Pagina-afbeeldingen
PDF
ePub

acts. Whether it be irritation excited by the contact of the blood, by the influx of the ner, vous fluid, or whatever elfe be the cause of its motion, it is fomething which is capable of producing, in a living mufcular fibre, reciprocal contraction and relaxation, This is the power we have to work with: and the enquiry is, how this power is applied in the inftance before us. There is provided in the central part of the body a hollow mufcle, invefted with spiral fibres, running in both di rections, the layers interfecting one another; in fome animals, however, appearing to be. femicircular rather than fpiral. By the contraction of these fibres, the fides of the mufcular cavities are neceffarily fqueezed together, fo as to force out from them any fluid which they may at that time contain: by the relaxa tion of the fame fibres, the cavities are in their turn dilated; and, of course, prepared to admit every fluid which may be poured into them. Into thefe cavities are inferted the great trunks, both of the arteries which carry out the blood, and of the veins, which bring it back. This is a general account of the apparatus and the fimpleft idea of its action is, that, by each contraction, a portion of blood.

is forced as by a fyringe into the arteries; and, at each dilatation, an equal portion is received from the veins. This produces, at each pulse, a motion and change in the mafs of blood, to the amount of what the cavity contains, which in a full grown human heart, I understand, is about an ounce, or two table fpoons full. How quickly these changes fucceed one another, and by this fucceffion how fufficient they are to support a stream or circulation throughout the fyftem, may be understood by the following computation, abridged from Keill's Anatomy, p. 117, ed. 3, "Each ventricle will at least contain one ounce of blood. The heart contracts four thousand times in one hour; from which it follows, that there paffes through the heart, every hour, four thousand ounces, or three hundred and fifty pounds of blood. Now the whole mass of blood is faid to be about twenty-five pounds, fo that a quantity of blood equal to the whole mass of blood paffes through the heart fourteen times in one hour; which is about once every four minutes." Confider what an affair this is, when we come to very large animals. The aorta of a whale is larger in the bore than the main pipe of the water-works at London Bridge;

M 3

Bridge; and the water roaring in its paffage through that pipe, is inferior, in impetus and velocity, to the blood gushing from the whale's heart. Hear Dr. Hunter's account of the diffection of a whale.. "The aorta measured a foot diameter. Ten or fifteen gallons of blood is thrown out of the heart at a stroke with an immenfe velocity, through a tube of a foot diameter. The whole idea fills the

mind with wonder *."

The account which we have here ftated, of the injection of blood into the arteries by the contraction, and of the correfponding reception of it from the veins by the dilatation, of the cavities of the heart, and of the circulation being thereby maintained through the blood-veffels of the body, is true, but imperfect. The heart performs this office, but it is in conjunction with another of equal curiofity and importance. It was neceffary that the blood should be fucceffively brought into contact, or contiguity, or proximity with the air. I do not know that the chymical reason, upon which this neceffity is founded, has

* Dr. Hunter's account of the diffection of a whale. Phil. Tranf.

been

[ocr errors]

been yet fufficiently explored. It feems to be made appear, that the atmosphere which we breathe is a mixture of two kinds of air; one pure and vital, the other, for the purposes of life, effete, foul, and noxious: that when we have drawn in our breath, the blood in the lungs imbibes from the air, thus brought into contiguity with it, a portion of its pure ingredi ent; and, at the fame time, gives out the effete or corrupt air which it contained, and which is carried away, along with the halitus, every time we expire. At leaft; by comparing the air which is breathed from the lungs, with the air before it enters the lungs, it is found to have loft fome of its pure part, and to have brought away with it an addition of its impure part. Whether thefe experiments fatisfy the queftion, as to the need which the blood stands in, of being visited by continual acceffes of air, is not for us to lenquire into; nor material to our argument: it is fufficient to know, that, in the conftitution of moft animals fuch a neceffity exifts, and that the air, by fome means or other, must be introduced into a near communication with the blood. The lungs of animals are conftructed for this purpose. They confilt

[ocr errors]

of blood-veffels and air-veffels lying close to each other; and wherever there is a branch of the trachea or windpipe, there is a branch accompanying it of the vein and artery, and the air-vessel is always in the middle between the blood-veffels*. The internal furface of thefe veffels, upon which the application of the air to the blood depends, would, if collected and expanded, be, in a man, equal to a fuperficies of fifteen feet fquare. Now in order to give the blood in its course the benefit of this organization (and this is the part of the subject with which we are chiefly concerned), the following operation takes place. As soon as the blood is received by the heart from the veins of the body, and before that it is fent out again into its arteries, it is carried, by the force of the contraction of the heart, and by means of a feparate and fupplementary artery, to the lungs, and made to enter the veffels of the lungs ; from which, after it has undergone the action, whatever it be, of that vifcus, it is brought back by a large vein once more to the heart, in order, when thus concocted and prepared, to be from thence diftributed anew into the

[merged small][ocr errors][merged small][merged small]
« VorigeDoorgaan »