CLASS III. PHYSIOLOGICAL PROEM. On treating of the very important and extensive range of of CL. III. Proposed diseases included under the present class, let us first take scope of a brief survey of the sanguineous function which is the inquiry. immediate theatre of their operation, and the means and instruments by which it is maintained. This comprehensive subject may be most conveniently General discussed under the three following divisions :--- I. THE MACHINERY OF THE SANGUINEOUS SYSTEM. II. ITS MOVING POWERS. III. THE NATURE OF THE FLUID CONVEyed. division. chinery of I. The importance of the blood to the general health I. Maof the animal system; and its existence in every part of the sanalmost every organ, have been known in every country guineous in which medicine has been studied from the first dawn system. of its cultivation. It is not necessary to retrace the wild Unsatisfactory hypoand idle hypotheses that were started in ancient times theses of the to account for the means by which this universal fluid ancients. travels from one part to another, and appears in every quarter. It is enough to observe that, till the great and transcendent doctrine of the circulation of the blood was completely established, the acutest physiologists wandered about in darkness and uncertainty, seldom satisfying themselves, and, still more rarely, the world around them insomuch that I am not acquainted with a single : I. Ma- tion loosely Proofs to which it ap present day. Arteries generally terminate in veins: conjecture that was ever vented upon the subject that is in the least degree worthy of repetition. The opinion, indeed, of a circulation of the blood through the system was loosely started by various writers even of very early times; but under every modification it was found to be accompanied with so many difficulties as always to be dropped almost as soon as it was revived, and rarely, till the middle of the seventeenth century, to show itself to any effective purpose. Hippocrates guessed at it, Aristotle assented to it, Serveto, who was burnt as a heretic in 1553, imperfectly taught it by pointing out the smaller circulation, or that through the lungs; and our own illustrious countryman, Harvey, about a century afterwards, gave a finish to the inquiry, by establishing the larger circulation, or that over the whole frame. The principal proofs of a circulation of the blood offered by Harvey, and those, indeed, on which we chiefly rely in the present day, are deduced from the disposition of the valves of the heart; the range of the arteries and the veins, and from what occurs when either the arteries or veins are opened, compressed, tied, or injected. Thus, if we open an artery, the blood that jets from the puncture flows in a direction from the heart; and in a direction to the heart, if we open a vein. A compression or ligature upon an artery, puts a stop to the blood that flows from above the ligature; but the same upon a vein puts a stop to the blood from below it, in which direction the vein immediately becomes distended. In like manner, an acid liquor injected into the veins coagulates the blood in the direction towards the heart, proving that the venous blood is every where travelling in this course. While an examination by the microscope of the half-transparent vessels of frogs and other cold-blooded animals confirms the view laid open by these phænomena, and shows to us a continual flow of the blood from the heart into the arteries, thence into the veins, and thence to the heart again; thus completing the circular career. The arteries, therefore, generally speaking, terminate in veins; but by no means the whole of them, for many system. others per are exhalant or secretory, and terminate in minute orifi- I. Maces on the surface of membranes and other organs; chinery of which the sanno microscope, however, has yet discovered, but whose guineous existence we have every reason to believe, as we perceive Many of a perpetual oozing of fluids, whose flow we cannot them in exotherwise account for, into all the cavities of the body; halants: which keeps their surfaces moist, and makes motion easy. While, according to M. Magendie, whose experiments, haps in however, seem to want confirmation, other minute arte- lymphatics. ries terminate in lymphatics, which he makes as much a part of the sanguiferous system as the veins; the lymphatics conveying the more attenuate part of the arterial blood, slightly tinged of an opaline or rose-coloured hue, though sometimes of a madder-red; such as the fluid which oozes upon puncturing the lymphatics, or the thoracic duct after a long fast. It is not necessary to examine into the correctness of this hypothesis in the present place, as we shall have occasion to notice it more at large when treating of the excernent system, which will be found to embrace both the absorbent and secretory vessels. It should, however, be remarked, that in M. Magendie's hypothesis the veins, and not the lymphatics, are the absorbents of the body*. of the san Omitting then for the present the consideration of the Instruments lymphatics, the machinery by which the circulation of guiferous the blood is principally effected consists of the heart itself, the arteries, and the veins. machinery. Heart in the more classes of animals. The heart in the more perfect classes of animals, as mammals, birds, and most, though not all, amphibials, is perfect a very compound organ; for in all these the blood, when received from the veins, is first sent from this central organ to the lungs to be duly aërated, or, according to Mr. Ellis's hypothesis, to be unloaded of its excess of carbone, and is afterwards returned from the lungs to the same organ before its general circulation over the system commences. These classes, therefore, are said to possess a double circulation. And as the heart itself consists of • Précis Elementaire de Physiologie, Tom. 11. 1. Machinery of the san guineous system. Heart double, and circulation double. Seat and appendages four cavities, a pair, composing what is called an auricle and a ventricle, belonging to each of the two circulations; and as each of these pairs is divided from the other by a strong membrane, these classes are also said to have not only a double circulation, but a double heart; a pulmonary and corporeal circulation, and a pulmonary and corporeal heart. The heart is well known to be situated in the chest, of the heart. between the lungs, above the diaphragm, and to be influenced by all the motions of the diaphragm. It is loosely surrounded by a dense and fibrous membrane, named, from its situation, pericardium, possessing little sensibility, closely connected with the substance of the diaphragm, and reflected over the heart and its large vessels. Its use is to confine the heart in its proper post; and to lubricate it, in its state of unceasing activity, with a peculiar fluid, denominated liquor pericardii, supposed to be secreted by peculiar glands, but more probably exhaled from the capillary arteries of the internal surface. In a state of health this fluid is small in quantity and of a reddish hue, some portion of the red parts of the blood being intermixed with it; but, in a morbid state of the membrane, it is apt to accumulate, change its properties, and lay a foundation for various complaints*. Restrictive power of dium. The power possessed by the pericardium of restrainthe pericar- ing the heart to its proper post, is obvious from the following fact. If, after detaching the sternum and opening the chest, an incision be made into the pericardium of a living animal wide enough for the purpose, the heart will often be found to leap out of its sac through this aperture, and to fall on the right or the left side of the How far the thorax. And hence the common and colloquial expresheart leaps sion derived from common feeling, of the leaping of the heart for joy-and it might as well be said for grief or terror-is founded on actual fact. The heart, which is loosely confined by its vessels, often leaps as far as its for joy. * See Bostock's Elementary System of Physiology, Vol. 1. p. 363. 8vo. |