is stated on this subject in the article ANATOMY.

The admission of matter into the orifices of the absorbing vessels has been accounted for in various ways. Some physiologists consider it as a case of capillary attraction. But a little reflection is sufficient to shew that the absorbents are not, like capillary tubes, immersed in a fluid. Besides, were such attraction the cause of absorption, that process should be carried on with regularity. On the contrary, absorption is occasionally very deficient, when abundance of fluids is presented to the mouths of the vessels, as in ædema; and in other cases, after being for a long time inactive, it is suddenly exerted to a great extent; thus large abscesses have been dispersed in one night. Others have endeavoured to discover some propelling power, which should protrude the matter subject to absorption into the mouths of the vessels. The pressure of the atmosphere on the surface of the body has been considered adequate to this effect, and the deposition of new matter by the secerning artery has been assigned as the cause of the propulsion of the old particles into the orifice of the absorbent. On this theory, secretion and absorption should correspond more exactly than they are known to do. Mr. J. Hunter acknowledged that he was unable to account for the effects produced, unless by attributing to the mouths of the absorbing vessels powers similar to those which a caterpillar exerts when feeding on a leaf.

Some suppose that the absorbents cannot take up any matter that is not fluid; consequently, that animal solids must be converted into fluids before they can become fit subjects for absorption; and that probably some solvent fluid is secreted for this purpose. The latter fact rests on no direct proof, and the whole hypothesis is very unlike the simplicity observable in other parts of the animal economy. It seems better in these difficult investigations, to note facts, than to form theories; and whoever contemplates the things done in the animal body will be astonished at the power of the vessels, by whose agency they must be effected; a whole bone may perish, as, for example, that of the thigh, and may be increased by a new one; the vascular lining of the new bone will altogether remove the dead one.

Besides the great and leading office of the absorbents in conveying the chyle into the venous system, their agency is

discerned in various other parts of the animal economy. The nearly transparent fluid that lubricates the interstices of the cellular substance, and the serous exhalation poured into circumscribed cavities, are taken up by the lymphatics, which must commence in all parts of the body by open orifices. When the due balance does not exist between the absorbing and secreting vessels, the cellular substance becomes loaded with fluid (anasarca), or circumscribed cavities are rendered dropsical. Together with the lymph or fluid which the absorbing vessels derive from the sources just mentioned, they convey from every part of the body the old constituent materials of our organs, in proportion as new particles are deposited by the arteries; and these different elements are intimately mingled and combined in their passage through the absorbent glands, and the plexures of lympathic

vessels.

It has been a disputed point, whether absorption goes on from the surface of the skin, while the cuticle is entire; the arguments on the affirmative sides are an

alleged increase of weight in the body after a walk in damp weather; the abun dant secretion of urine after remaining for some time in a bath; the evident swelling of the inguinal glands after a long immersion of the lower extremities in warm water; the effects of mercury administered by friction, fumigation, &c. It has been stated in opposition, that oil of turpentine has not been absorbed after long immersion of the arm; that solutions of medicated substances have not been taken up under similar circumstances, &c. We think it is sufficiently proved, that absorption from the surface does take place in the human body; but whether this extends, as a modern physiologist has stated, to gaseous bodies, cannot yet be decided.

It appears probable that the internal surface of the bronchi and pulmonary vesicles is an absorbing surface. For when a person breathes air loaded with the vapour of turpentine, that substance very speedily shews itself in the urine, although the skin will not take it up. If the body really increase in weight in damp air, it might be accounted for by means of pulmonary absorption. It must probably be rather in this way, or by the skin, that contagious matters affect the constitution.

The absorbents are concerned in pro ducing changes in the different secreted Aluids: they remove the aqueous portions

of the bile and urine, and often take up even the colouring parts of the former, and convey them into blood, whence they are deposited in all our organs, and produce the yellowness of jaundice. They introduce various diseases into the human frame, as syphilis, hydrophobia, inoculated small-pox, &c.; and in other instances act in a curative manner, by taking up extravasated blood, by reducing swollen parts, &c.

The circulation is the motion by which the blood setting out from the heart, is constantly carried to all parts of the body in the arteries, and returns to the same point in the veins. The uses of this circulatory motion are, to submit the blood altered by the mixture of lymph and chyle to the contact of the atmosphere in the lungs (respiration), to convey it to several organs in which various animal fluids are separated from it (secretion); and to every part of the body, for supplying its growth, and repairing its losses, by means of its nutritive particles when completely assimilated (nutrition). The conveyance or transport of our fluids, rather than their elaboration, is the office of the organs of circulation. In this view they may be compared to those labourers, who, in a large manufactory, from which various products issue, carry the materials to the workmen employed in the actual fabrication. As among the latter there are some who purify and bring to perfection the materials furnished by others, so the lungs and secretory glands are constantly employed in separating from the blood all those heterogeneous matters, which could not be assimilated to the substance of our organs.

