CHAP, iv.] THE VASCULAR MECHANISM. 247 



sure taken by other methods ; and indeed, as shewn in Fig. 37 and 

 in others which we shall give, corresponding features occur in 

 curves of other changes in the heart. All these curves shew a 

 flattening maintained, with smaller variations, during the con- 

 tinuance of the systole ; this is so characteristic that it has been 

 called the 'systolic plateau.' It is true that curves of ventri- 

 cular pressure taken by certain methods, that of Frey and Krehl's 

 for instance, do not shew this 'plateau,' the curve in such cases 

 rising gradually to a maximum and immediately beginning to fall, 

 so that the summit is a simple peak. And it is argued that such 

 a curve is the true curve of ventricular pressure always obtained 

 so long as the blood in the ventricle has free access to the interior 

 of the catheter, and that the plateau is only seen when the end of 

 the catheter is too near the apex, and its opening closed, at the 

 height of the systole, by the ventricular walls coming together ; the 

 top of the true curve is thus as it were cut off. But the evidence 

 is on the whole opposed to this view, and we shall accept the 

 plateau as being a true representation. 



Though the curves given above agree in these main features, 

 they differ in many minor features, and other features also of minor 

 value appear in curves of endocardiac pressure according to the 

 various circumstances in which the heart finds itself. Some of 

 these minor features we shall presently find useful in discussing 

 the mechanism of the beat. 



132. The output. Since the use of the pressure exerted by 

 the ventricle is to drive a quantity of blood out of the ventricle 

 into the aorta (or pulmonary artery) it is important to study the 

 ' output ' or quantity of blood so driven out ; and since, under 

 normal circumstances, the quantity ejected by the right ventricle 

 is the same as that ejected by the left ventricle we may confine 

 our attention to the latter. 



The normal or average output has been calculated in various 

 ways, by help of certain assumptions ; but these we may put on 

 one side since the matter has now been made the subject of direct 

 experimental determination. 



Methods. Method of Stolnikow. This consists in allowing the 

 blood to flow from the carotid into a vessel until a certain measured 

 quantity has escaped, and then returning this blood to the right 

 auricle while the blood from the carotid is flowing into a second 

 similar vessel to be similarly returned, and in repeating this manoeuvre 

 a certain number of times. One carotid is tied (the animal being a 

 dog), and the arch of the aorta plugged beyond (Fig. 44 p). The 

 circulation is thus confined to the lungs and the coronary system. 

 Into the other carotid is tied a tube connected by a forked branching 

 la and 2a with two vessels I. and II., which also communicate by a 

 similar forked branching Iv and 2v with the right auricle. The 

 blood is allowed to flow through la into I. until a certain quantity has 

 escaped. Then la is closed, while 2a and Iv are opened. The blood 



