SECTION IV 

 THE MECHANISM OF THE HEART PUMP 



IN the mammal the two sides of the heart are only in communica- 

 tion by means of the blood-vessels of the systemic and pulmonary 

 area. Each side consists of an auricle into which the veins open, 

 and a ventricle which receives the blood from the auricle and dis- 

 charges it into the arterial trunk either aorta or pulmonary artery. 

 Since the auricles have to act merely as a receptacle for part of the 

 blood which enters during the relaxation or diastole of the heart, their 

 cavities are smaller than those of the ventricles, and their walls are 

 thin, corresponding to the small amount of work thrown on them 

 in propelling blood into the relaxed ventricle. The ventricles have 

 the office of carrying on the main work of the circulation and of forcing 

 blood through the peripheral resistance. Their walls are much 

 thicker than those of the auricles. The right ventricle has a wall 

 which is only about one-fourth the thickness of the left ventricle, in 

 conformity with the much heavier work to be done by the latter. 

 On cutting a section through the two ventricles in a contracted con- 

 dition the thin wall of the right ventricle is seen to lie in the 

 form of a 'crescent round the circular left ventricle. The capacity 

 of both ventricles is approximately equal, and amounts in man to 

 about 140 c.c. for each ventricle when the heart is completely 

 relaxed. 



The auricles are separated from the ventricles by a fibrotendinous 

 ring. From this ring take origin most of the muscular fibres of the 

 heart walls. The muscular fibres of the auricles run in both circular 

 and longitudinal directions, the circular fibres being continued round 

 both auricles, and special rings of circular fibres surrounding the 

 openings of the great veins. From the fibrotendinous ring between the 

 auricle and the left ventricle and from the sides of the aorta the 

 muscular fibres forming the superficial layer of the ventricular wall 

 pass obliquely downwards to the left towards the apex of the ventricle. 

 Here they loop round into the interior of the ventricle and pass up 

 near its inner surface to end either in the papillary muscles or in the 

 auriculo-ventricular ring of fibrous tissue. Between these two layers 

 we find a third median layer of muscular fibres which is in the form 

 of a muscular cone. The fibres of this layer form complete loops round 



1004 