The word circulation, when used absolutely, comprehends the whole course of the blood, as well in the lungs, as in the arteries and veins of the body at large. The greater circulation is the passage of the blood from the left side of the heart, through the arteries, to the extremities of the body, and its return through the veins to the right side of the same viscus. The lesser circulation is the transmission of the blood from the right to the left side of the heart, through the lungs.

The course, which the blood takes, has been already explained in the article ANATOMY. We subjoin the proofs and experiments, by which the facts there stated are supported.

The passage of the blood through the heart, i. e. from the right auricle to the left ventricle, by the medium of the lungs, is manifest from the structure of the VOL. X.

heart itself. The valves, which are plac ed at its various apertures, actually will not admit of the blood's motion in any other direction than what we have de scribed. That this fluid passes from the heart into the trunk of the aorta, thence into its branches, and so on to the most minute ramifications, is evinced; 1. By the effect of ligatures on these vessels; the artery becomes turgid between the heart and the ligature, and empty between the ligature and its distribution. 2. By opening an artery when tied, above and below the ligature; the blood in this case flows from that opening only, which is nearest to the heart. 3. By ocular testimony; the passage of the blood can be seen by the aid of glasses in frogs, fishes, &c. The passage of the blood through the veins, in a contrary course to that in which it flows along the arteries, i. e. from the minute ramifications towards the trunks, and thence to the heart, is proved. 1. By the structure and disposition of the valves, which afford an invincible impediment to all retrograde motion. 2. By ligatures on these vessels, which make the vein turgid between the extremities of the body and the ligature, and empty in the rest of its course. 3. By opening a vein, when tied above and below the ligature. 4. By microscopical observation in animals.

The passage of the blood from the ar teries into the veins seems to flow as a corollary, from what we have stated concerning the proofs of its course in these two systems of vessels. We have shown that the ultimate arteries are continuous with the origins of the veins; that the blood moves from the heart to the extremities in the former vessels, and that it passes from the extremities to the heart in the latter. The intermediate passage is a direct consequence of these facts. But it may be demonstrated by direct proofs, independently of this argument. If we tie the artery of a part, its correspondent vein receives no blood; if we take off the ligature, the vein is again filled. The quantity of blood expelled from the aortic ventricle is so considerable, that the supply can only be kept up by its return to the heart. We calculate that two ounces are sent into the aorta at each pulsation; if we suppose 80 pulsations in a minute, 9,600 ounces will be thrown out in an hour and 14,400 pounds in a day. The same blood, therefore, which the aorta received from the heart, must be returned to this viscus; and the only passage by which it can return, is through

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the veins. Lastly, the passage of the blood from the arteries into the veins, may be proved by the direct testimony of the senses in living animals. The use of the microscope affords this proof in the transparent parts of cold-blooded animals, as the mesentery and web of the foot in frogs, the tail of fishes, &c.

The motions of any part of the heart, considered singly and individually, consist in a constant series of alternate contractions and dilatations; or, as they are technically named, alternate states of systole and diastole. The contractions take place as in other muscles; the dilating cause consists in the forcible entrance of blood into the cavity. The auricles and ventricles, when viewed in relation to each other, are successively contracted and dilated; the corresponding parts acting at the same time on both sides of the heart. Thus, when the auricles contract, in order to expel the blood which they have received from the system at large, and from the lungs, the ventricles are relaxed, and therefore in a state fit for receiving this blood. When, in the following moment, the recently filled ventricles contract, in order to urge forwards the blood into the two arterial trunks, the auricles are relaxed, and become immediately distended by the current of venous blood.

The action of the heart, and of the vessels connected with it, may therefore be distributed into successive periods. In the first of these, the vena cava and pulmonary veins pour their blood into the two auricles, and thus cause a diastole of these cavities. The systole of the auricles transmits the blood into the ventricles in the second period; and these latter cavities expel their contents into the arteries in the third portion of time. Thus the action of the veins takes place at the same point of time with that of the ventricles; and the contraction of the auricles is synchronous with that of the arteries.

