The clamour that this publication excited was inconceivable. It was either not true, or the ancients had already taught the same. Riolan, a more respectable antagonist than the common herd, was alone honoured with an answer: and the venerable antagonist retired with some disgrace from the field, accused of cavilling, unmeaning quibbles, rash unfounded assertions, and even unfaithful experiments. The number and names of his other antagonists which lie before us would fill our page, with little advantage or satisfaction to our readers.

During the dilatation of the heart, when the blood enters the ventricles the coronary arteries receive that fluid, contrary to all the other arteries of the body, and thus supply the muscular fibres of the heart with the blood; for the passage of the blood is freer through the arteries during the heart's inflation than at the contraction, because those vessels are then less convoluted. That the heart is not the one and sole cause of circulation appears, because the arteries all perform their diastole at the same instant in healthy people. If the heart's propelling the blood was the sole cause of the circulation, the pulsation of the artery would be an undulation, and in different parts it would be perceived at different times, as the impulse at different distances of the artery from the heart would be in succession.

That some other power than the velocity of the blood dilates the capillary arteries to give passage to the globules, seems evident also from the experiments of Dr. Hales. He poured water into the aorta and other arteries of dogs; and though its force and velocity were equal to that given to the blood by the heart, yet it never passed by the anastomoses of the arteries and veins, but through the sides of the arteries; and this seems to prove that the arteries arc totally stopped by the contraction of their fibres after the vital power no longer continues to act, and that the force of the heart hath not a power equal to what is required to dilate them. To this experiment indeed objections may be made; but strong arguments may be adduced to render the principal position highly probable. It is supported with great force by Dr. Whytt.

The whole arterial tube, therefore, contributes to the emotion of the blood; and the heart, instead of moving a weight equal to the whole mass in this way, impels no more than about two ounces, the quantity supposed to be contained by the ventricle in each diastole. Sec Animalis motus, also Haller's Physiology.

The laws of the circulation, or the general circumstances that influence its various modifications, must detain us a little longer. Physiologists have anxiously endeavoured to ascertain the quantity of blood thrown out by each contraction of the heart, as well as its velocity. But these circumstances are of little real importance, and we need not examine nor attempt to refute the calculations and errors. It is enough that the left ventricle contains about two ounces; and that probably somewhat less is thrown out at each contraction. Whatever the velocity may be, it is gradually diminishing; for the areas of all the branches exceed that of the aorta, and the angles must sometimes impede rectilineal motion. A proportion of the impetus is in this way lost; nor is the resistance of the coats of the arteries, or the friction, to be wholly overlooked; though the elasticity and the muscular power, as well as the mucous secretion on the internal surfaces, greatly lessen its effects.

The velocity of the blood in the arteries will be in proportion to the frequency of contraction; and this in proportion to the return of blood in the veins, which is influenced by a variety of causes, chiefly exercise and agitation of mind. The frequency of the contraction, which arises from irritability, does not increase the velocity of the blood, since, in such instances, the left ventricle contracts before it is filled, and this state is not attended with a determination to the surface. In a healthy state the arteries are always full, and consequently each impulse gives successive momenta to the whole mass: but this succession is so rapid, and the action of the arterial coats so immediate, that the pulsation, which is the consequence, is apparently synchronous over the whole body. As, however, the velocity diminishes from the causes mentioned, this pulsation must be at last imperceptible; and at some distance from the heart, and more particularly in the veins, it is of course lost. The velocity of the blood will therefore vary in proportion to the power of the heart, to the distance from it, to the causes influencing the action of the arteries, and to the direction as affected by gravity.

