Stagyrite, as they were to his follower Cesalpino, they were not only none to our independent inductive philosopher, but are adduced by him as physiological necessities in the nature of things.

That there is the freest possible communication between the arteries and veins of the animal body is amply demonstrated, as Galen showed, by the fact that both are drained of their contents whether a branch of one or other be divided and suffered to bleed for a sufficient length of time. "Now," says Harvey, "if, when performing phlebotomy, we assume the blood abstracted to represent the quantity that would have flowed naturally through the limb during the time employed in the business, double this to get at so much as would have passed through the other arm, calculate how much more would have gone through the lower limbs, the abdominal viscera, the lungs and the head-we shall, in another way, arrive at something like an idea of the quantity of blood that must leave and return to the heart in a given interval of time, and reach the conviction that its motion in a ceaseless circuit upon itself, as it were, will alone account for the large amount proclaimed by our figures." The premisses of no physiologist before Harvey could possibly have led him to an induction such as this.

In what precedes, we are led by reasoning upon obvious phenomena to conclude, that the blood

propelled by the heart returns to it again and again by the veins. Happily the structure of the veins makes this conclusion a readily demonstrable truth; for within them, in those of the extremities especially, there are found numerous loose folds or floating portions of their inner membrane, of a semilunar shape, and having their free edges turned towards the trunks of the vessels. Sometimes single but more commonly double or in pairs, they then arise from opposite sides of the vein, and meet by their free edges when brought into action. The arrangement of these folds is such that, whilst they present not the slightest impediment to the flow of the blood from the smaller to the larger branches, they effectually oppose its motion in the opposite sense, or from trunks to branches.

These membranulæ, ostiola, or valves of the veins were observed, as we have seen, by more than one of the earlier anatomists-by Sylvius, Eustachius, Paul Sarpi, and others; but they were first particularly described and figured by Fabricius of Aquapendente, who is generally credited with having discovered them. But neither Fabricius nor any of his predecessors understood their function or their real importance-Cesalpino does not even notice them in his account of the arm bound by the bleeding fillet, and this he would hardly have failed to do had he entertained the ideas of the circulation that have been ascribed to him. The ostiola, by Fabricius and others his contemporaries, were believed merely to retard

the flow of the blood in the veins, from the centre to the periphery of the body, so as to give the several parts time to appropriate the nutriment they required; or they were held to prevent the blood from overburthening dependent parts by its weight, or even from gravitating entirely into these.

In the lower animals, however, there are valves in situations where no effect of gravity from position was to be apprehended. The main duty of the valves of the veins, in a word, is to prevent the blood from flowing from trunks to branches, from central towards peripheral parts: offering no impediment to the current inwards, they obstinately resist efforts to force it outwards, and accurately represent the floodgates at the mouth of a stream, which give way to the efflux but close against the reflux.

This truth, Harvey goes on to say, is readily demonstrated in the arm bound as for blood-letting. The veins are then seen turgid and with knots or swellings at intervals in their course, at the points especially where one branch joins another. The knots in question mark the positions of valves; a fact which is immediately made manifest if an attempt be made to force the blood in one of the vessels from above downwards by the pressure of a finger. The valve nearest the part below the point of pressure starts at once into action, and can be felt distinctly as a hard, resistant knot. If the pressure be now reversed, the vein being compressed by the point of a finger beyond

a valve, the blood within it will then be easily streaked upwards till it passes the valve above, when the part of its canal between this and the point of pressure will not only be emptied, but will so remain whilst the pressure is continued. The pressure below being now withdrawn, the empty vein fills instantly and looks turgid as before.

The valves of the veins, consequently, act no other

wise than do the sigmoid valves at the roots of the aorta and pulmonic artery, and as the mitral and tricuspid valves between the ventricles and auricles of the heart: they offer no impediment to the flow of the blood in the direction of their trunks, but oppose effectual barriers to its course towards their branches; the effect of which is, that the blood poured by the terminal capillaries of the arteries into the initial

capillaries of the veins, can only move in one direction outward ever from the heart by the arteries, the efferent vessels; inward ever towards the heart, by the veins, the afferent vessels. In the words of our author, we must therefore conclude that 66 THE BLOOD IN THE ANIMAL BODY IS SUBJECTED TO A CERTAIN CIRCULAR MOTION; THAT THIS MOTION IS INCESSANT; AND THAT THE PULSE OR CONTRACTION OF THE HEART IS ITS SOLE EFFICIENT CAUSE.

of the heart as the impelling power in the circulation of the blood; a conclusion in which it is to be

1 "Necessarium est concludere circulari quodam motu in circuitu agitari in animalibus sanguinem, et esse in perpetuo motu; et hanc esse actionem sive functionem cordis quam pulsu peragit, et omnino motus et pulsus cordis causam unam esse." (Exerc., cap. xiv. ad fin.)