Blood-Pressure in the Veins.-The blood-pressure gradually lessens as we proceed from arteries near the heart to those more remote, and again from these to the capillaries, and thence along the veins to the right auricle. The blood-pressure in the veins is nowhere very great, but is greatest in the small veins, while in the large veins towards the heart the pressure becomes negative, or, in other words, when a vein is put in connection with a mercurial manometer, the mercury will fall in the arm furthest away from the vein and will rise in the arm nearest the vein, which has a tendency to suck in rather than to push forward. In the large veins of the neck this tendency to suck in air is especially marked, and is the cause of death in some surgical operations in that region. The amount of pressure in the brachial vein is said to support 9 mm. of mercury, whereas the pressure in the veins of the neck is about equal to a negative pressure of 3 to 8 mm.

The variations of venous pressure during systole and diastole of the heart are very slight, and a distinct pulse is seldom seen in veins except under very extraordinary circumstances.

The formidable obstacle to the upward current of the blood in the veins of the trunk and extremities in the erect posture supposed to be presented by the gravitation of the blood, has no real existence, since the pressure exercised by the column of blood in the arteries, will be always sufficient to support a column of venous blood of the same height as itself: the two columns mutually balancing each other. Indeed, so long as both arteries and veins contain continuous columns of blood, the force of gravitation, whatever be the position of the body, can have no power to move or resist the motion of any part of the blood in any direction. The lowest blood-vessels have, of course, to bear the greatest amount of pressure; the pressure on each part being directly proportionate to the height of the column of blood above it hence their liability to distension. But this pressure bears equally on both arteries and veins, and cannot either move, or resist the motion of, the fluid they contain, so long as the columns of fluid are of equal height in both, and continuous.

Velocity of the Blood Current.

The velocity of the blood-current at any given point in the various divisions of the circulatory system is inversely proportional to their sectional area at that point. If the sectional area

of all the branches of a vessel united were always the same as that of the vessel from which they arise, and if the aggregate sectional area of the capillary vessels were equal to that of the aorta, the mean rapidity of the blood's motion in the capillaries would be the same as in the aorta and largest arteries; and if a similar correspondence of capacity existed in the veins and arteries, there would be an equal correspondence in the rapidity of the circulation in them. But the arterial and venous systems may be represented by two truncated cones with their apices directed towards the heart; the area of their united base (the sectional area of the capillaries) being 400-800 times as great as that of the truncated apex representing the aorta. Thus the velocity of blood in the capillaries is not more than of that in the aorta.

(a.) In the Arteries.-The velocity of the stream of blood is greater in the arteries than in any other part of the circulatory system, and in them it is greatest in the neighbourhood of the heart, and during the ventricular systole. The rate of movement diminishes during the diastole of the ventricles, and in the parts of the arterial system most distant from the heart. Chauveau has estimated the rapidity of the blood-stream in the carotid of the horse at over 20 inches per second during the heart's systole, and nearly 6 inches during the diastole (520-150 mm.).

Estimation of the Velocity.-Various instruments have been devised for measuring the velocity of the blood-stream in the arteries. Ludwig's "Stromuhr" (fig. 141) consists of a U-shaped glass tube dilated at a and a', the ends of which, 7 and i, are of known calibre. The bulbs can be filled by a common opening at k. The instrument is so contrived that at b and b', the glass part is firmly fixed into metal cylinders, attached to a circular horizontal table, e c', capable of horizontal movement on a similar table d d' about the vertical axis marked in figure by a dotted line. The opening in cc', when the instrument is in position, as in fig., corresponds exactly with those in d d'; but if c c' be turned at right angles to its present position, there is no communication between h and a, and i and a', but h communicates directly with i; and if turned through two right angles c' communicates with d, and c with d', and there is no direct communication between h and i. The experiment is performed in the following way :The artery to be experimented upon is divided and connected with two

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cannulæ and tubes which fit it accurately with h and i-h the central end, and i the peripheral; the bulb a is filled with olive oil up to a point rather lower than k, and a' and the remainder of a is filled with defibrinated blood; the tube on k is then carefully clamped; the tubes d and d' are also filled with defibrinated blood. When everything is ready, the blood is allowed to flow into a through h, and it pushes before it the oil, and that the defibrinated blood into the artery through i, and replaces it in a'; when the blood reaches the former level of the oil in a, the disc e c' is turned rapidly through two right angles, and the blood flowing through d into a' again displaces the oil which is driven into a. This is repeated several

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Fig. 142.-Diagram of Chauveau's Instrument. a. Brass tube for introduction into the lumen of the artery, and containing an index-needle, which passes through the elastic membrane in its side, and moves by the impulse of the blood-current. c. Graduated scale, for measuring the extent of the oscillations of the needle.

times, and the duration of the experiment noted. The capacity of a and a' is known; the diameter of the artery is also known by its corresponding with the cannula of known diameter, and as the number of times a has been filled in a given time is known, the velocity of the current can be calculated.