The systole of the ventricles, which is supposed to occupy one-third of the time of the whole pulsation of the heart, is accomplished by an approximation of the sides of the cavities to the middle partition, and of the apex to the basis of the heart. The whole viscus by this means becomes shorter and more obtuse. The well known fact of the heart striking against the left breast in its contraction, may seem on the first glance to refute this account of the systole of the ventricles. But, on a further examination, it can have no such effect; since the phenomenon in question depends on two causes amply sufficient to produce the

effect. The swelling of the auricles, which are at the back of the heart, and particularly of the left auricle, which is interposed between the spine and the base of the left ventricle, necessarily causes the point of the heart to advance towards the side; and this motion may be imitated in the dead body by injecting or inflating the muscles. The other cause consists in the connection of the large arteries, particularly of the aorta, with the base of the heart. A curved and flexible tube, when suddenly distended, becomes in some measure straightened. Thus when the blood is impelled into the aorta, the curve of that vessel approaches more nearly to a straight line. Its posterior end being firmly attached to the vertebræ, is immovable; to its anterior and moveable part is fixed the heart, which, by the straightening of the vessel, is obliged to describe a portion of a circle, in doing which, the apex strikes against the side. These two circumstances occur simultaneously: the venous blood rushes into the auricles at the same time that the ventricle fills the aorta. The impulse of the blood expelled by the aortic ventricle is felt in the whole arterial system; and it produces in all arteries, which come within the sphere of the touch, and which have an area of not less than one-sixth of a line in diameter, an obvious and perceptible effect, called the pulse, which is a real state of diastole of the artery, and which is ascertained to correspond exactly, and to be perfectly synchronous with the systole of the heart. The number of pulsations in a given space of time varies infinitely in healthy persons. Age is the chief cause of these varieties; but other circumstances, which constitute the peculiar state of health of each individual, have considerable effect; so that no standard can be settled which shall prove generally correct. The following numbers afford, we believe, as near an approximation as can be expected amidst so much uncertainty: they will serve at least as a comparative view in subjects of different ages. The heart of an infant sleeping tranquilly performs in the first days of existence about 140 pulsations in a minute; at the end of the first year the pulsations are, in the same space of time, 124.

At the end of the second year
Third and following years
Seventh and following
Time of puberty

Manhood

Sixtieth year

110

96

86

80

75

65

The pulsations of the heart proceed in

a regular and continued succession to the last period of life, and then all its parts do not cease to act at once; but the right auricle and ventricle survive the opposite cavities for some little time, so that the former part has been called the ultimum moriens. The blood, which returns by the vena cava after the last expiration, no longer finds the usual passage through the lungs, which are contracted, but it is still urged on from behind by that which the aorta has recently propelled. Hence it is forced into the right auricle, and excites contraction in that part, by the stimulus of its presence, some time after the left side has been at rest. This congestion on the right side of the heart in the last agony explains the empty state of the arteries, particularly the larger ones, after death.

It is hardly possible to determine the velocity of the blood's motion in the healthy state; for individuals differ from each other in this respect, and considerable variety probably takes place in different parts of the body. It is generally supposed, that the blood flows in a more gentle stream through the small arteries than in the arterial trunks; and that the velocity of its current is somewhat less in the veins than in the arteries of the body. These differences have, however, been exaggerated by former physiologists. The mean velocity of the blood in the aorta is calculated at eight inches for each pulsation, which gives about fifty feet in a minute. If we reflect that the systole of the ventricle, which gives the whole impulse to the blood, occupies only onethird of the whole pulse, the velocity of the blood's motion must be trebled in that division of the time. It is said that this velocity, which we have assigned to the blood's motion at its departure from the heart, becomes speedily diminished in its further progress; and the diminution has been deduced from various causes. The first and most powerful of these is the constantly increasing area of the branches, when compared with the trunk of an artery. (See ANATOMY.) It is a well-known law in hydraulics, that the velocity of a fluid passing through an inverted cone constantly decreases, and that the diminution of velocity is in the ratio of the increase of area. The mathematical physiologists have also noticed the effects of friction, deducing these from a comparison with the course of fluids in dead tubes. Other causes have been derived from the same source; hence the serpentine course of some arteries, the

unfavourable angles by which they sometimes arise,and their communications with each other, are enumerated an.ong the circumstances which retard the course of the arterial blood. But it must be remembered, that in viewing these retarding causes we are considering their action on the blood, as if this fluid were contained in inanimate tubes, and influenced merely by the contraction of the heart, without taking into the account any accessory impulse, which may be, and probably is, derived from the arteries. This retardation has been variously estimated by different calculators, who have all made it very considerable. Hales supposes the blood to flow through the capillary arteries of a frog at the rate of two-thirds of an inch in a minute, which will be about 650 times slower than in the human aorta. Robinson and Whytt have gone still further: the former stating, that the velocity of the blood's motion in the aorta is to that in the smallest vessels as 1100 to 1. We mention these calculations, to shew what absurdities have been committed by men of the greatest abilities, when they have applied the laws which regulate the properties of dead matter to the living functions of the animal machine. Haller's observations on the circulation in living animals, (Elem. Phys. lib. vi. sec. 1. § 30.) entirely overthrow these calculations. He found by his microscopical experiments that the blood flowed generally as rapidly through the small as through the larger vessels. He states also, that in living animals it is poured out as far from a small as from a large artery. The numerous and diversified experiments of Spallanzani afford additional evidence of the same truth.

