in dogs fed on vegetable substances it amounted to over 90 per cent. In some instances,' where the animals (rabbits and fowls) were fed on bread and grain exclusively, the proportion of expired oxygen amounted to 101 or even 102 per cent.; that is, more oxygen was actually contained in the carbonic acid exhaled, than had been absorbed in a free state from the atmosphere. A portion, at least, of the carbonic acid must therefore have been produced by other means than direct oxidation.

IV. It has already been shown, in a previous chapter, that the carbonic acid which is exhaled from the lungs is not primarily formed in the blood, but makes its appearance in the substance of the tissues themselves; and furthermore, that even here it does not originate by a direct oxidation, but rather by a process of decomposition, similar to that by which sugar, in fermentation, is resolved into alcohol and carbonic acid. We understand from this how to explain the singular fact alluded to in the last paragraph, viz., the abundant production of carbonic acid, under some circumstances, with a comparatively small supply of free oxygen. The statement made by Liebig, therefore, that starchy and oily matters taken with the food are immediately oxidized in the circulation without ever being assimilated by the tissues, is without foundation. It never, in fact, rested on any other ground than a supposed probability; and as we see that carbonic acid is abundantly produced in the body by other means, we have no longer any reason for assuming, without direct evidence, the existence of a combustive process in the blood.

V. The evolution of heat in the animal body is not general, as it would be if it resulted from a combustion of the blood; but local, since it takes place primarily in the substance of the tissues themselves. Various causes will therefore produce a local elevation or depression of temperature, by modifying the nutritive changes which take place in the tissues. Thus, in the celebrated experiment of Bernard, which we have often verified, division of the sympathetic nerve in the middle of the neck produces very soon a marked elevation of temperature in the corresponding side of the head and face. Local inflammations, also, increase very sensibly the temperature of the part in which they are seated, while that of the general

'Annales de Chimie et de Physique, 3d series, xxvi. pp. 409–451.

mass of the blood is not altered. Finally it has been demonstrated by Bernard that in the natural state of the system there is a marked difference in the temperature of the different organs and of the blood returning from them. The method adopted by this experimenter was to introduce, in the living animal, the bulb of a fine thermometer successively into the blood vessels entering and those leaving the various internal organs. The difference of temperature in these two situations showed whether the blood had lost or gained in heat while traversing the capillaries of the organ. Bernard found, in the first place, that the blood in passing through the lungs, so far from increasing, was absolutely diminished in temperature; the blood on the left side of the heart being sometimes a little more and sometimes a little less than one-third of a degree Fahr. lower than on the right side. This slight cooling of the blood in the lungs is owing simply to its exposure to the air through the pulmonary membrane, and to the vaporization of water which takes place in these organs. In the abdominal viscera, on the contrary, the blood is increased in temperature. It is sensibly warmer in the portal vein than in the aorta; and very considerably warmer in the hepatic vein than in either the portal or the vena cava. The blood of the hepatic vein is in fact warmer than that of any other part of the body. The production of heat, therefore, according to Bernard's observations, is more active in the liver than in any other portion of the system. As the chemical processes of nutrition are necessarily different in the different tissues and organs, it is easy to understand why a specific amount of heat should be produced in each of them. A similar fact, it will be recollected, was noticed by Dutrochet, in regard to the different parts of the vegetable organization.

VI. Animal heat has been supposed to stand in a special relation to the production of carbonic acid, because in warm-blooded animals the respiratory process is more active than in those of a lower temperature; and because, in the same animal, an increase or diminution in the evolution of heat is accompanied by a corresponding increase or diminution in the products of respiration. But this is also true of all the other excretory products of the body. An elevation of temperature is accompanied by an increased activity of all the nutritive processes. Not only carbonic acid, but the

'Gazette Hebdomadaire, Aug. 29 and Sept. 26, 1856.

ingredients of the urine and the perspiration are discharged in larger quantity than usual. An increased supply of food also is required, as well as a larger quantity of oxygen; and the digestive and secretory processes both go on, at the same time, with unusual activity.

