ACTION OF THE HEART. 643 



The chief cause of the motion of the blood, is the contraction of the 

 muscular walls of the heart upon its fluid contents. But the move- 

 ment of the blood is modified by the elasticity and muscular contrac- 

 tility of the coats of the arteries ; it is aided by the pressure of the 

 muscles of the body upon the veins, and likewise, to a certain degree, 

 by the movements of the walls of the chest in respiration. Perhaps 

 also the changes incidental to nutrition, secretion, excretion, and res- 

 piration, which occur in the blood, in the systemic and pulmonary 

 capillaries, may influence its motion through them. The direction of 

 the blood-current, is primarily determined by the valves within the 

 heart, and is aided by those within the veins. 



Action of the Heart. 



The heart, the great agent concerned in the circulation, propels the 

 blood from its interior, into the body and lungs, by means of succes- 

 sive contractions of its ventricular walls. The contractility of its mus- 

 cular tissue is the immediate source of the motor power which impels 

 the blood through the body. The suspension of the heart within the 

 smooth pericardium, facilitates its movements; whilst the equally 

 smooth endocardium diminishes the friction of the blood against the 

 walls of its cavities. The heart has been likened to a force-pump, but 

 this is a rude comparison ; for a pump is a passive apparatus, through 

 which some extrinsic force operates ; whereas the heart, alternately 

 dilating and contracting, first receives the blood from the veins, and 

 then drives it into the arteries, by means of a force resident in its own 

 walls. In this action, the right and left auricles dilate and con- 

 tract together; and the right and left ventricles dilate and contract 

 together. The contraction of the two ventricles always takes place 

 immediately after that of the two auricles ; and when the ventri- 

 cles have contracted, an interval, or pause, occurs, before another 

 contraction of the auricles takes place, and during this pause both sets 

 of cavities are gradually dilating. The contraction of the auricles 

 and ventricles, is named their systole, whilst their dilatation is called 

 their diastole ; the order of the successive dilatation and contraction of 

 these parts, was compared by Harvey to the successive movements of 

 deglutition. Thus, when the auricles are dilating, they receive blood 

 from their respective veins, and when they contract, they force the 

 blood into the ventricles, which are then dilating to receive it ; when 

 the ventricles are filled, they in turn contract, and propel the blood 

 into the great arteries. But the right auricle receives the blood from 

 the body generally, and the right ventricle propels it into the lungs ; 

 whilst, of the left cavities of the heart, parted off completely from the 

 corresponding cavities of the right side, the left auricle receives the 

 blood from the lungs, and the left ventricle propels it into the body. 

 As the auricles merely propel the blood into the ventricles, their walls 

 are comparatively thin, whilst the thicker-walled ventricles frave re- 

 lation to the greater work they have to perform. The proportionately 

 greater thickness of the walls of the left ventricle, and the greater 

 strength of the mitral and aortic semilunar valves, as compared with 

 the tricuspid and pulmonary semilunar valves on the right side of the 



