Source: Snow, John. London Med. Gazette, vol. 31, Mar. 3, 1843, pp. 810-816.

On the circulation in the capillary blood-vessels, and on some of its connections with pathology and therapeutics

By John Snow, M.R.C.S.

For the London Medical Gazette.

(*This article has been selected and re-arranged from two papers read by the author at the Westminster Med. Soc. On Jan. 21st, and Feb. 4th.)

The heart not the only cause of the circulation.--The various functions assist the capillary circulation.--Circulation in plants--in the more simple animals--in asphyxia.--Contraction of vessels does not assist the circulation.--Diapnets, or medicines which promote respiration.--Cutaneous transpiration.--Inflammation.

The blood, after leaving the left ventricle of the heart, passes through tubes which divide and subdivide until they become of microscopic smallness, before they re-unite to convey the blood to the right side of the heart; whence the whole of that fluid has again to pass through a similar arrangement of vessels in the lungs, before it reaches the point from which we set out. It is in the minute branches that all the changes in the blood, and the reciprocal changes between that fluid and the rest of the organism, take place; the heart and larger vessels merely serving the mechanical purpose of transmitting the blood. These ultimate or smallest branches, which have obtained the name of capillaries, seem in most of the tissues of the body, quite distinct in arrangement and appearance from the arteries which bring the blood from the heart, and veins which return it to that organ. Viewed by the microscope, they are seen to be of equal diameter throughout, intercommunicating with each other so as to form a net-work, and forming a union between the arteries and veins, but having no other openings or communications with any other parts.

There is considerable diversity of opinion with respect to the forces which circulate the blood. Many physiologists of great eminence maintain that the contraction of the ventricles is the sufficient and sole power which forces the blood through the capillaries and back again to the heart; whilst others are of opinion that various causes assist the heart to propel the blood through the vessels. I feel convinced that, besides the action of the heart, there is one other power of importance engaged in the circulation of the blood, a power which has been suggested and described with more or less of distinctness by Dr. Allison, Baumgartner, Koch, Dr. John Reid, and, I believe, other authors, but has not yet received that general assent, or even consideration, which it deserves. The opinion to which I allude I will define as follows:--that the mutual changes which take place at the capillaries, between the blood and the tissues, are attended with attractions and repulsions which assist to impel the blood in a definite direction. Some of the fluid part of the blood is continually passing through the invisible pores of the capillary vessels to nourish the surrounding solids, whilst, at the same time, a fluid, containing in solution the tissues decomposed in supporting the vital functions, enters the capillaries from without: in various parts exhalation and imbibition of liquids and gases are going on, and in others special secretions are separated from the blood. The opinion, then, which I advocate is, that all these actions assist the circulation of the blood; in short, that every function promotes the capillary circulation of the part in which it takes place.

Towards establishing these views, I shall first shew that the causes to which I have alluded are alone sufficient to move a nutritious fluid in capillary tubes.

The isolated cells of chara, and of most simple vesicular plants, are filled with a liquid which contains globules visible by the microscope. These globules move round in their respective [810/811] cells, each cell having a separate circulation of its contained fluid. There is no contraction of the walls of the cells and indeed, there is no conceivable cause for this circulation, except the process of nutrition going on in the cells. In the higher plants, in which the sap moves in capillary tubes and spaces, there is no contraction of the vessels, nor any other mechanical force to cause the circulation. Absorption by the spongioles of the root can raise the ascending sap to a limited extent only; when the root is cut off, and the stem put in water, part of the water is raised into the plant; and indeed, the ascent of the sap may be observed in spring to commence first at the buds, the fluid lower down afterwards following: it follows, then, that the functions, going on in the leaves are the chief cause of the ascent of the sap. In the leaves the course of the fluid is changed, and the elaborated sap returns towards the root, nourishing the plant, and forming various secretions in its course; and this course is not always a descending one, for in a great portion of many plants the returning sap has to travel in opposition to the force of gravitation. It is evident, therefore, that the circulation through the leaves, and back again towards the root, is accomplished by the exhalation and imbibition of the leaves, the change of composition and other functions performed there, and by the nutrition and secretion taking place as the sap returns--by some or all of these functions--since there are no other causes, except, indeed, capillary attraction, which cannot cause the circulation, but only aid it to a small extent.