Animal heat, then, is a phenomenon which results from the simultaneous activity of many different processes, taking place in many different organs, and dependent, undoubtedly, on different chemical changes in each one. The introduction of oxygen and

the exhalation of carbonic acid have no direct connection with each other, but are only the beginning and the end of a long series of continuous changes, in which all the tissues of the body successively take a part. Their relation is precisely that which exists between the food introduced through the stomach, and the urinary ingredients eliminated by the kidneys. The tissues require for their nutrition a constant supply of solid and liquid food which is introduced through the stomach, and of oxygen which is introduced through the lungs. The disintegration and decomposition of the tissues give rise, on the one hand, to urea, uric acid, &c., which are discharged with the urine, and on the other hand to carbonic acid, which is exhaled from the lungs. But the oxygen is not directly converted into carbonic acid, any more than the food is directly converted into urea and the urates.

Animal heat is not to be regarded, therefore, as the result of a combustive process. There is no reason for believing that the greater part of the food is "burned" in the circulation. It is, on the contrary, assimilated by the substance of the tissues; and these, in their subsequent disintegration, give rise to several excretory products, one of which is carbonic acid.

The numerous combinations and decompositions which follow each other incessantly during the nutritive process, result in the production of an internal or vital heat, which is present in both animals and vegetables, and which varies in amount in different species, in the same individual at different times, and even in different parts and organs of the same body.

CHAPTER XIV.

THE CIRCULATION.

THE blood may be regarded as a nutritious fluid, holding in solution all the ingredients necessary for the formation of the tissues. In some animals and vegetables, of the lowest organization, such as infusoria, polypes, algæ, and the like, neither blood nor circulation is required; since all parts of the body, having a similar structure, absorb nourishment equally from the surrounding media, and carry on nearly or quite the same chemical processes of growth and assimilation. In the higher animals and vegetables, however, as well as in the human subject, the case is different. In them, the structure of the body is compound. Different organs, with widely different functions, are situated in different parts of the frame; and each of these functions is more or less essential to the continued existence of the whole. In the intestine, for example, the process of digestion takes place; and the prepared ingredients of the food are thence absorbed into the blood vessels, by which they are transported to distant tissues and organs. In the lungs, again, the blood absorbs oxygen which is afterward to be appropriated by the tissues; and carbonic acid, which was produced in the tissues, is exhaled from the lungs. In the liver, the kidneys, and the skin, other substances again are produced or eliminated, and these local processes are all of them necessary to the preservation of the general organization. The circulating fluid is therefore, in the higher animals, a means of transportation, by which the substances produced in particular organs are dispersed throughout the body, or by which substances produced generally in the tissues are conveyed to particular organs, in order to be eliminated and expelled.

The circulatory apparatus consists of four different parts, viz: 1st. The heart; a hollow, muscular organ, which receives the blood at one orifice and drives it out, in successive impulses, at another. 2d. The arteries; a series of branching tubes, which convey the blood from the heart to the different tissues and organs of the body.

3d. The capillaries; a network of minute inosculating tubules, which are interwoven with the substance of the tissues, and which bring the blood into intimate contact with the cells and fibres of which they are composed; and, 4th. The veins; a set of converging vessels, destined to collect the blood from the capillaries, and return it to the heart. In each of these four different parts of the circulatory apparatus, the movement of the blood is peculiar and dependent on special conditions. It will therefore require to be studied in each one of them separately.

THE HEART.

The structure of the heart, and of the large vessels connected with it, varies considerably in different classes of animals, owing to the different arrangement of the respiratory organs. For the respiratory apparatus being one of the most important in the body, and

the one most closely connected by anatomical relations with the organs of circulation, the latter are necessarily modified in structure to correspond with the former. In fish, for example (Fig. 76), the heart is an organ consisting of two principal cavities: an auricle (a) into which the blood is received from the central extremity of the vena cava, and a ventricle (b) into which the blood is driven by the contraction of the auricle. The ventricle is considerably larger and more powerful than the auricle, and by its contraction drives the blood into the main artery supplying the gills. In the gills (cc) the blood is arterialized; after which it is collected by the branchial veins.

These veins unite upon the median line to form the aorta (d) by which the blood is finally distributed throughout the frame. In