Chauveau's instrument (fig. 142) consists of a thin brass tube, a, in one side of which is a small perforation closed by thin vulcanised indiarubber. Passing through the rubber is a fine lever, one end of which, slightly flattened, extends into the lumen of the tube, while the other moves over the face of a dial. The tube is inserted into the interior of an artery, and ligatures applied to fix it, so that the movement of the blood may, in flowing through the tube, be indicated by the movement of the outer extremity of the lever on the face of the dial.

The Hæmatochometer of Vierordt, and the instrument of Lortet, resemble in principle that of Chauveau.

(b.) In the Capillaries.-The observations of Hales, E. H. Weber, and Valentin agree very closely as to the rate of the bloodcurrent in the capillaries of the frog; and the mean of their estimates gives the velocity of the systemic capillary circulation at about one inch (25 mm.) per minute. The velocity in the capil

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laries of warm-blooded animals is greater. In the dog to 180 inch (5 to 73 mm.) a second. This may seem inconsistent with the facts, which show that the whole circulation is accomplished in about half a minute. But the whole length of capillary vessels, through which any given portion of blood has to pass, probably does not exceed from th to th of an inch (5 mm.); and therefore the time required for each quantity of blood to traverse its own appointed portion of the general capillary system will scarcely amount to a second.

(c.) In the Veins.-The velocity of the blood is greater in the veins than in the capillaries, but less than in the arteries: this fact depending upon the relative capacities of the arterial and venous systems. If an accurate estimate of the proportionate areas of arteries and the veins corresponding to them could be made, we might, from the velocity of the arterial current, calculate that of the venous. A usual estimate is, that the capacity of the veins is about twice or three times as great as that of the arteries, and that the velocity of the blood's motion is, therefore, about twice or three times as great in the arteries as in the veins, 8 inches (about 200 mm.) a second. The rate at which the blood moves in the veins gradually increases the nearer it approaches the heart, for the sectional area of the venous trunks, compared with that of the branches opening into them, becomes gradually less as the trunks advance towards the heart.

(d.) Of the Circulation as a whole.—It would appear that a portion of blood can traverse the entire course of the circulation, in the horse, in half a minute. Of course it would require longer to traverse the vessels of the most distant part of the extremities than to go through those of the neck: but taking an average length of vessels to be traversed, and assuming, as we may, that the movement of blood in the human subject is not slower than in the horse, it may be concluded that half a minute represents the average rate.

Satisfactory data for these estimates are afforded by the results of experiments to ascertain the rapidity with which poisons introduced into the blood are transmitted from one part of the vascular system to another. The time required for the passage of a solution of potassium ferrocyanide, mixed with the blood, from one jugular vein (through the right side of the heart, the pulmonary vessels, the left cavities of the heart, and the general circulation) to the jugular vein of the opposite side,

varies from twenty to thirty seconds. The same substance was transmitted from the jugular vein to the great saphena in twenty seconds; from the jugular vein to the masseteric artery, in between fifteen and thirty seconds; to the facial artery, in one experiment, in between ten and fifteen seconds; in another experiment in between twenty and twenty-five seconds; in its transit from the jugular vein to the metatarsal artery, it occupied between twenty and thirty seconds, and in one instance more than forty seconds. The result was nearly the same whatever was the rate of the heart's action.

In all these experiments, it is assumed that the substance injected moves with the blood, and at the same rate, and does not move from one part of the organs of circulation to another by diffusing itself through the blood or tissues more quickly that the blood moves. The assumption is sufficiently probable, to be considered nearly certain, that the times above mentioned, as occupied in the passage of the injected substances, are those in which the portion of blood, into which each was injected, was carried from one part to another of the vascular system.

Another mode of estimating the general velocity of the circulating blood, is by calculating it from the quantity of blood supposed to be contained in the body, and from the quantity which can pass through the heart in each of its actions. But the conclusions arrived at by this method are less satisfactory. For the total quantity of blood, and the capacity of the cavities of the heart, have as yet been only approximately ascertained. Still the most careful of the estimates thus made accord very nearly with those already mentioned; and it may be assumed that the blood may all pass through the heart in from twenty-five to fifty seconds.

Local Peculiarities of the Circulation.

The most remarkable peculiarities attending the circulation of blood through different organs are observed in the cases of the brain, the erectile organs, the lungs, the liver, and the kidneys.

1. In the Brain.-For the due performance of its functions, the brain requires a large supply of blood. This object is effected through the number and size of its arteries, the two internal carotids, and the two vertebrals. It is further necessary that the