THE CIRCULATION OF THE BLOOD 283 



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, its duration and frequency 

 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 

 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 



FIG. 125. V. CURVE OF THE PRESSURE IN THE VENTRICLE OF THE DOG. A. CURVE or 

 THE PRESSURE IN THE AORTA. The curves were taken simultaneously. 5, 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 auriclo-ventricular valve. (Hiirthle.) 



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 defined. 



The intra-ventricular pressure has been obtained by specially devised 

 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 Hurthle. 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 

 the jugular vein or into the left ventricle through the carotid artery. Both 



