POWERS MOVING VIE BLOOD.
In proceeding to the third division of our subject, viz. the phenomena of the circulation and the powers by which the blood is moved,we would remark, that, however desirable it might appear in a systematic work of this kind to treat of these two subjects under distinct heads, such a separation would have the effect of detaching inconveniently the facts from the legitimate conclusions which may be drawn from them. We shall first state the phenomena and causes of the motion of the blood which belong strictly to the organs of circulation themselves, and afterwards shall treat of various circumstances connected with the other functions by which the circulation is modified. In this view it is our chief object that the facts adduced should bear upon the explanation of the motion of the blood in the human body, but from the nature of the investigation the facts themselves must be drawn chiefly from experiments made upon the lower animals. Of course those experi ments and observations which have been made on Mammiferous animals have most value in relation to such a view of the function as that which it is our intention to give. The order which we shall follow is founded on the course which the blood pursues. We shall treat, 1, of the passage of the blood through the heart ; 2, of its flow in the aiteries ; 3, of its.passage from the arteries to the veins through the ca pillaries; and 4, of its flow in the veins.
1. Flow ql the blood through the heart.— That the muscular contraction of the heart is, in man and in all animals in which this organ exists, the principal source of the power by which the blood is propelled in its course, seems to be satisfactorily proved by the facts, that whenever the action of the heart ceases or is impeded, the whole circulation ceases, and that, when an obstruction prevents the action of the heart from reaching the blood in any of the bloodvessels, the flow of blood ceases almost instantaneously in all the branches proceeding from the obstructed vessel. The constant and regular persistence of the contractions of this muscular organ from the commencement of life to its termination, the early period at which it begins to act in the fcetus, viz. before any re gular circulation of blood takes place, and the existence of a heart or some shnilar contractile organ in all those animals in which a regular circulation of blood or nutritious fluids occurs, are confirmatory of the view suggested by direct observation and experiment. Under the article HEART will be found a detailed account of the structure and functions of this or,9,an ; in this place we shall only- state, in as few words as we can, what seems to have been best ascer tained regarding its action, in so far as this appears to have a reference to the force of im pulsion and direction which it coinmunicates to the blood.
The action of the heart may be observed by opening the chest of a living animal, or for a short time in one immediately after death, or best of all in an animal deprived of sense and motion by poison, and in which artificial respira tion is maintained ; it has also been seen in chil dren born with ectopia cordis, or in persons in whom from accident a part of the' heart has been exposed to view. When observed under
one or other of these circumstances, the action or contraction of the whole heart is seen to consist of two motions, viz. 1, the contraction or systole of the auricular part, and 2, that of -the ventricular part of the organ. The con traction of the auricle immediately precedes that of the ventricle and seems to be continued into it, and the systole of each cavity is imme diately followed by its diastole or relaxation.*• After the relaxation of the ventricle, there is a period of repose, or a pause in the action of the heart, during which motion seems to be nearly suspended. At the moment when the systole of the ventricle takes place, the heart appears to be diminished in all its dimensions, and exactly at the same instant of time, the apex is seen to be moved towards the sternum, in whatever position the animal is placed. This tilting forwards of the apex gives the heart a pulsation against the ribs that can be felt externally. This pulsation probably de pends on the arrangement of the muscular fibres of the heart, as the raising of the apex occurs when the heart is removed from the body and is empty of blood. At the time of the systole the heart is thicker and more conical in its figure than during the diastole; when held in the hand it feels hard, and the ventricles appear to have propelled the whole of the blood out of their interior, as far as one can judge from the great diminution in their size. In the inferior animals, as Reptiles and Fishes, its colour is lighter from the expulsion of the blood. During the relaxation or dias tole, the heart appears to fall away from the chest, its parietes become flaccid, and it as sumes a flattened form. The pulse in the arteries, which is in truth nothing more than the communication of the impulse of the heart along the blood in these vessels, corresponds, at least in the larger arteries near the heart, very exactly in time with the ventricular systole and the beat on the walls of the chest. The action of the heart is accompanied by two sounds, that can be heard on applying the ear to the cardiac region. The first of these sounds is synchronous with the systole of the lientricles, the second with their diastole; the second follows the first immediately, and is succeeded by an interval of silence. Of the space of time in which a full action of the heart is completed, the systole of the ventricle occu pies nearly a third, the systole of the auricle less than a quarter; the dilatation of the ven tricle and repose taken together must be effected in the remainder.