146 THE CIRCULATION OF THE BLOOD 



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 sec- 

 tions across the heart in one or other of these conditions. For such purposes the heart in 

 diastole is easily obtained, but for the heart in systole it is necessary to use the some- 

 what artificial means of injecting the heart with hot chromic acid solution just be- 

 fore 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, and 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 distention or collapse of the vessel; and partly by cal- 

 culation 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. 



Intracardiac Pressure Curves 



The earliest method for taking such curves consisted in introducing 

 into the cardiac chambers and the blood vessels of the horse, so-called 

 cardiac sounds. These consisted of a more or less rigid tube furnished at 

 one end with a little elastic bag or ampulla and connected at the other 

 with a tambour, by means of rubber tubing. One of these little bags 



