HEART 



2745 



HEART 



in climbing a mountain 2,500 feet high. The 

 force expended by the heart in one hour would 

 raise a man weighing 160 pounds seventy-two 

 feet. A day's work of this wonderful organ has 

 been computed to be equal to that of about 

 7.5 horse power. 



Description. The heart is a flattened, pear- 

 shaped pouch about the size of a closed fist, 

 averaging five and one-half inches from base 

 to apex and three and one-half inches across 

 its broad surface. In men its approximate 

 weight is eleven ounces; in women, nine 

 ounces. It lies obliquely within the chest, with 

 its broader end, or base, in the direction of the 

 right shoulder and its apex pointing downward, 

 forward and toward the left. As the lower 

 part can be felt beating the more easily, for 

 at each stroke the narrow end strikes against 

 the wall of the chest, it is commonly believed 

 that the heart is on the left side of the body. 

 On the contrary it lies very nearly in the 

 middle line. The narrow end can be felt beat- 

 ing if the hand is placed between the carti- 

 lages of the fifth and sixth ribs to the left of 

 the breastbone. The larger end extends along 

 the right side of this bone upward to the third 

 rib. The whole organ is enclosed in a bag 

 of membrane called the pericardium, and is 

 nourished by 

 blood conveyed 

 to it by the 

 coronary arteries. 



The heart is 

 ingeniously con- 

 structed to pump 

 two different 

 streams of blood 

 at the same time, 

 and is divided 

 into right and 

 left cavities, with 

 no opening what- 

 ever in the mus- 

 cular partition 

 which runs down 

 its center 

 top 



Each of these 

 cavities is in turn (/) 

 divided into 

 upper and a lower 

 chamber, called 



FRONT VIEW 

 from () Superior vena cava 



(b) Right auricle 

 to bottom. ( C ) Right ventricle 



(d) Left ventricle 



(e) Left auricle 

 Pulmonary vein 



(g) Pulmonary artery 

 an (h) Aorta 



Right subclavian artery 

 Right carotid artery 



O) 



(fc) Left carotid artery 



respectively right and left auricles and right 

 and left ventricles. All four chambers have 

 elastic, muscular walls, but those of the ven- 

 tricles are much the stronger and thicker, 



because the latter are the pumping chambers 

 of the heart and do the harder work. Further- 

 more, as the left side pumps blood throughout 

 the body, and the right side only through the 

 lungs, the walls of the left ventricle are three 

 times as thick as those of the right ventricle. 



Circulation in the Heart. This section 

 should be read with reference to the article 

 CIRCULATION OF THE BLOOD, page 1387, where 

 there is a color plate. The auricles are the 

 receiving chambers of the heart, the ventricles 

 the delivery rooms. The blood which is to 

 circulate through the lungs enters the right 

 auricle through two large veins, the venae 

 cavae. From the right auricle it is forced by 

 the contraction of the walls of that chamber 

 into the right ventricle, passing through an 

 opening guarded by a valve consisting of three 

 little triangular flaps of membrane (tricuspid 

 valve). This valve is so constructed as to 

 permit the blood to flow into the ventricle, 

 but prevents its flowing backward, acting some- 

 thing like double-swinging doors which open 

 in but one direction. Between the left auricle 

 and left ventricle is a similar valve having two 

 flaps, and called, because of its resemblance to 

 a bishop's miter, the mitral valve. 



When the right ventricle is filled its muscles 

 contract and squeeze the blood into the pul- 

 monary artery, which leads to the lungs. The 

 walls of the auricle, at the same time, are 

 relaxing and receiving another supply of blood. 

 The pressure exerted by these movements 

 closes the tricuspid valve and opens three 

 valves between the ventricle and the pulmo- 

 nary artery. These are called semilunar valves, 

 because they are shaped like half-moons, and 

 they are also so constructed as to prevent 

 the blood from flowing backward. When all 

 of that fluid is expelled from the ventricle 

 its walls relax and the semilunar valves close. 

 The blood then flows into the lungs, where it 

 goes through a purifying process, returning to 

 the left auricle through the pulmonary veins. 



Now begins the circulation of the purified 

 blood through the left side of the heart. From 

 the left auricle the blood passes into the left 

 ventricle through the mitral valve, and out 

 of the left ventricle through another set of 

 semilunar valves into the largest artery in the 

 body, the aorta. This artery, by means of its 

 numerous branches, sends the blood to all 

 parts of the body; when the circulation is 

 completed it is returned to the right auricle 

 to start again on its journey through the 

 lungs. It requires only about half a minute 



