78 EVENTS DURING A CARDIAC CYCLE. 



into the cardiac veins is prevented by valves. The movement of the 

 heart causes a regular pulsatile phenomenon in the blood of the vense 

 cavse, which under abormal circumstances may produce a venous pulse 

 (see Venous pulse). 



(&.) The chief motor effect of the contraction of the auricles is the 

 dilatation of the relaxed ventricle, which has already been dilated to a 

 slight extent by the elastic force of the lungs. 



The dilatation of the ventricles has been ascribed to the elasticity of the 

 muscular walls the strongly contracted ventricular walls (like a compressed india- 

 rubber bag), in virtue of their elasticity, are supposed to return to their normal 

 resting form, and thereby to suck in or aspirate the blood under a negative pres- 

 sure. Such suction power on the part of the ventricle is, however, only effective 

 to a very slight extent. 



(c.) When the ventricles are distended by the inflowing blood, the 

 auriculo-ventricular valves are floated up, partly by the recoil or 

 reflexion of the blood from the ventricular wall, and partly owing to 

 their lighter specific gravity, whereby they easily float into a more or 

 less horizontal position. The valves are also raised to a slight extent 

 by the longitudinal muscular fibres, which pass from the auricles into 

 the cusps of the valve (Paladino). 



(C.) The Ventricles now Contract, and simultaneously the auricles 

 relax, whereby 



(1.) The muscular walls contract forcibly from all sides, and thus 

 diminish the ventricular cavity. 



(2.) The blood is at once pressed against the under-surface of the 

 auriculo-ventricular valves, whose curved margins are opposed to each 

 other like teeth, and are pressed hermetically against each other (Sand- 

 borg and Worm Miiller). It is impossible for the blood to push the 

 cusps backwards into the auricle, as the chordae tendinice hold fast their 

 margins and surfaces like a taut sail. The margins of the neighbouring 



cusps are also kept in apposition by the 

 chordae tendiniae from one papillary muscle 

 always passing to the adjoining edges of 

 two cusps (John Reid). The extent to 

 which the ventricular wall is shortened is 

 compensated by the contraction of the 

 papillary muscle, and also of the large 

 muscular chordae, so that the cusps cannot 

 be pushed into the auricle. 



pig, 24. " When the valves are closed their surfaces 



The closed semi-lunar are horizontal, so that even when the 

 valves of the pulmonary ventricles are contracted to their greatest 

 artery seen from below. extentj ft small amount of Uood remainSj 



which is not expelled (Sandborg and Worm Miiller). 



