THE MECHANISM OF THE CIRCULATION 



439 



the orifice of which is also guarded by three semilunar or sigmoid 

 valves. 



The heart of the horse beats in health about forty times in the minute. 

 The order of succession in the contraction is that the two auricles beat 

 simultaneously, and then the two ventricles. Then there is a pause. The 

 duration of the contraction is nearly the same as the pause. The con- 

 traction of the heart is named the systole. During the systole the whole 

 heart becomes shorter and more conical in form, and twists a little upon 

 its long axis towards the right. This contraction of the heart commences 

 above at the opening of the great veins, which, being here surrounded 

 by muscular bands, nearly close, and thus greatly impede, but do not 

 altogether prevent, the backward 

 flow of the blood from the heart 

 into the venous system. From 

 the great veins a wave of con- 



o 



traction instantly spreads over the 

 auricles, driving the blood they 

 contain through the auriculo- 

 ventricular valves into the ven- 

 tricles, which, already containing 

 a little blood that has gravitated 

 into them, are now distended to 

 the utmost. There is no appreci 

 able interval between the contrac- 

 tion of the auricles and that of 

 the ventricles, but the wave of 

 contraction continuing, without 

 stay or stop, to spread from the 



auricles, makes the ventricles close upon their contents. The first effect 

 of this is to raise the auriculo-ventricular valves and approximate their 

 edo-es, and thus to prevent any return of blood into the auricles. The 

 next is to propel the blood contained by the ventricles into the pulmonary 

 artery and the aorta, for distribution through the lungs in the one case and 

 over the general system in the other. In so doing the blood forces open 

 the semilunar and sigmoid valves in these vessels and compels the whole 

 mass of blood to move onwards. But as the column of blood resting on 

 the valves just before they are forced open is quiescent, a brief period is 

 required to overcome its inertia, and a remarkable provision against the 

 jar that would otherwise be felt through the body, from the vigour and 

 suddenness of the heart's systole, is found in the elasticity of the coats 

 of the large vessels. Whilst, then, a part of the heart's force drives the 



Fig. 191. Section of the Heart, showing the Valvular 

 Apparatus 



A, Aorta. B, Pulmonary Artery. C, Mitral Valve. 

 D, Tricuspid Valve. 



