the direct application of the laws of filtration and diffusion to the explanation of the composition of lymph, but it is a point upon which more information is necessary before it alone can be accepted as a basis for a secretion theory. Meanwhile it seems evident that in spite of the very valuable work of Heidenhain, which has added so much to our knowledge of the conditions influencing the formation of lymph, the existence of a definite secretory activity of the endothelial cells of the capillaries has not been proved.

Summary of the Factors Controlling the Flow of Lymph.—We may, therefore, adopt, provisionally at least, the so-called mechanical theory of the origin of lymph. Upon this theory the forces in activity are, first, the intracapillary pressure tending to filter the plasma through the endothelial cells composing the walls of the capillaries; second, the force of diffusion depending upon the inequality in chemical composition of the blood-plasma and the liquid outside the capillaries, or, on the other side, between this liquid and the contents of the tissue-elements; third, the force of osmotic pressure. These three forces acting every where control primarily the amount and composition of the lymph, but still another factor must be considered. For when we come to examine the flow of lymph in different parts of the body striking differences are found. It has been shown, for instance, that in the limbs, under normal conditions, the flow is extremely scanty, while from the liver and the intestinal area it is relatively abundant. In fact, the lymph of the thoracic duct may be considered as being derived almost entirely from the latter two regions. Moreover, the lymph from the liver is characterized by a greater percentage of proteids. To account for these differences Starling suggests the plausible explanation of a variation in permeability in the capillary walls. The capillaries seem to have a similar structure all over the body so far as this is revealed to us by the microscope, but the fact that the lymph-flow varies so much in quantity and composition indicates that the similarity is only superficial, and that in different organs the capillary walls may have different internal structures, and therefore different permeabilities. This factor is evidently one of great importance. From the foregoing considerations it is evident that changes in capillary pressure, however produced, may alter the flow of lymph from the blood-vessels to the tissues, by increasing or decreasing, as the case may be, the amount of filtration; changes in the composition of the blood, such as follow periods of digestion, will cause diffusion and osmotic streams tending to equalize the composition of blood and lymph; and changes in the tissues themselves following upon physiological or pathological activity will also disturb the equilibrium of composition, and, therefore, set up diffusion and osmotic currents. In this way a continual interchange is taking place by means of which the nutrition of the tissues is effected, each according to its needs. The details of this interchange must of necessity be very complex when we consider the possibilities of local effects in different parts of the body. The total effects of general changes, such as may be produced experimentally, are simpler, and, as we have seen, are explained satisfactorily by the physical and chemical factors enumerated.

III. CIRCULATION.

PART I.-THE MECHANICS OF THE CIRCULATION OF THE BLOOD AND OF THE MOVEMENT OF THE LYMPH.

A. GENERAL CONSIDERATIONS.

THE metaphorical phrase "circulation of the blood" means that every particle of blood, so long as it remains within the vessels, moves along a path which, no matter how tortuous, finally returns into itself; that, therefore, the particles which pass a given point of that path may be the same which have passed it many times already; and that the blood moves in its path always in a definite direction, and never in the reverse.

The discoverer of these weighty facts was "William Harvey, physician, of London," as he styled himself. In the lecture notes of the year 1616, mostly in Latin, which contain the earliest record of his discovery, he declares that a "perpetual movement of the blood in a circle is caused by the beat of the heart" ("perpetuum sanguinis motum in circulo fieri pulsu cordis ").1 For a long time afterward the name of the discoverer was coupled with the expression which he himself had introduced, and the true movement of the blood was known as the "Harveian circulation."

Course of the Blood.-The metaphorical circle of the blood-path may be shown by such a diagram as Figure 8.

If, in the body of a warm-blooded animal, we trace the course of a given particle, beginning at the point where it leaves the right ventricle of the heart, we find that course to be as follows: From the trunk of the pulmonary artery (PA) through a succession of arterial branches derived therefrom into a capillary of the lungs (PC); out of that, through a succession of pulmonary veins, to one of the main pulmonary veins (PV) and the left auricle of the heart (LA); thence to the left ventricle (LV); to the trunk of the aorta (A); through a succession of arterial branches derived therefrom into any capillary (C) not supplied by the pulmonary artery; out of that, through a succession of veins (V) to one of the vena cava or to a vein of the heart itself; thence to the right auricle (RA), to the right ventricle (RV), and to the trunk of the pulmonary artery, where the tracing of the circuit began.

1 William Harvey: Prelectiones Anatomia Universalis, edited, with an autotype reproduction of the original, by a committee of the Royal College of Physicians of London, 1886, p. 80.

2 Harvey's discovery of the circulation was first published in the modern sense in his work Exercitatio anatomica de motu cordis et sanguinis in animalibus, Francofurti, 1628. This great classic can be read in English in the following: On the Motion of the Heart and Blood in Animals. By William Harvey, M. D.; Willis's translation, revised and edited by Alex. Bowie, 1889.

76

It must be noted here that a particle of blood which traverses a capillary of the spleen, of the pancreas, of the stomach, or of the intestines, and enters the portal vein, must next traverse a series of venous branches of diminishing size, and a capillary of the liver, before entering the succession of veins which will conduct the particle to the ascending vena cava (compare Figs. 8 and 9).

An

rangement is of extreme importance for the physiology of nutrition. arrangement of the same order, though less conspicuous, exists in the kidney.

