218 CIRCULATION OF THE BLOOD. 



stream at the commencement of the aorta in other words, 

 with the quantity of blood delivered by the heart per minute. 

 The work done by the heart in maintaining the circulation, 

 manifests itself in the aorta in two modes, those of pressure 

 and progressive motion of the blood. These two phenomena 

 are not, however, collateral results, i. e., they do not stand in 

 the same relation to the agent which produces them. The 

 former is rather the efficient cause of the latter ; for so long 

 as the arterial pressure continues, i. e., so long as the pressure 

 in the aorta is greater ilmn that in the venae cavre, progressive 

 movement also continues. As soon as equilibrium is estab- 

 lished, circulation stops. Systemic death consists in decline 

 of aortic pressure. This decline may occur rapidly, as in 

 syncope; but usually, even in deaths by violence, it is very 

 gradual. In deaths from disease it may last for days, weeks, 

 or even months. 



SECTION I. ARTERIAL PRESSURE. 



33. The arterial pressure, although in the mean remarkably 

 constant, almost as constant as the temperature of the body, 

 is subject to recurring variations i. e., alternate augmenta- 

 tions and diminutions, which are of three orders. Of these, 

 the first is dependent on the rhythmical injection of blood into 

 the arteries by the contraction of the heart ; the second, on the 

 influence which the respiratory movements, or rather the alter- 

 nate acts of breathing, exercise on the circulation ; the third, 

 on augmentations or diminutions of what is called the tonusof 

 the arteries, by virtue of which they are constantly undergoing 

 changes of diameter, consequent on varying conditions of the 

 nervous system. 



In the measurement of the arterial pressure we have, there- 

 fore, two distinct problems. The first is the determination of 

 the mean or average pressure, which, as I have said before, is 

 almost as constant as the temperature in the same animal so 

 long as it remains in a natural state ; the second is the investi- 

 gation of the variations due to the heart's action, to respira- 

 tion, or to arterial contractility, respectively. 



For the determination of the mean arterial pressure, and of 

 those variations which belong to the second and third class, 

 preference is to be given to the ordinary mercurial manometer, 

 one branch of which is connected with the artery to be investi- 

 gated, while the other is open. This instrument, as so applied, 

 constitutes what Poiseuille designated by the term hsemadyna- 

 mometer. It was employed in this simple form until Ludwig, 

 in 1848, by his invention of the kymograph, laid the foundation 

 of the more exact methods of investigating blood-pressure 

 which are now in use. Just as the first method of Poiseuille 

 originated in the ruder experiments of our countryman Hales, 



