from without to become fit to be incorporated with pre-existing organic tissues, a mixture with the products of living action is a usual preliminary.

The respiration of plants, like the respiration of animals, consists in the evolution of carbonic acid, and the consequent development of temperature. By this evolution of carbon, which appears to take place at all times, though in minute proportion, some purification of the proper sap must be effected; and there also appears to be other means of excretion, by which the same end is still further promoted.

Of Reproduction.-The continual renovation of the tissues composing the frame of an adult animal, and of the leaves of trees in each successive spring, bears a striking analogy to the reproduction of species by the individual, whether animal or plant.

A germ separates from the body of the parent, which, under the application of certain conditions, different in different divisions of organic nature, becomes developed into a new individual.

The whole process is of the most wonderful character from beginning to end, and in whatever part of organic nature it is studied. The least complicated mode of reproduction is found in such organisms as the red snow (protococcus nivalis), which consists of a simple aggregation of vesicles, without any definite arrangement,-sometimes united, but capable of existing separately. In simple organisms of this kind, simple

ORGANIC NATURE.-No. III.

F

rupture gives independent existence to the rudiments of new individuals contained within them.

The successive development of the several structures belonging to the mature individual in the higher parts of both kingdoms, is not less wonderful than the varied primitive development of the germs.

The progress of the development of the chick during incubation affords one of the most interesting examples of this ulterior stage of reproduction.

Let us briefly review the anatomy of the egg at the commencement of incubation. Beneath the shell there is a membrane consisting of two layers, which, by their separation

a, cicatricula; b, yolk-bag; c, membrane lining the shell; d, attachment of chalazæ; f, air space; g, albumen.

at the larger end, form a space filled with air, especially rich in oxygen, this air vesicle being destined to serve for respiration. Within the inner layer of this membrane lies the white, and within the white, enclosed in its proper membrane, the yolk. From each extremity of the yolk-bag proceeds into the white a knotty body, terminating in a flocculent extremity. These two bodies are termed the chalaza, and their effect is to keep the yolk uppermost in the white; for by shak

ing an egg violently, the connexion of these with the white is destroyed, the yolk sinking to whatever end of the shell is downwards; and this is the secret of making an egg stand upon end, without proceeding to the violent expedient reported to have been employed by Columbus. On the surface of the yolk-bag is a small round milkwhite spot, called the cicatricula, surrounded by one or more whitish concentric circles. The cicatricula is the blastoderm, or germinal membrane, from which the future being is developed. Beneath the germinal membrane there is a canal, which leads to a chamber in the centre of the yolk, and which is filled with a whitish granular substance. Such is the description of the egg in the fowl, and in its general character it represents the matured ovum in vertebrate animals.

As soon as incubation commences, the germinal membrane becomes distinctly separate from the yolk and yolk-bag, spreading and assuming the form of a central pellucid spot, surrounded by a broad dark ring. At the same time it becomes thickened and prominent, and is soon separable into three layers; of these the exterior is a serous layer, the internal a mucous layer, and between the two is situated a vascular layer in which vessels soon become apparent. From the first, all the serous structures of the future animal are developed, as from the mucous layer are all the mucous structures, and from the middle all the vascular structures.

Towards the close of the first day, the serous or outer layer has become thickened into the first rudiment of the dorsal portion of the future embryo, while the two other