148 THE CIRCULATION OF THE BLOOD 



sure changes occur as must serve to produce opening or closing- of the 

 various valves; and (2) the contour of the curves obtained from each 

 cavity may be examined in order to find out exactly how the pressure 

 in that particular cavity is behaving. 



Comparison of the Curves 



Before using the curves for ascertaining the relative pressure in the 

 different cavities, they must be graduated according to some scale, for 

 it is clear that by the use of instruments like those we have been describ- 

 ing, the absolute pressure value of each curve will vary according to the 

 construction of the instrument (thickness of membrane, etc.), and in- 

 deed instruments of varying degrees of resistance must be employed in 

 taking curves from places having such different pressures as exist in 

 the auricles and ventricles. The graduation is, however, a very easy 

 matter, and consists, as already explained (page 126), in connecting the 

 instrument by means of a T-piece with a mercury manometer and a pres- 

 sure bottle and then marking on the tracing, the points corresponding to 

 each 10, 20 or 50 millimeters of increase of pressure, as the case may be. 



To ascertain the time relationship between the opening and the closing 

 of the auriculoventricular valve, the tracings should be taken from the 

 right auricle and the right ventricle, and to ascertain the same with re- 

 gard to the semilunar valve, from the left ventricle and the aorta.* 



By comparing the curves it is now an easy matter to ascertain the 

 exact moment at which the pressure in the one cavity comes to equal 

 that in the other. This moment, read on the accompanying time tracing, 

 will obviously indicate that at which the particular valve is just about to 

 open or close. From the results of such experiments, the curves may be 

 superimposed as in Fig. 34. , 



In the first place let us compare the curves from the right auricle and 

 ventricle. The curves begin at the very end of diastole, and they show 

 that a distinct increase in pressure is occurring in both auricle and ven- 

 tricle and lasting about 0.05 second. This is of course caused by auric- 

 ular systole, and since it occurs in both cavities, it indicates that the 

 passage between them, the auriculoventricular orifice, must be open. 

 The ventricular curve then suddenly shoots away beyond the auricular 

 because of the onset of systole in the ventricle, and the point at which 

 the two curves begin to separate indicates the moment at which the 

 auriculoventricular valves close. From this time on until ventricular 

 systole has given place to diastole, (about 0.2 second), the auricle is 



*The connections with the heart may be made by pushing long cannulae down the large veins or 

 arteries, or in the case of the ventricles by inserting a cannula with a sharp point directly through 

 the wall of the ventricle. 



