about one-fifth of all the heat produced in the body during twenty-four hours.

The cutaneous secretion may be greatly increased by temporary causes. An elevated temperature or unusual muscular exertion, will increase the circulation through the skin and largely augment the amount of fluid discharged. It then exudes more rapidly than it can be carried off by evaporation, and collects upon the skin as a visible moisture, whence it is known as the sensible perspiration. The amount of perspiration discharged during violent exercise has been known to rise as high as 350 or 380 grammes per hour; and Dr. Southwood Smith' found that the laborers employed in heated gas-works sometimes lost, by both cutane ous and pulmonary exhalation, nearly 1600 grammes in the course of an hour. The evaporation of this increased quantity of fluid consumes a large portion of the caloric derived from the heated atmosphere, and thus prevents an undue rise in the temperature of the bodily organs.

It is possible that certain influences transmitted through the nerves may also have the power of controlling directly the molecular activity of the tissues, and may thus diminish the amount of internal heat at the source of its production; but the experimental evidence of this action is yet incomplete, and its mode of operation comparatively obscure.

The production of heat in the animal body and the regulation of its temperature, by which it is maintained at or near a normal standard, are two of the most important phenomena presented by the living organism. They are the result of an associated series of vital actions, and at the same time essential conditions for the continuance of life.

1 Philosophy of Health. London, 1838, chap. xiii.

CHAPTER XV.

THE CIRCULATION.

THE blood is a nutritious fluid, holding in solution the ingredients necessary for the formation of the tissues. In all the higher animals and in man, the structure of the body is compound, consisting of various organs, with widely different functions, situated in different parts of the frame. In the intestine the process of digestion is accomplished, and the prepared ingredients of the food are thence absorbed into the bloodvessels, by which they are transported to distant parts. In the lungs the blood absorbs oxygen, which is afterward appropriated by the tissues; and the carbonic acid produced in the tissues is finally exhaled from the lungs. In the liver, the kidneys, and the skin, other substances are produced or eliminated, and these local processes are all necessary to the preservation of the general organization. The circulating fluid is therefore a means of transportation, by which substances produced in particular organs are dispersed throughout the body, or by which substances produced in the tissues generally are conveyed to particular organs, in order to be eliminated.

The circulatory apparatus consists of four different parts, namely, 1st. The heart; a hollow, muscular organ, which propels the blood. 2d. The arteries; a series of branching tubes, which convey it from the heart to different parts of the body. 3d. The capillaries; a network of inosculating tubules, interwoven with the substance of the tissues, which bring the blood into intimate contact with their component parts; 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 different parts of the circulatory apparatus, the movement of the blood is peculiar and dependent on special conditions.

The Heart.

The structure of the heart and of the adjacent vessels varies in different classes of animals, owing to the different arrangement of the respiratory organs.

In man and the mammalians the process of respiration is not only much more active than in cold-blooded animals, but the lungs are also the only special organs of aeration. The whole of the blood, accordingly, after returning from the general system, passes through the lungs before it is again distributed to the system. It thus traverses in succession the general circulation for the whole body, and the special circulation for the lungs. The mammalian heart (Fig. 99), consists of ( 318 )

a right auricle and ventricle (a, b), receiving the blood from the vena cava (i), and driving it to the lungs; and a left auricle and ventricle

(f, g) receiving the blood from the lungs and propelling it outward through the arterial sys

tem.

Fig. 99.

In the mammalian heart, the different parts of the organ present certain peculiarities and bear certain relations to each other, which influence its action and movements. The heart itself is suspended somewhat freely in the cavity of the chest, attached to the spinal column mainly by the great blood vessels passing through the superior and posterior mediastinum. It is of a more or less conical form; its base, situated upon the median line, being directed upward and backward, while its apex points downward, forward, and to the left, surrounded by the pericardium, but capable of a certain degree of lateral and rotatory motion. The auricles, which have a smaller capacity and thinner walls than the ventricles, are situated at the upper and posterior part of the organ (Figs. 100 and 101); while the ventri

CIRCULATION IN

Right auricle. b. Right ventricle. c. Pulmonary artery. d d Lungs. e. Pulmonary vein. f. Left auricle. g. Left ventricle. h. Aorta.

i.

Vena cava.

over, are not situated on the same plane. The right ventricle occupies

a position somewhat in front and above that of the left; so that in an anterior view of the heart the greater portion of the left ventricle is concealed by the right (Fig. 100), and in a posterior view the greater portion of the right ventricle is concealed by the left (Fig. 101); while in both positions the apex of the heart is constituted altogether by the point of the left ventricle.

The different cavities of the heart and of the adjacent blood vessels on each side, though continuous with each other, are partially separated by certain constrictions. The orifices by which they communicate are known by the names of the auricular, auriculo-ventricular, and aortic and pulmonary orifices; the auricular orifices being the passages from the vena cava and pulmonary veins into the right and left auricles; the auriculo-ventricular orifices leading from the auricles into the ventricles; and the aortic and pulmonary orifices leading from the ventricles into the aorta and pulmonary artery respectively.

The auriculo-ventricular, aortic, and pulmonary orifices are furnished with valves, which allow the blood to pass readily from the auricles to the ventricles, and from the ventricles to the arteries, but shut back in such a manner as to prevent its return in the opposite direction. The course of the blood through the heart is, therefore, as follows (Fig. 102):

Valves closed.

From the vena cava it passes into the right auricle; and from the right auricle into the right ventricle. On the contraction of the right ventricle, the tricuspid valves shut back, preventing its return into the auricle (Fig. 103); and it is thus driven through the pulmonary artery to the lungs. Returning from the lungs, it enters the left auricle, thence passes into the left ventricle, from which it is finally delivered into the aorta, and distributed throughout the body. The two streams of blood,

arterial and venous, in their passage through the heart, follow a course which is, in each case, curvilinear and more or less spiral in direction;

Valves open.

the axes of the currents crossing each other in the right and left cavities of the organ respectively (Fig. 104). The venous blood, received Fig. 104.

COURSE OF BLOOD THROUGH THE HEART.-a, a. Vena cava, superior and inferior. b Right ventricle. c. Pulmonary artery. d. Pulmonary vein. e. Left ventricle. f. Aorta. by the right auricle from the two venæ cave, passes downward and forward from the auricle into the ventricle. In the body of the right