SECTION IV 

 THE MECHANISM OF THE HEART PUMP 



IN the mammal the two sides of the heart are only in communication 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 discharges 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 ven- 

 tricles in a contracted condition 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 the left ventricle. 

 The middle layer is connected by many strands of muscular fibres with both 

 inner and outer layers. 



Mall divides the muscular fibres of the mammalian heart into four groups, two super- 

 ficial and two deep, as follows : 



(1) The superficial bulbo-spiral fibres. These arise from the conus arteriosus, the 



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