THE ACTION OF THE HEART 145 



through the thoracic walls into the heart walls, one of them so placed 

 that it pierces the apex of the ventricle, the other so that it pierces the 

 base. The needles then act as levers with their fulcra at the. chest wall, 

 and if the movements of their outer free ends, produced by the movements 

 of the heart, are observed, they will be found to confirm the observations 

 made on the exposed heart. 



More particular investigations of the changes occurring in the shape 

 of the heart cavity during systole and diastole have been undertaken by 

 making measurements of sections across the heart in one or other of 

 these conditions. For such purposes the heart in diastole is easily ob- 

 tained, but for the heart in systole it is necessary to use the somewhat 

 artificial means of injecting the heart with hot chromic acid solution 

 just before the death of the animal. The chromic acid causes the cardiac 

 muscle to contract and maintains it in this condition. The outcome of 

 these investigations is, however, not of much practical importance. 



Although it is now common knowledge that the direction of the flow 

 of the blood is from the veins to the arteries, yet it may be of interest 

 to consider for a moment the general principle of the methods by which 

 William Harvey succeeded in making this discovery. His evidence was 

 partly anatomic, partly experimental. He pointed out that the walls of 

 the veins, arid of the auricles to which they lead, are very thin, whereas 

 those of the arteries and ventricles are very thick, and he concluded that 

 in the veins the blood must flow gently from the tissues toward the 

 heart, to which the valves in the veins direct it, and that in the arteries 

 it must be propelled by pulses with each systole through the arteries 

 towards the tissues by the contraction of the walls of the ventricles. The 

 experimental support for this hypothesis he furnished partly by clamping 

 the large vessels, veins and arteries leading to or from the heart, and 

 observing the resulting distension or collapse of the vessel ; and partly by 

 calculation of the amount of blood which must be expelled from the 

 ventricles in a given period of time. 



Harvey's discoveries concerning the events of the cardiac cycle were 

 not much added to until experimental methods were devised by which 

 the pressure changes occurring in the various cavities could be measured 

 and compared. Until such measurements were elaborated, it was impos- 

 sible to investigate the mechanism by which the various valves between 

 the heart cavities and the vessels connected with them perform their 

 function, or to describe with any degree of accuracy the events occurring 

 in the heart chambers during the various phases of the cardiac cycle. 

 It is for the purpose of ascertaining the exact time relationship of these 

 changes that intracardiac pressure curves are studied. 



