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.