SECTION IV 

 THE MECHANISM OF THE HEART PUMP 



IN the mammal the two sides of the heart are in communication only 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 ventricles 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 arlerlosus, the 

 ft side of the aorta and the left side of the auriculo-ventricular ring, and take an 



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