CIRCULATION OF THE BLOOD. Ill 



a fine cord of fibrous tissue, is a small fibrous nodule, the corpus Arantii, 

 and from this and from the attached border fine fibres extend into every 

 part of the mid substance of the valve, except a small lunated space just 

 within the free edge, on each side of the corpus Arantii. Here the valve 

 is thinnest, and composed of little more than the endocardium. Thus 

 constructed and attached, the three semilunar valves are placed side by 

 side around the arterial orifice of each ventricle, so as to form three little 

 pouches, which can be separated by the blood passing out of the ventricle, 

 but which immediately afterward are pressed together so as to prevent 

 any return (7, Fig. 93, and 7, Fig. 99). This will be again referred to. 

 Opposite each of the semilunar cusps, both in the aorta and pulmonary 

 artery, there is a bulging outward of the wall of the vessel: these bulg- 

 ings are called the sinuses of Valsalva. 



Structure of the Valves. The valves of the heart are formed es- 

 sentially of thick layers of closely woven connective and elastic tissue, over 

 which, on every part, is reflected the endocardium. 



THE ACTION OF THE HEART. 



The heart's action in propelling the blood consists in the successive 

 alternate contraction (systole) and relaxation (diastole) of the muscular 

 walls of its two auricles and two ventricles. 



Action of the Auricles. The description of the action of the heart 

 may best be commenced at that period in each action which immedi- 

 ately precedes the beat of the heart against the side of the chest. For at 

 this time the whole heart is in a passive state, the walls of both auricles 

 and ventricles are relaxed, and their cavities are being dilated. The auri- 

 cles are gradually filling with blood flowing into them from the veins; and 

 a portion of this blood passes at once through them into the ventricles, 

 the opening between the cavity of each auricle and that of its correspond- 

 ing ventricle being, during all the pause, free and patent. The auricles, 

 however, receiving more blood than at once passes through them to the 

 ventricles, become, near the end of the pause, fully distended; and at the 

 end of the pause, they contract and expel their contents into the ventricles. 



The contraction of the auricles is sudden and very quick; it commences 

 at the entrance of the great veins into them, and is thence propagated 

 toward the auriculo-ventricular opening; but the last part which contracts 

 is the auricular appendix. The effect of this contraction of the auricles is 

 to quicken the flow of blood from them into the^ ventricles; the force of 

 their contraction not being sufficient under ordinary circumstances to 

 cause any back-flow into the veins. The reflux of blood into the great 

 veins is, moreover, resisted not only by the mass of blood in the veins and 

 the force with which it streams into the auricles, but also by the simulta- 

 neous contraction of the muscular coats with which the large veins are 