In many species of the lowest tribes of animals, the circulation of the blood which takes place in capillary tubes is independent of contractions and all mechanical forces, and must arise from the functions taking place in the vessels: for instance, the trematoda, an order of intestinal worms, possess two vessels on each side of the body, in which the blood moves in opposite directions; and, according to the observations of Ehrenberg and Von Nordman, these vessels do not contract in the least. The greater number of animals in the lowest classes have, it is true, some vessels which contract; but these contractions are often very feeble compared to the extent and vigour of the circulation. The motion of the ambient liquid over the surface and through the canals of sponges, likewise tends greatly to strengthen the views under consideration; for since no contractions or ciliary motions can be detected in these beings, the currents must be caused by the nutrition, respiration, and other functions taking place on the surfaces of the sponge. And it may be mentioned here that when development first commences in the higher animals the circulation is independent of any mechanical impulse: it has been clearly shown by Von Baer that the blood is formed before the vessels, and that its first motion is towards the heart, and not from it.

The next point in this inquiry which I shall take up, will be, to ascertain whether the action of the heart is of itself sufficient to effect the circulation; and it is only necessary to pay attention to the phenomena of asphyxia to perceive that it is not. When those changes in the blood which take place at the lungs are prevented by stopping the access of oxygen, the passage of the blood through the lungs is at once greatly diminished, and is shortly arrested, whilst the heart continues to act some time longer. It matters not whether the lungs are collapsed or distended, or whether the respiratory motions are continued or not; if the characteristic change in the blood is prevented, the right ventricle of the heart is unable to propel the blood properly through the lungs. There is no mechanical impediment, the blood is not coagulated, as the fact of recovery, if the cause of asphyxia be timely removed, proves; the only conclusion, then, is, that the change in the blood from venous to arterial being necessary to its free passage through the lungs this change must generate forces which assist its passage. In his experiments on asphyxia in dogs, Dr. John Reid found (Edin. Med. and Surg. Journ. No. 147.), by the aid of M. Poiseulle's instrument, the hæmadynameter, that during the first moments of asphyxia the pressure on the systemic arteries was increased, and consequently that there was obstruction in the capillaries of the general circulation: from which it appears that the small quantity of blood which does pass through the lungs, being but little altered from its [ 811/812] venous state , is not fitted to undergo those opposite changes which take place in the body at large; and the left ventricle is but ill able to propel it through the capillaries for want of the auxiliary forces arising from those changes.

On the other hand, a circulation sufficient to support growth and formation of parts to a considerable extent, may be maintained without the assistance of the heart. The proof of this consists in the existence of human acephalous monsters without heart, many of which have been developed in a uterus which contained no other fœtus at the same time, the contraction of whose heart might be supposed, by a union of the placental vessels, to aid the circulation; and in these cases there was consequently a double capillary circulation, the placental and general one, both independent of a heart. In one remarkable case related by Emmert, the uterus merely contained a limb attached to the placenta by a funis.

That other agencies are at work, assisting or controlling the circulation, is evident from a single fact, admitted, I believe, by all recent observers with the microscope, viz. that the colourless corpuscles of the blood do not move through the capillary vessels with the same speed as the red globules, but roll slowly along near the sides. And to complete the proof of the opinion with which we set out, it is only necessary to shew that there are no other causes which can assist the heart to move the blood besides those we have stated.

Many authors, perceiving the immense force that would be required to impel a fluid through such small tubes as the capillary blood-vessels; considering the portal circulation, where the blood is collected from these minute vessels and again passes through a similar set before it returns to the heart; considering the circulation in fishes, in which the blood all passes through the capillaries of the gills, and afterwards through those of the system, before it again reaches the heart; and considering, likewise, the circulation of monsters without hearts, and perceiving difficulties in all these and other considerations, have admitted the inability of the heart to circulate the blood, but have suggested contractions of the capillary vessels, or of the arteries, as the forces which assist the circulation. Now contraction of the capillaries would impede the entrance of blood into these vessels as much as it would facilitate its exit on the other side; and consequently could be no assistance to the circulation--unless, indeed, it were, as Mr. Julius Jeffreys suggests (Med. Gaz. March 18, 1842), a vermicular contraction; and that no such contraction as this exists is evident on observing the capillary circulation with the microscope. The same is true of the arteries; the pulse proves that there is no vermicular or progressive contraction in them, and any other contraction would resist the action of the heart quite as much as it could assist the flow of blood towards the veins. The elasticity of the arteries modifies the impulse of the heart exerted on the blood, and causes its current to be continuous; whereas if it had to pass through rigid tubes it would stop between each stroke of the heart. But this elasticity can apply no additional power to the circulation; it only acts the part of a regulator of the motion. The arteries, in addition to their elasticity, which continues after death, possess a tonic or vital contractility, which enables them to accommodate themselves to the altered quantity of blood and to different states of the circulation. This contractility I believe to be under the influence of that system of nerves which accompanies the arteries everywhere and is distributed to their coats,--I mean the ganglionic system, or great sympathetic,--and I consider that it has great influence over the quantity of blood which reaches different parts, and over the state of the circulation, both in health and disease; but it cannot assist the circulation by adding any power to it; it can only modify the application of other forces. The capillaries themselves can alter their size, and thus modify the circulation which they cannot assist; they do so on the application to them of various medicinal and physical agents.

