138 COUESE OF THE BLOOD IN THE HEART. 



and are especially characterized by their disposition to anastomose with 

 one another, as represented in Fig. 60. In the ventricles, the arrange- 

 ment is such that the fibres of the external and internal surfaces decus- 

 sate. 



The motions of the heart consist in the relaxations and contractions of 

 Eelaxations the muscular walls of its cavities. The two auricles contract 

 dons^fThe" at tf 16 same moment, as do also the two ventricles, but the 

 heart. contractions of the auricles coincide with the relaxations of the 



ventricles. 



The course of the blood through the heart is this. The venous blood, 

 Course of the brought by the ascending and descending cava3, flows into 

 blood in the the right auricle as it is dilating, and for the moment pushes 

 movements of forward to the ventricle, but the auricle, being of less capac- 

 the valves. fry t h an ^Q ventricle, is rilled to distention first ; at this in- 

 stant it contracts, forcing its contents past the tricuspid valve into the 

 ventricle, and fills it completely. The blood can not regurgitate into the 

 veins to any extent while this is going on, because of the almost perfect 

 closure of their valves. The right ventricle now commences to contract ; 

 its fleshy columns shorten so as to pull upon the tendinous cords attach- 

 ed to the flaps of the tricuspid valve : this enables the blood to get be- 

 hind them, and they quietly close the aperture between the auricle and 

 ventricle ; the closure is riot, however, under all circumstances, perfect, 

 the mechanism being such as to permit leakage or regurgitation to a lim- 

 ited extent. The blood now rushes into the pulmonary artery, passing 

 by its semilunar valves, which, the moment the ventricular pressure 

 ceases, shut, so as to prevent any return to the heart. 



Having passed through the lungs and been submitted to the air, the 

 blood now returns to the left auricle, which forces it into the left ventri- 

 cle, the action on this side of the heart being the same as on the other; 

 the mitral valve, which closes the opening from the auricle into the ven- 

 tricle, is worked in the same manner as the tricuspid, and the blood is 

 pressed into the aorta, the semilunar valves of which, at that instant, 

 shut abruptly with an audible sound, and prevent any regurgitation. In 

 this manner the distribution to the system is accomplished. 



On both sides of the heart, as soon as the auricles have finished their 

 contraction, they begin to dilate, and continue to do so during the peri- 

 od that the ventricles are contracting. Thus there is an accumulation 

 in them when the ventricles are ready to dilate, and, as soon as that oc- 

 curs, the blood flows freely forward into those cavities, the complete dis- 

 tention of which is then accomplished by the contraction of the auricles, 

 as before explained. 



Movements of The mode of action of the two sets of cavities is different, 

 and ventricles. Tlie auricle s contract suddenly, first at the place of junction 