The quantity of blood distributed to any part of the system will differ in proportion to the action of the arteries of that part; an action increased by resistance of every kind: but the effects of that resistance, at first owing to the action of the arteries of that part, and afterwards to the general consent of every part of the circulating system, is little affected by the state of the circulation in a distant part. The contrary idea has induced the most singular and preposterous practice. It is the parent of the doctrines of derivation and revul-sion, which we shall afterwards notice. Thus, if the head was affected, blood was drawn from the feet; but it will be at once obvious, that sixteen ounces of blood from a vein, if affecting one thousand six hundred arteries, will lessen the quantity of blood in each only 0.01; consequently, on the contraction of the left ventricle, only one hundredth part less of blood will be sent to the head. But if the sixteen ounces be taken from the temporal arteries, or jugular veins, the head will be depleted in the same proportion, without any diminution. It is to be regretted that this idea continues to prevail among practitioners; and we still find blisters and cataplasms applied to the legs to relieve congestions in the head tor lungs.

The quantity of blood distributed to different parts varies at different periods. In the growing state, the heart evidently increases in its bulk in a less proportion than the capacity of the arterial system. As age approaches, the number of the arteries lessens, and the proportion of the heart gains the ascendancy. In this state the venous system is proportionably fuller than the arterial. In young animals the head is large, and its vessels full. Diseases of the head, from fulness and haemorrhages from the nose, are then common. At a subsequent period the determination is to the lungs, and soon afterwards to the genital system in both sexes; at a more advanced period to the haemorrhoidal vessels. When the number of arteries diminishes from age, we find venous plethora in the head, with serous apoplexies and palsies; in the lungs, with humoral asthmas and catarrhus suffocativus; in the abdomen, with discharges of black bile; in the extremities, with varices.

Any general increase of the action of the arteries determines the blood rather to the surface than the internal parts; but, if checked in its determination to the surface, or irregularly accelerated or retarded, the viscera chiefly suffer. If, from the continued action of any cause, a fixed determination to any part is established, it becomes a necessary part of the constitution, and cannot without danger be altered.

The whole of the blood sent from the heart is not returned to that organ by the veins. The exhalations from the arteries into the cellular substance employ a part of it; the various secretions also greatly lessen it. The arterial system, however, always continues full, in consequence of the contractility of its muscular coat. The venous system has not this advantage, but the motion of the blood is slower in these; and, as it is kept up by the pulsation of the arteries,muscular action, etc. while advantage is taken of every action by the frequent interposition of valves, these vessels must continue full, since, from the want of any active force, a portion must be discharged into the heart, before that below can be propelled forward.

Some other circumstances respecting the circulation can only be understood when the structure of the heart is known. See Cob.

We have remarked, that there is some doubt whether arteries terminate by continuous vessels in veins: in the corpora cavernosa penis they do not, and the veins there certainly absorb the effused blood. The course of the circulation also, when minutely examined, is not regularly progressive. It sometimes is retrogade for a little way, favoured by an anastomosis, chiefly when the vessel will not admit the red globules. The veins too do not all pass immediately to the heart; for, as we have remarked, those of the abdomen unite in forming the vena portae dispersed in the liver, apparently for the secretion of bile.

The circulation of the blood in the foetus hath some peculiarities different from what is observed in adults. 1st, The blood does not all pass through the lungs; a very small part only takes that course each time that it returns to the heart. 2dly, The blood brought by the two venae cavae into the right auricle of the heart passes chiefly into the right ventricle, but not entirely; for some portion goes immediately through the foramen ovale into the left auricle, and especially that brought up by the cava inferior. Suppose, then, two-thirds of the blood passes into the right ventricle, in order to circulate through the pulmonary artery; yet all the blood that flows into it in the foetus will not circulate through the

3 L 2 lungs, for a considerable part must necessarily pass by the ductus arteriosus, directly to the aorta, before it hath arrived at the lungs; so that probably not above one-third of the blood circulates through the lungs every time it is brought back to the heart. That blood which was thrown out directly from the right to the left auricle, goes thence to the left ventricle, and so on to the aorta, without touching at either the right ventricle or pulmonary artery, and consequently not arriving at the lungs. After the child is born, and a little grown up, the foramen ovale is closed up in most subjects; though, in some instances, it is found to continue more or less open during the whole life of the person.