THE CIRCULATION OF THE BLOOD 283 



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 dis- 

 charge of blood from its cavities and hence remain in a state of permanent 

 diastole. 



2. Negative Pressure. It has also been demonstrated by the employ- 

 ment of the minimal valve that there is brief moment in the cycle when the 

 intra-ventricular pressure is less than the pressure of the atmosphere, be- 

 coming indeed negative to it. This moment must be that between the be- 

 ginning of the relaxation of the ventricles and the opening of the auriculo- 

 ventricular valves. The extent of the negativity, as well as its duration 

 have never been satisfactorily determined. Goltz, however, found in the 

 left ventricle of the dog a minimal pressure of 23 to 50 mm. of mercury. 

 The cause of the negative pressure, its influence on the opening of the 



FIG. 122. 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, a-b, line of atmospheric pressure. The ordinates 

 0-5 correspond in the two records, o, Closure of the auriculo- ventricular valve; i, opening 

 of the semilunar valves; 2, point of maximum pressure; 3, beginning of the ventricular relaxa- 

 tion; 4, closure of the semilunar valves; 5, opening of the auriculo-ventricular valve. (Hurthle.) 



auriculo-ventricular valves, and on the entrance of blood into the ventricles 

 are equally unknown. A probable cause is an expansion of the base of 

 the ventricles due to the enlargement of the aorta and pulmonic artery. 

 That it is not due to the expansion of the thorax is evident from the fact 

 that it occurs when the thorax is open and the heart exposed. 



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 venae cavae and auricles on the 

 one hand, and in the aorta, on the other hand, and their relations one to 

 another accurately denned. 



