Chapter VII: Of the Circulation (2)

301. It was found by Dr. Hales, that, under ordinary circumstances, the blood rises in a tube connected with a vein to the height only of six inches, while it has been shown (290) that in the artery it ascends as high as ten feet. This prodigious difference between the venous and the arterial tension led to the conclusion that the impulsive force of the heart was all but exhausted before the blood reached the veins, and set physiologists on the search for other powers to carry on the venous circulation. It was overlooked that the blood has an open and ready escape from the great trunks of the veins through the right chambers of the heart, and that in consequence of this free escape of their fluid, these vessels indicate no greater tension than is just sufficient to lift the blood to the heart, and to overcome friction[7]. M. Magendie having laid bare the chief artery and vein of a living limb, and having raised the vessels in such a manner that he could place a ligature around the former, without including the latter, found that the flow of blood from a puncture made below a ligature on the vein, was rapid or slow, according as the heart was allowed to produce a greater or less degree of tension in the artery, which tension was regulated by compressing the artery between the fingers. After a similar preparation of a limb, a ligature was placed around the vein; a tube was then inserted into it; it was found that the blood ascended in the tube from the obstructed vein just as high as from the artery.

302. Thus we are able to trace the action of the heart from the beginning to the end of the circle. Of this circle it is the sole moving power; but it is a living engine acting in combination with living vessels. The force it exerts is a vital force, economized by the agency of a vital property communicated to the vessels, by virtue of which they spontaneously and completely remove all physical obstruction to the progress of the stream through its channels.

303. Some German physiologists of great eminence, after a careful and patient observation of the blood, have satisfied themselves that in addition to the contraction of the heart, it is necessary to admit a second original and independent motive force, namely, a self-moving power inherent in the particles of the blood itself. The blood we know is a living substance. No reason can be assigned why the power of originating motion should not be communicated to such a substance as well as to the muscular fibre, of which, indeed, one constituent of the blood affords the basis. Such a power, if found to be inherent in the particles of the blood, would explain some phenomena connected with the circulation not yet clearly elucidated; but the proof of the self-moving power of the blood does not yet seem to be complete. It is, however, impossible to explain the phenomena of the circulation, or to obtain a satisfactory view of some of the other functions of the economy, without supposing the particles of the blood to be endowed with a vital power of repulsion, in consequence of which they are prevented from uniting when in contact, and the fluidity of the mass is maintained.

In this account of the powers that move the blood, no notice has been taken of the physical agents supposed to act as auxiliaries to the heart, in carrying on the circulation, such as the suction power of the thorax, and of the auricles of the heart, and the capillary attraction of the vessels; because, without questioning the existence of such agents, or denying that advantage may be taken of them, it seems pretty clear that their influence is but trivial, and they assumed importance only when the vital endowments of the tissues were not well understood.

304. The ultimate end for which the apparatus of the circulation is constructed, and for which all its action is exerted, is to convey arterial blood to the capillary arteries. These vessels are totally distinct in structure and in office from the larger arterial tubes. All the tunics of these minute vessels diminish in thickness and strength as the tubes lessen in size, but more especially the middle or the fibrous coat; which, according to Wedemeyer, may still be distinguished by its colour in the transverse section of any vessel whose internal diameter is not less than the tenth of a line; but that it entirely disappears in vessels too small and too remote to receive the wave of blood in a manifest jet. But while the membranous tunics diminish, the nervous filaments distributed to them increase: the smaller and thinner the capillary, the greater the proportionate quantity of its nervous matter; and this is most manifest in organs of the greatest irritability. The coats of the capillaries successively becoming thinner and thinner, at length disappear altogether, and the vessels ultimately terminate in membraneless canals formed in the substance of the tissues. "The blood in the finest capillaries," says Wedemeyer, "no longer flows within actual vessels; it is not contained in tubes whose parietes are formed by a membranous substance distinguishable by its texture and compactness from the adjoining cellular tissue: it is contained in the different tissues in channels which it forms in them for itself; and, under the microscope, the stream is seen easily and rapidly to work out for itself a new passage in the tissues which it penetrates."