Direct observations made with the microscope favour the doctrine I advocate, since Spallanzani and many later physiologists have seen the blood move in the capillaries of the frog for a length of time after the removal of the heart; and I conceive the conclusion to be drawn from these observations cannot be weakened by the negative [812/813] evidence of others, who have observed the blood not to move under similar circumstances.

There is an experiment by Magendie to which it will be well to allude, since it is considered by that eminent physiologist, and many others, to amount to a proof that the heart's action is the sole cause of the motion of the blood in the veins; but a careful examination of that experiment will shew that it is not unfavourable to the views here entertained, and that, at the utmost, it only proves, what no one would attempt to deny, namely, that, when the blood is prevented from reaching the capillaries, it cannot flow through them. The following is an account of the experiment in the author's own words. "After having passed a ligature round the thigh of a dog, as I now described, that is, without including the crural artery or vein, apply a ligature separately upon the vein near the groin, and then make a slight opening in this vessel. The blood will immediately escape, forming a considerable jet. Then press the artery between the fingers to prevent the arterial blood from reaching the member. The jet of venous blood will not stop on this account; it will continue some instants; but it will become less and less, and the flowing will at last stop, though the whole length of the vein is full. If the artery be examined during the production of these phenomena, it will be seen to contract by degrees, and will become completely empty. The blood of the vein then stops; and at this period of the experiment, if you cease to press the artery, the blood injected by the heart will enter, and as soon as it has arrived at the last divisions, it will begin to flow again at the opening of the vein, and by little and little the jet will be established as before." The movements of the chest in respiration give a little assistance to the return of blood to the heart; but it is only on the large veins in the immediate vicinity of the chest that these movements can have any influence; in natural respiration this influence is very small, and under no circumstances can it extend to the capillaries; therefore it need not occupy much attention in this paper. I have now, I think, considered this subject in all its bearings, and have, I trust, proved that the heart, although, undoubtedly, the chief cause of the circulation, is yet materially assisted by other forces situated at the capillaries; not by any contractions of these vessels, or by any other mechanical power, but by forces arising out of the functions in the capillaries and in their immediate vicinity. I have nothing to advance respecting the intimate nature of the attractions and repulsions which accompany the changes of composition at the capillaries, and which tend to move the blood in a definite direction. I have carefully avoided such terms as chemical, electrical, and vital, both in order that I might not be misunderstood, and because I look upon chemical affinity, electricity and vitality, rather as expressions which are useful to us in the infancy of science than as forces which have a separate and defined existence. Our ignorance, however, of the ultimate cause of these attractions is no argument against their existence; since we admit many laws in science of the causes of which we are ignorant.

The views of the circulation entertained in this article will enable us to explain many phenomena in physiology and medicine which would otherwise be a mystery; of which I will give two or three examples. Assafœtida, ether, various essential oils, camphor, and other volatile medicines, relieve difficult and impeded respiration; they are sometimes called antispasmodics, but they give relief in various cases of obstructed pulmonary circulation when no spasm is even supposed to exist, and I do not know by what modus operandi they can be beneficial except by one in accordance with the above views of the capillary circulation. They are all separated from the blood in the lungs and escape with the breath; they leave the lining membrane of the air-cells in the gaseous form with the carbonic acid gas and watery vapour, increasing very much the quantity of vapour which exhales from the pulmonary capillaries, and thus give additional impetus to the blood; in this way lessening congestion and relieving its attendant distressing symptoms. As this class of medicines promote the function of respiration I will venture to call them diapnetics, from ??? and ????. To call them expectorants would not be correct, as expectoration is not respiration. As the greater number of diuretics are voided with the urinary excretion, the above class of medicines resembles them [813/814] in this respect, since they pass out of the body with the excreted carbonic acid gas and aqueous vapour. A dry atmosphere gives relief in some forms of asthma; it promotes the excretion of watery vapour, and acts in the same way as the above medicines.

