278 TEXT-BOOK OF PHYSIOLOGY. 



coming 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 con- 

 siderably at times, the heart must meet and exceed each rise, within limits if 

 the circulation is to be maintained. This it does by calling on the reserve 

 power with which it is endowed. 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 of its walls, but in so doing it encroaches on the reserve 

 power proportionally and when the latter has become exhausted the heart 

 may, on some sudden rise of pressure in the aorta, be unequal to the discharge 

 of blood from its cavities and hence become paralyzed. 



The Intra-ventricular Pressure Curve of the Dog. It was stated in 

 a previous paragraph that the contraction of the auricles and ventricles of 

 animals other than the horse have been graphically recorded. This is 

 especially true of the heart of the dog. A graphic record of the intra- 

 ventricular pressure, its course, its variations, and time relations is necessary 

 for the interpretation of the heart mechanisms. With such a record may be 

 compared the records of the pressures in the vense cavae and auricles 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 devised 

 manometers or tonometers or tonographs, as they are variously termed, the 



FIG. 128. V. CURVE OF THE PRESSURE IN THE VENTRICLE OF THE DOG. A. CURVE OF 

 THE PRESSURE IN THE AORTA. The curves were taken simultaneously, s, Tuning-fork vibrations 

 each corresponding to i/ioo of a second. The ordinates 0-5 correspond in the two records, 

 o, Closure of the auriculo- ventricular valve; i, opening of the semi-lunar valves; 2, point of 

 maximum pressure; 3, beginning of the ventricular relaxation; 4, closure of the semi-lunar valves; 

 5, opening of the auriclo-ventricular valve. (Hurthle.} 



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 diameter, 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 

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

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