305. Some of these fine capillaries, before they entirely lose their membranous tunics, communicate directly with veins. Of the capillaries which terminate by direct communication with veins, some are large enough to admit of three or four of the red particles of the blood abreast; the diameter of others is sufficient to admit only of one; while others are so small that they can transmit nothing but the serum of the blood. As long as the capillary is of sufficient magnitude to receive three or four of the particles abreast, it is evident that it possesses regular parietes; but by far the greater number, before they communicate with veins, lose altogether their membranous coats. There are no visible openings or pores in the sides or ends of the capillaries by means of which the blood can be extravasated, preparatory to its being imbibed by the veins. There is nowhere apparent a sudden passage of the arterial into the venous stream; no abrupt boundary between the division of the two systems. The arterial streamlet winds through long routes, and describes numerous turns before it assumes the nature and takes the direction of a venous streamlet. The ultimate capillary rarely passes from a large arterial into a large venous branch.

306. The vital power which it has been shown (298) is possessed by the arterial trunks and branches, is still more intense in the minute capillaries. If alcohol, strong acetic acid, naphtha, and other stimulating fluids, be injected into the arteries of a living animal, it is found that they are not transmitted through the capillaries at all, or, at all events, that they make their way through them with extreme difficulty; whereas mild, unirritating fluids pass with rapidity and ease. Wedemeyer exposed and divided the main artery in the fore-leg of a horse, together with the corresponding vein in the shoulder. Several syringes-full of tepid water were now injected into the lower end of the artery. The gentlest pressure was sufficient to force the fluid through the capillaries. At each injection the water issued in a full stream from the aperture of the vein, the flow of the fluid ceasing as soon as the injection was stopped. Next, instead of water, four syringes-full of pure cold brandy were injected. To propel this fluid through the capillaries, so as to render its smell and taste perceptible at the aperture of the vein, required a great degree of pressure; and when at last the fluid issued from the vein, it merely trickled in a feeble stream.

The experiment being repeated on another horse with vinegar, six syringes-full of which being injected in rapid succession, at first this fluid passed as easily as water, afterwards it flowed with greater difficulty and in a small stream; before long the force required to propel it was extreme, and at last the obstruction to its passage became complete, so that no fluid whatever issued from the vein.

These experiments, whenever repeated, afforded the same result, and they demonstrate that the capillaries are capable of being stimulated to contract upon their contents, and that they can contract with such force as to stop the current. It is manifest that the power by which they do this is vital, because after death all fluids, the mildest and the most acrid, pass through them with equal facility.

307. Drs. Thompson, Philip, Hastings, and others in this country, have applied stimulants of various kinds to the capillary arteries, in order to observe with the microscope the changes which the vessels undergo. The results of these experiments, performed independently, agree with each other; and all the observers concur in stating that those results are so obvious and decisive as to admit of no question. Wedemeyer, fully aware of all that had been done on this subject by the English physiologists, repeated their experiments with his usual patience and care, vigilantly watching the effects with his microscope. His observations completely coincide with those of our countrymen. The circulation being observed in the mesentery of the frog and in the web of its foot, it was apparent that no change whatever took place in the diameter of the small arteries, nor in that of the capillaries, as long as the circulation was allowed to go on in its natural state; but as soon as stimulants were applied to them, an alteration of their diameter was visible. Alcohol, without much apparent contraction of the vessels, stopped the flow of the blood. Muriate of soda, in the course of three or four minutes, caused the vessels to contract one-fifth of their calibre, which contraction was followed by dilatation and gradual retardation and stoppage of the blood. Ammonia caused immediate and direct dilatation, and the effect of galvanism was still more striking. In a space of time varying from ten to thirty seconds, nay, sometimes immediately after the completion of the galvanic circle, the vessels contracted, some a fourth, others half, and others three-fourths, of their calibre. The flow of the blood through the contracted vessels was accelerated. The contraction sometimes lasted a considerable time, occasionally several hours; in other instances the contraction ceased in ten minutes, and the vessels resumed their natural diameter. A second application of galvanism to the same capillaries seldom caused any material contraction.

