86 PHYSIOLOGY FOR BEGINNERS CHAP. 



body except the lungs. It gives off a large branch on the 

 right side almost at once. There are semilunar valves at its 

 commencement from the left ventricle just like the semilunar 

 valves at the origin of the pulmonary artery from the right 

 ventricle. Hold the aorta under a tap and let the water run 

 gently down it towards the ventricle. Notice that the three 

 pockets of the semilunar valve swell out, and meeting in the 

 middle, block the way so that the water will not run in. 

 The valve will allow blood to pass from the left ventricle into 

 the aorta, but not back again. Cut open the aorta with 

 scissors, from the ventricle upwards, and see that the pockets 

 of the semilunar valve open away from the ventricle towards 

 the artery. Behind each of two of the pockets is a small 

 opening from the aorta. These are the openings of two 

 arteries which give branches to the substance of the heart 

 itself. They are called the coronary arteries. After the 

 blood which these carry from the aorta has been sent 

 through the capillaries in the substance of the heart it flows 

 back into the right auricle by a vein, the coronary vein, 

 which has a rather large opening into that chamber just below 

 the septum between the auricles. Some of the coronary vessels 

 can be seen running in the grooves on the surface of the heart. 

 We have seen that there are valves where each auricle opens 

 into its corresponding ventricle, and where each ventricle opens 

 into the artery leaving it ; there are, however, none where the 

 veins open into the auricles. 



The Human Heart. The human heart is a little smaller 

 than that of the sheep, being about the size of the person's fist. 

 It does not differ in any important respect from the heart of 

 the sheep except that there are four pulmonary veins opening 

 into the left auricle instead of two ; so that the description 

 given is true also for the heart of man. 



The Tissues of the Heart. The heart is, as we have 

 seen, composed of muscular tissue of a special kind. This is 

 covered on the outer side by the thin shining membrane, the 

 visceral layer of the pericardium, closely adherent to the 

 muscular tissue. On the inner side the muscular tissue is 

 covered, and so the chambers of the heart lined, by a thin 

 transparent membrane, called the endocardium, which is 

 also closely adherent to the muscular tissue. The valves are 