We have stated, that the blood is thrown into the arteries by separate contractions of the heart; yet these vessels are constantly full, as may be proved by opening them during the heart's diastole. For the blood flows on in such a way, that the subsequent quantity discharged from the right ventricle, overtakes that which is before, and thus causes the pulsation of the arteries. The excess of ve locity in the blood coming from the heart, over that contained in the arteries, becomes constantly less; and at a certain point ceases altogether. Here the pulse ceases also: hence in microscopical observations on the course of the blood in small vessels, its stream appears to be uniform; and it is commonly stated, that the pulsation ceases in vessels of about one-sixth of a line in diameter.

The motion of the blood in the minute veins seems to be equal to its velocity in the small arteries; this increases in the larger trunks; and there is a constant acceleration in the blood's course until it arrives at the heart. This fluid is passing through tubes which constantly decrease in area; and it follows of necessity, that by diminishing the channel of a fluid its course must be accelerated. Hence the trunks of the vena cavæ return to the heart, within a given time, as much blood as the aorta carried out of this viscus.

The motion of the blood along the veins must be derived from the impulse which it receives from the heart, and from the action (if there be any) of the arteries. Its circulation in these vessels is aided by the contraction of the muscles, which must urge on the continued fluid towards the heart, since their valves prevent any retrograde motion.

We shall readily perceive, that no certain calculation can be formed of the powers of the heart, when we consider that neither the quantity of blood expelled at one pulsation, nor the distance through which it passes in a given time, nor the velocity of its course, can be defined with any certainty; much less can we form any accurate estimate of the obstacles which occur to the blood's motion, which must considerably affect such a calculation. We may however approach in some degree to the truth, by collecting and comparing the results of probable conjecture. If we calculate the blood contained in the body at thirty pounds, the number of pulsations in one minute at seventy-five, and the quantity expelled from the left ventricle at each pulsation at two ounces and a half, the whole quantity will pass through the heart about twenty-two times in the course of an hour; and it will perform the circulation once in less than three minutes. velocity with which this blood is propelled by the systole of the left ventricle may be collected from the violence with which it is ejected from a wounded artery, and the altitude to which it ascends. Blumenbach has seen it projected more than five feet from the carotid of an adult during the first contractions of the heart. Our countryman Hales calculated from his experiments, in which he measured the height of the blood's ascent in a glass tube, inserted into a large artery, that it would be thrown seven feet and a half from the human carotid: he estimates the surface of the ventricle at fifteen square inches; and thus finds that one

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thousand three hundred and fifty cubic inches, or about fifty-one pounds weight, press upon the left ventricle, and must be overcome by its systole. Many other calculations of the powers of the heart have been formed upon mathematical principles; but different persons have been led to such opposite results, that we are warranted from this circumstance in disregarding them altogether. Borelli makes the powers of the heart equal to an hundred and eighty thousand pounds; Keil to eight ounces. Senac observes, that if a weight of fifty pounds be attached to the foot, with the knee of that side placed on the opposite one, the weight will be elevated at each pulsation: this weight is placed at a considerable distance from the centre of motion; and, allowing for this circumstance, he estimates the moving power at four hundred pounds.

This power of the heart, so wonderful both in extent and duration, must be referred to the irritability of the organ, in which point of view it seems far to exceed all other muscular parts of the body. That the immediate cause of contraction in this viscus arises from the presence of blood in its cavities, is shewn by the celebrated experiment of Haller; in which the longer duration of action in the right or left cavities was varied by influencing the supply of blood.

In the action of those muscles that depend on the will, a supply of nerves, and a distribution of blood to the moving fi bres, seem to be essential conditions. It has been disputed, whether or not these circumstances are necessary in the heart, and what share they may contribute to the heart's action. We may observe in the first place, that the actions of the heart are completely involuntary; that no exertion of the will can produce the smallest effect in accelerating, retarding, or otherwise affecting the actions of this part. Yet various arguments prove that the nerves exert an influence over this organ. Not to mention the peculiar arrangement of the cardiac nerves, the sympathy between the heart's action, and nearly every other function, even of the most different classes, suffices to demonstrate the connection. The vehement disturbance of the heart from the passions of the mind, must be familiar to every person from his own experience; its action is also strongly influenced by various states and affections of the alimentary canal.

The action of the heart is intimately