308. The evidence, then, is abundant that stimulants are capable of modifying to a great extent the action of the capillary arteries, sometimes causing them to contract, at other times to dilate; sometimes quickening the flow of blood through them, at other times retarding it, and frequently altogether arresting its motion. This contractile power of the capillaries must be a vital endowment, for no such property is possessed by any substance destitute of life, and there is satisfactory evidence that it is communicated, regulated, and controlled by the organic nerves, which, as has been shown, increase as the size of the vessels and the thickness of their membranous tunics diminish. The powerful influence of these nerves upon the capillary vessels is placed beyond doubt or controversy by the obvious local changes produced in the capillary circulation by sudden, and even by mental, impressions, by the flush of the cheek and the sparkle of the eye, at a thought conceived or a sound heard; changes which can be effected, as far as we have any knowledge, by no medium excepting that of the nerves. The part performed by electricity, the physical agent by which it is conceived the nerves operate, will be considered hereafter.

309. Exerting upon each other a vital force of repulsion, under a vital influence derived from the organic nerves, urged by the vital contraction of the heart, the particles of the blood reach the extreme capillaries. Most of these capillaries terminate (304) in canals, which they work out for themselves in the substance of the tissues. The tissues are endowed with a vital attractive force, which they exert upon the blood—an elective as well as an attractive force: for in every part of the body, in the brain, the heart, the lung, the muscle, the membrane, the bone, each tissue attracts only those constituents of which it is itself composed. Thus the common current, rich in all the proximate constituents of the tissues, flows out to each. As the current approaches the tissue, the particles appropriate to the tissue feel its attractive force, obey it, quit the stream, mingle with the substance of the tissue, become identified with it, and are changed into its own true and proper nature. Meantime, the particles which are not appropriate to that particular tissue, not being attracted by it, do not quit the current, but passing on, are borne by other capillaries to other tissues, to which they are appropriate, and by which they are apprehended and assimilated, When it has given to the tissues the constituents with which it abounded, and received from them particles no longer useful, and which would become noxious, the blood flows into the veins to be returned by the pulmonic heart to the lung, where, parting with the useless and noxious matter it has accumulated, and, replenished with new proximate principles, it returns to the systemic heart, by which it is again sent back to the tissues.

310. Particles of blood are seen to quit the current and mingle with the tissues; particles are seen to quit the tissues and mingle with the current. But all that we can see, with the best aid we can get, does but bring us to the confines of the grand operations that go on, of which we are altogether ignorant. Arterial blood is conveyed by the arteries to the capillaries; but before it has passed from under the influence of the capillaries it has ceased to be arterial blood. Arterial blood is conveyed by the carotid artery to the brain; but the cerebral capillaries do not deposit blood, but brain. Arterial blood is conveyed by its nutrient arteries to bone, but the osseous capillaries do not deposit blood, but bone. Arterial blood is conveyed by the muscular arteries to muscle, but the muscular capillaries do not deposit blood, but muscle. The blood conveyed by the capillaries of brain, bone, and muscle is the same, all comes alike from the systemic heart, and is alike conveyed to all tissues; yet in the one it becomes brain, in the other bone, and in the third muscle. Out of one and the same fluid these living chemists manufacture cuticle, and membrane, and muscle, and brain, and bone; the tears, the wax, the fat, the saliva, the gastric juice, the milk, the bile, all the fluids, and all the solids of the body.

311. And they do still more; for they are architects as well as chemists; after they have manufactured the tissue, they construct the organ. The capillaries of the eye not only form its different membranes and humours, but arrange them in such a manner as to constitute the optical instrument; and the capillaries of the brain not only form cerebral matter, but build it up into the instrument of sensation, thought, and motion.

