THE CIRCULATION OF THE BLOOD. 279 



sures of 23 to 52 mm. for the left ventricle have also been 

 obtained. 



The maximal pressure in the ventricles during the systole, though 

 always higher than that in the arteries, is not a fixed or an invariable 

 pressure, as it rises and falls with the latter from moment to moment. 

 Within limits the cardiac power, and therefore the intra-cardiac 

 pressure, is capable of considerable increase. The function of the 

 heart is to drive the blood through the vessels with a given velocity. 

 This is only possible by first overcoming the resistance to the flow 

 offered by the vessels, as indicated by the arterial pressure. As this 

 is a variable factor, rising and falling very considerably at times, the 

 heart must meet and exceed each rise, if the circulation is to be main- 

 tained. The power put forth by the heart is proportional to the 

 work it has to perform. If the arterial pressure continues- higher 

 than the average for any length of time, the heart meets the condition 

 by an hypertrophy oi JtsJEfllls* ^ 



The Ihtra- ventricular Pressure Curve. An accurate interpre- 

 tation of the play of the heart mechanism necessitates the obtaining of 

 a graphic record of the course of the intra- ventricular pressure, its varia- 

 tions and time relations. With such a record may be compared the 

 records of the pressures in the venae cavae, on the one hand, and in the 

 aorta, on the other hand, and their relations one to another accurately 

 defined. 



The intra- ventricular pressure has been obtained by specially de- 

 vised manometers or tonometers or tonographs, as they are variously 

 termed, the construction of which is such as to enable them to respond 

 instantly to the very rapid variations of the pressure which occur 

 during the brief cardiac cycle. One of the best is that of Hiirthle. 

 This consists of a small metallic tambour 5 or 6 millimeters in diam- 

 eter, covered by a thin rubber membrane. A small button resting 

 on the membrane plays against an elastic steel spring, by the tension 

 of which the pressure of the blood is counterbalanced. The move- 

 ments of the membrane are taken up, magnified, and recorded by a 

 suitable lever. A long cannula is inserted into the right ventricle 

 through the jugular vein or into the left ventricle through the carotid 

 artery. Both cannula and tambour are filled with an alkaline solu- 

 tion to prevent coagulation of the blood, and then joined air-tight. 

 The pressure of the blood in the ventricle is thus transmitted by a 

 liquid column to the tambour and to its attached lever. With such 

 a manometer a curve is registered similar to that shown in Fig. 123. 

 To obtain the absolute value of this curve in millimeters of mercury 

 it is necessary to previously graduate the instrument. An examination 

 of the curve shows that previous to the ventricular contraction there 

 is a very slight rise of pressure above that of the atmosphere, repre- 

 sented by the line a b. This may be due to the inflow of blood 