MM. Breschet and Becquerel have recently found that on applying impermeable substances to the skins of animals, and thereby preventing the cutaneous transpiration, the animals gradually decreased in temperature, and appeared to die of a kind of cutaneous asphyxia. I do not know that these gentlemen have offered any explanation of this circumstance, and I do not see that it is capable of any, except that afforded by the views which it is the object of this paper to support. Exudation of water and watery vapour being one important function of the skin, it follows that, this being stopped, the cutaneous circulation is slackened, and along with it the other functions of the skin, the most important of which is the evolution of caloric. We need not expect sanguineous congestion of the skin under these circumstances, since there are plenty of other channels for the blood; the skin being in this respect very differently situated from the lungs. The evaporation of the cutaneous transpiration is attended with the abstraction of caloric, and this evaporation can preserve the body under extraordinary circumstances from being raised above the temperature compatible with life; as I believe, was first shewn by Franklin: but we learn from the experiments of Breschet and Becquerel that, under ordinary circumstances, the cutaneous transpiration is the cause of a generation of caloric much more than sufficient to over-balance the abstraction of heat arising from its own evaporation. Dr. Willis has been good enough to direct my attention to a note to his translation of Wagner's Physiology in which he states his opinion that one use of the cutaneous secretion is to assist the capillary circulation. That such is the case, and that the fall of temperature in animals experimented on by Breschet, was the direct effect of the arrest of transpiration from the skin, and not a consequence of some injury to the system at large, I satisfied myself by folding one of my arms in oil-skin under the usual clothing; for I experienced a sensation of coldness all the afternoon it was applied, and on its removal the arm was found to be evidently colder than the other. It is generally taught that in fever with a hot and dry skin there is diminished cutaneous transpiration, but, reasoning by analogy from the above facts, we may conclude that such is not the case, and that the secretion from the skin is at least equal to, if not greater than, what it is in health; and, that such is really the case, a little consideration will prove; for the patient drinks a good deal, voids but little urine, does not increase in bulk, but on the contrary, gets thinner; therefore, there must be increased evaporation from the skin and lungs, for which the elevated temperature of the body is sufficient cause. The heat of the surface, which is sometimes considered as a consequence of diminished evaporation, accounts satisfactorily for the dryness of the skin, where the cutaneous secretion is not very great indeed, by the increased rapidity of evaporation it must produce. The moisture of surface produced by antimony and some other diaphoretics, I conceive, arises from the property these medicines have of reducing the temperature of the body, and not from increased determination to the skin. I admit that this position respecting the perspiration in fever requires to proved by experiment; but in the meantime, I feel so satisfied that the evaporation from the lungs, which is confined by physical laws, cannot be sufficient to dispose of all the supplementary fluid, that I have no doubt, in my own mind, on the point.

The above views of the circulation afford, I think, a better explanation of the phenomena of inflammation than do any other. Increased function, which, according to these views, necessarily accelerates the circulation of a part, may lead to inflammation; as, where excitement of the passions of the mind causes phrenitis, or, where the intensity, or long-continued application of light causes, inflammation of the eyes: now, according to Dr. Kaltenbrunner and other eminent microscopical observers, the velocity of the capillary circulation is greatly increased in the early stage of inflammation. All observers, I believe, agree that the capillary vessels of an inflamed part are increased in size, or dilated: and [814/815] some authors, who consider the heart's action the only cause of the circulation, say that the current of blood through an inflamed part must be retarded in proportion to the dilation of the capillaries, since when a liquid is forced through a tube having one portion dilated, its motion is slower in the dilated portion. Were the contractions of the heart the sole cause of the circulation, this would be so, but then the temperature of inflammation would be the normal temperature; and if this disease consisted in a stoppage of the capillary circulation, as other authors say, then an inflamed part would be colder than the others: for whether we consider that the blood merely warms the body by distributing heat which it acquires from some internal source, or whether we adopt the view, which has been proved by various facts, that animal heat is chiefly evolved in the systemic capillaries, where the arterial is changed into venous blood; still, to account for the elevated temperature of inflamed parts, the passage of an increased quantity of blood through the vessels is necessary, since the external air and contiguous bodies are constantly conducting away the heat. One function, to which the capillary circulation is subservient, is the maintenance of sensibility; and when the circulation is accelerated to the amount of active inflammation, the sensibility of the inflamed part is increased into pain. The pain in this disease, I think, is generally in proportion to the natural sensibility; those parts which are most sensible in health being most painful in inflammation. It is probable that there are no living parts that have not an obtuse sensibility, although they may usually not have the opportunity of evincing it: thus, an alimentary bolus is generally not felt after it is swallowed; but if it be unusually hot or cold, or harder or larger than common, it may be felt all the way down the œsophagus till it reaches the stomach.