Causes of the Blood-flow.-The force by which the blood is driven from the right to the left side of the heart through the capillaries which are related to the respiratory surface of the lungs, is nearly all derived from the contraction of the muscular wall of the right ventricle, which narrows the cavity thereof and ejects the blood contained in it; the force by which the blood is driven from the left to the right side of the heart through all the other capillaries of the body, often called the "systemic" capillaries, is derived nearly all from the contraction of the muscular wall of the left ventricle, which narrows its cavity and ejects its contents. The contractions of the two ventricles are simultaneous. The force derived from each contraction is generated by the conversion of potential energy, present in the chemical constituents of the muscular tissue, into energy of visible motion; a part also of the potential energy at the same time becoming manifest as heat. In the maintenance of the circulation the force generated by the heart is to a very subordinate degree supplemented by the forces which produce the aspiration of the chest and by

the force generated by the contractions of the skeletal muscles throughout the body (see p. 95).

Mode of Working of the Pumping Mechanism.—During each contraction or "systole" of the ventricles the blood is ejected into the arteries only, because at that time the auriculo-ventricular openings are each closed by a valve. During the immediately succeeding "diastole" of the ventricles, which consists in the relaxation of their muscular walls and the dilatation of their cavities, blood enters the ventricles by way of the auricles only, because at that time the arterial openings are closed each by a valve which was open during the ventricular systole; and because the auriculo-ventricular valves which were closed during the systole of the ventricles are open during their diastole. During the first and longer part of the diastole of the ventricles the auricles, too, are in diastole; the whole heart is in repose; and blood is not only entering the auricles, but passing directly through them into the ventricles. Near the end of the ventricular diastole a brief simultaneous systole of both auricles takes place, during which they, too, narrow their cavities by the muscular contraction of their walls, and eject into the ventricles blood which had entered the auricles from the "systemic" and pulmonary veins respectively. The systole of the auricles ends immediately before that of the ventricles begins. The brief systole of the auricles is succeeded by their long diastole, which corresponds in time with the whole of the ventricular systole and with the greater part of the succeeding ventricular diastole. During the diastole of the auricles blood is entering them out of the veins. Thus it is seen that the direction in which the blood is forced is essentially determined by the mechanism of the valves at the apertures of the ventricles; and that it is due to these valves that the blood moves only in the definite direction before alluded to. In the words, again, of Harvey's note-book, at this point written in English, the blood is perpetually transferred through the lungs into the aorta as by two clacks of a water bellows to rayse water."

66

Pulmonary Blood-path.-In the birds and mammals the entire breadth of the blood-path, at one part of the physiological circle, consists in the capillaries spread out beneath the respiratory surface of the lungs. The right side of the heart exists only to force the blood into and past this portion of its circuit, where, as in the systemic capillaries, the friction due to the fineness of the tubes causes much resistance to the flow. This great comparative development of the pulmonary portion of the blood-path in the warm-blooded vertebrates is related to the activity, in them, of the respiration of the tissues, which calls for a corresponding activity of function at the respiratory surface of the lungs, and for a rapid renewal in every systemic capillary of the internal respiratory medium, the blood. This rapid renewal implies a rapid circulation; and that the speed is great with which the circuit of the heart and vessels is completed has been proven by experiment, the method being too complicated for description here.2 1 Prelectiones, etc., p. 80.

2 Karl Vierordt: Die Erscheinungen und Gesetze der Stromgeschwindigkeiten des Blutes. 2te Ausgabe, 1882.

Rapidity of the Circulation.-By experiment the shortest time has been measured which is taken by a particle of blood in passing from a point in the external jugular vein of a dog to and through the right cavities of the heart, the pulmonary vessels, the left cavities of the heart, the commencement of the aorta, and the arteries, capillaries, and veins of the head, to the startingpoint, or to the same point of the vein of the other side. This time has been found to be from fifteen to eighteen seconds. Naturally, the time would be different in different kinds of animals and in the different circuits in the same individual.

Order of Study of the Mechanics of the Circulation.-The significance and the fundamental facts of the circulation have now been indicated. Its phenomena must next be studied in detail. As the blood moves in a circle, we may, in order to study the movement, strike into the circle at any point. It will, however be found both logical and instructive to study first the movement of the blood in the capillaries, whether systemic or pulmonary. It is only in passing through these and the minute arteries and veins adjoining that the blood fulfils its essential functions; elsewhere it is in transit merely. Moreover, it is only in the minute vessels that the blood and the nature of its movement are actually visible.

After the capillary flow shall have become familiar, it will be found that the other phenomena of the circulation will fall naturally into place as indicating how that flow is caused, is varied, and is regulated.

B. THE MOVEMENT OF THE BLOOD IN THE CAPILLARIES AND IN THE MINUTE ARTERIES AND VEINS.

Characters of the Capillaries.-Each of the vessels which compose the Emmensely multiplied capillary network of the body is a tube, commonly of less than one millimeter in length, and of a few one-thousandths only of a millimeter in calibre, the wall of which is so thin as to elude accurate measure

FIG. 10-A capillary from the mesentery of the frog (Ranvier).

The calibre of each capillary may vary from time to time. These facts indicate the minute subdivision of the blood-stream in the lungs, and among the tissues-that is, at the two points of its course where the essential functions of the blood are fulfilled. These facts also show the shortness of

1 The following is a very valuable book of reference: Robert Tigerstedt: Lehrbuch der Physiologie des Kreislaufes, 1893.