The Creation of God by Jacob Hartmann (color ebook reader TXT) 📖

The heart is a hollow muscular organ divided by a longitudinal septum into a right and a left half, each of which is again subdivided by a transverse constriction into two compartments communicating with each other, and named auricle and ventricle.

The heart is inclosed in the pericardium and placed behind the sternum and costal cartilages on the border end or base, by which it is attached, being directed upwards, backwards, and to the right, and extending from the level of the fourth to that of the eighth dorsal vertebra, the apex downwards, forwards, and to the left.

In size, it is about five inches long, three and a half in its greatest width, and two in its extreme thickness from the anterior to the posterior surface. The weight is from nine to ten ounces.

The circulation of the blood.—

The body is divided into two chief cavities, the chest or thorax, and abdomen, by a curved muscular partition called the diaphragm or midriff. The chest is almost entirely filled with lungs and heart, the latter being fitted in, so to speak, between the two lungs, nearer to the front than the back of the chest, and partly overlapped by them.

In the living body the heart and lungs are in constant rhythmic movement, the result of which is an unceasing stream of air through the trachea alternately into and out of the lungs, and an unceasing stream of blood into and out of the heart.

The blood is conveyed away from the heart by the arteries and returned to it by the veins; the arteries and veins being continuous with each other, at one end by means of the heart, and at the other by a fine network of vessels called capillaries. The blood, therefore, in its passage from the heart passes first into the arteries, then into the capillaries, and lastly into the veins, by which it is conveyed back again to the heart—thus completing a revolution, or circulation.

There are two circulations by which all the blood must pass—the one a shorter circuit from the heart to the lungs and back again, which is called the pulmonic; the other the larger circuit, from the heart to all parts of the body and back again, which is called the systemic; and a subordinate stream of blood, that has been collected by the blood-vessels of the intestines, passes by means of the portal vein through the liver, and is called the portal circulation.

The principal force provided for constantly moving the blood on this course, is that of the muscular substance of the heart; other assistant forces are (2) those of the elastic walls of the arteries, (3) the pressure of the muscles among which some of the veins run, (4) the movements of the walls of the chest in respiration, and (5) probably to some extent the interchange of relations between the blood and the tissues which ensues in the capillary system during the nutritive processes. The right direction of the blood’s course is determined and maintained by the valves of the heart.

The heart is divided into two chief chambers or cavities—right and left. Each of these chambers is again divided into an upper and lower portion called respectively auricle and ventricle, which freely communicate with each other.

The right auricle communicates on the one hand with the veins of the general system and on the other with the right ventricle. The valvular curtain between the right auricle and the right ventricle is named the tricuspid; by it the auricle is guarded from the ventricle. The ventricle leads directly into the pulmonary artery and this in turn into the lungs. The pulmonary artery is guarded by three semilunar valves. The left auricle again communicates on the one hand with the pulmonary vein and on the other with the left ventricle, which is guarded by the mitral or bicuspid valve. The left ventricle leads directly into the aorta, which is also guarded by three semilunar valves. The aorta is a large artery which conveys the blood to the general system.

The arrangement of the heart’s valves is such that the blood can pass only in one definite direction, and this is—from the right auricle the blood passes into the right ventricle, and thence into the pulmonary artery, by which it is conveyed to the capillaries of the lungs. From the lungs, the blood, which is now purified and altered in color, is gathered by the pulmonary veins and taken to the left auricle. From the left auricle it passes into the left ventricle, and thence into the aorta, by which it is distributed to the capillaries in every portion of the body.

The Heart’s action. The heart’s action in propelling the blood consists in the successive alternate contractions and dilatations of the muscular walls of the two auricles and ventricles. The auricles contract simultaneously; so do the ventricles; their dilatations also are severally simultaneous; and the contractions of the one pair of cavities are synchronous with the dilatations of the other.

Valves—Bi and Tricuspid. During auricular contraction the force of the blood propelled into the ventricle is transmitted in all directions, but being insufficient to raise the semilunar valves, it is expended in distending the ventricle and in raising and gradually closing the auriculo-ventricular valves (tricuspid and bicuspid valves). These when the ventricle is full form a complete septum (partition) between it and the auricle.

The arterial or semilunar valves are brought into action by the pressure of the arterial blood forced back towards the ventricles, when the elastic walls of the arteries recoil after being dilated by the blood propelled into them in the previous contraction of the ventricle.

The sounds. When the ear is placed over the region of the heart two sounds may be heard at every beat of the heart, which follow in quick succession, and are succeeded by a pause or a period of silence. The first sound is dull and prolonged; its commencement coincides with the impulse of the heart and just precedes the pulse at the wrist. The second is a shorter and sharper sound, with a somewhat flapping character, and follows close after the arterial pulse.

First sound. The chief cause of the first sound of the heart appears to be the vibration of the auriculo-ventricular valve, and also, but to a less extent, of the ventricular walls, and the coats of the aorta and pulmonary artery, all of which parts are suddenly put into a state of tension at the moment of ventricular contraction.

The second sound is more complete than that of the first. It is probably due entirely to the sudden closure and consequent vibration of the semilunar valves when they are pressed down across the orifice of the aorta and pulmonary artery.

Pulse. The heart of a healthy adult man in the middle period of life acts from seventy to seventy-five times per minute. The frequency of the heart’s action gradually diminishes from the commencement to near the end of life.

In persons of sanguine temperament, the heart acts somewhat more frequently than in those of the phlegmatic; and in the female sex more frequently than in the male; in children, more frequently still.

Capacity. The capacity of the two ventricles is probably exactly the same. From the mean of various estimates taken, it may be inferred that each ventricle is able to contain on an average about three ounces of blood, the whole of which is impelled into the respective arteries at each contraction.

Every time the ventricles contract three ounces of blood are pumped out of the heart into the lungs and heart respectively.

Calculating seventy pulses per minute, the quantity of blood passing through the heart would be about 211 ounces, or 14¼ pints per minute; or 895 pints per hour, or 21,480 pints in 24 hours.

Velocity. The velocity of the stream of blood is greater in the arteries than in any other part of the circulatory system, and in them it is greatest in the neighborhood of the heart and during the ventricular systole; the rate of movement diminishes during the diastole of the ventricles, and in the parts of the arterial system most distant from the heart. The rate is calculated to be about from 10 to 12 inches per second in the large arteries near the heart.