The small arteries of an inflamed part become dilated as well as the capillaries, and often, also, the arteries leading to the part. This dilation of the arteries cannot be explained on mechanical principles, and must consequently depend on the nerves; most likely on the branches of the great sympathetic. Those who say that inflammation consists in obstruction of the capillary circulation, argue that this obstruction is the mechanical cause of the dilation and increased pulsation of the arteries; but obstruction could have no such effect unless it existed in all the capillaries of the system at once, as in asphyxia; for the pressure of the heart's action is exerted on all the arteries alike, and if the circulation were obstructed in one part, the blood would find an easy passage through others. Moreover, the compression of an artery causes no such dilation and pulsation above the compressed part as those of inflammation. The dilatation of the small arteries accounts sufficiently for the throbbing of many inflamed parts. In the state of health, the wave caused by the heart's impulse is stopped some distance before it reaches the capillaries; but in inflammation, the small arteries become so much enlarged, that the undulation caused by the heart's action reaches completely to the capillary vessels; and from the number of these arteries a pulsation is produced equal to that of a large artery in the normal state.

That inflammation is something more than an altered condition of the capillaries, or a loss of balance between these vessels and the heart, is proved by the fact, that the cornea, various articular cartilages, and other structures which contain no vessels of any kind are subject to this disease. Inflammation is a diseased process going on between the solids of the inflamed tissue and the blood circulating through it, or near it; and we find that this process has a tendency to change the structure and composition of both the solids and the blood; for there is good reason to conclude, from some observations made by Mr. Addison and others, that the increased formation of fibrine, in this disease, takes place chiefly in the inflamed part.

When inflammation proceeds so far as to produce disorganization of any kind, the circulation generally, I believe, becomes interrupted. But it is not my intention to enter at length into the subject of this disorder: I shall only make a few remarks on one or two points in the treatment before I conclude. According to the above theory of the circulation, it is very clear how perfect rest from the func-[815/816]tions of an inflamed organ, when it can be obtained, should greatly promote the resolution of the inflammation. The beneficial effects of local blood-letting are likewise apparent; for if the functions of an organ assist the circulation through it, when these functions are lowered by the abstraction of blood from the part, the capillary circulation cannot regain its wonted velocity unless some cause exist, or come into operation, to re-excite the morbidly increased functions of the part. On the other hand, if the circulation depended entirely on the vis a tergo, it is not evident how blood-letting from the vessels of the diseased part or those in its immediate vicinity, could do more good than the removal of the same quantity of blood from any other part of the body.

From what I have said respecting the effect of the cutaneous secretion on the capillary circulation, it follows that inflammation of the skin ought to be relieved by stopping the transpiration from the inflamed part; and this is an indication which may be fulfilled with safety and advantage in every case of inflammation on or near the surface. I believe it is on this principle that the benefit of water-dressings and poultices chiefly depends, as well as of lunar caustic and of flour to erysipelas; the former making a dead, and in a great measure impermeable membrane of the cuticle, and the latter likewise interfering with transpiration. The effects of stopping the exudation from the skin seem to reach some depth below the surface; for covering the mammary glands with large adhesive plaster is the most successful means I know of putting a stop to the secretion of milk, and thereby preventing the ill effects which might arise from its congestion in these glands.

Whilst, as was stated before, a dry atmosphere ought to give relief in asthma with passive congestion of the lungs, in inflammation of these organs the most suitable atmosphere should be one so charged with moisture that it can take up no more; and such an atmosphere is recommended by Mr. Jeffreys (Med. Gaz. Feb. 18, 1842), though on different principles, and has been found by him to be beneficial.

These are but a few of the subjects in medical science which are capable of receiving illustration and explanation from the above views of the capillary circulation; for one truth must always harmonize with all others.

Frith Street, Soho Square

Feb. 13th, 1843