312. The practical applications of these phenomena are numerous and most important; but they can be clearly and impressively stated only when the operation of the physical agents which influence the circulation, and which proportionally affect life and health, has been explained.

FOOTNOTES.

[1] Computationi in alimentis faciendæ hanc formam esse Ulpianus scribit, ut _à primâ ætate_ usque ad annum vicesimum quantitas alimentorum triginta annorum computetur, ejusque quantitatis Falcidia præstetur: _ab annis verò viginti_ usque ad annum vicesimumquintum annorum viginti octo: _ab annis vigintiquinque_ usque ad annos triginta, annorum vigintiquinque: _ab annis triginta_ usque ad annos trigintaquinque annorum viginti duo; _ab annis trigintaquinque_ usque ad annos quadraginta annorum viginti: _ab annis quadraginta_ usque ad annos quinquaginta tot annorum computatio fit quot ætate ejus ad annum sexagesimum deerit, remisso uno anno: _ab anno verò quinquagesimo_ usque ad annum quinquagesimumquintum annorum novem: _ab annis quinquagintaquinque_ usque ad annum sexagesimum annorum septem: _ab annis sexaginta_, cujuscunque ætatis sit, annorum quinque; eoque nos jure uti Ulpianus ait, et circa compu tationem ususfructus faciendam. Solitum est tamen _à primâ ætate_ usque ad annum trigesimum computationem annorum triginta fieri: _ab annis verò triginta_ tot annorum computationem inire, quot ad annum sexagesimum deesse videntur; nunquam ergo amplius quam triginta aunorum computatio initur. Sic denique, et si Reipublicæ ususfructus egetur, sive simpliciter, sive ad ludos, triginta annorum computatio fit. Si quis ex heredibus rem propriam esse contendat, deinde hereditariam esse convincatur: quidem putant ejus quoque Falcidiam non posse retineri; quià nihil intersit, subtraxerit an hereditariam esse negaverit. Quod Ulpianus rectè improbat. (Vide Justin. Pandect. lib. 35, tit. 2, ad Legem Falcidiam.)

[2] Which maximum is a little above the highest point hitherto any where attained.

[3] Hence in the preparation of jelly as an article of diet, the parts of young animals, as the feet of the calf, are principally employed; whereas soups made from beef contain a large proportion of albumen, while in those made from veal the proportion of jelly preponderates.

[4] Treatise on Ligaments, by Bransby B. Cooper, Esq.

[5] For these illustrations I am indebted to Mr. Lister, who has been so kind as to make drawings of the objects for me.

[6] Whenever there is any interruption to the ordinary flow of the circulating fluids, the powers of the anastomosing circulation are capable of being increased to a surprising extent. The aorta itself has frequently been tied in animals of considerable size without destroying life; in the human body it has also been found obliterated by disease in different parts of its course, in one case as high as the termination of its curvature. In the cure for aneurism the external iliac artery has been tied by Mr. Abernethy with success; the subclavian artery below the clavicle by Mr. Keate; the common carotid by Sir Astley Cooper; the subclavian artery above the clavicle by Mr. Ramsden; the internal iliac artery by Dr. Stevens; the arteria innominata by Dr. Mott, of New York; and lastly, the abdominal aorta itself, by Sir A. Cooper. Mr. Grainger tied the abdominal aorta of a dog; when the animal had recovered from that operation, the carotids and the great trunks of the anterior extremities were tied: in this manner the whole course of the circulation was altered. The dog, which was of very large size, survived all these operations, and appeared to enjoy its ordinary health. Grainger's General Anatomy, p. 251-253.

[7] See this matter very ably discussed in Dr. Arnott's excellent work on the Elements of Physics, vol. i.

END OF VOL. I.

London: Printed by W. CLOWES and SONS, Stamford Street.

TRANSCRIBER'S NOTES. 1. (Figure LXXIV.) was incorrectly labeled as (Figure LXXVI.). This has been corrected. 2. No Figure LXX in original book.