338 THE CIRCULATION. 



tinue to oscillate at or about this level. The height of the column of 

 fluid, thus supported outside the body, indicates the degree of pressure 

 to which the blood is subjected in the interior of the vessels. This 

 pressure, due to the reaction of the entire arterial system, is known as 

 the arterial pressure. 



The arterial pressure is best measured by connecting the open arteiy, 

 by a flexible tube, with a small reservoir of mercury, provided with a 

 narrow upright glass tube, open at its upper extremity. When the 

 mercury in the receiver is exposed to the pressure of the arterial blood, 

 it rises in the upright tube to a corresponding height. 



This pressure averages, in the dog and other animals of similar size, 

 150 millimetres of mercury. 



When such an instrument is connected with the carotid artery, the 

 level of the mercury in the upright tube, while indicating on the whole 

 an average pressure, exhibits two series of oscillations ; showing that 

 the degree of the blood-pressure is constantly changing, owing to two 

 different causes. One of these oscillations is synchronous with the move- 

 ments of respiration. At every inspiration, the level of the mercury 

 falls somewhat, with every expiration it rises. As the movement of in- 

 spiration consists in an expansion of the cavity of the chest, its effect is 

 to diminish the support afforded the heart and great bloodvessels, and of 

 course to lower in a similar degree the tension of the whole arterial 

 system. At the moment of expiration, on the other hand, the thoracic 

 parietes return to their former position, and the pressure upon the heart 

 and the arteries in the chest is re-established. These changes are indi- 

 cated by corresponding slow fluctuations in the arterial pressure and in 

 the height of the mercurial column. The oscillations of the mercury 

 due to respiration, however, are not at all uniform, but vary according 

 to the condition of the respiratory movements. When respiration is 

 active and somewhat labored, the oscillations may reach the extent of 

 30 millimetres ; when it is very quiet, as in an animal deeply etherized, 

 they may diminish so far as to be nearly or quite imperceptible. 



The other series of oscillations is a more constant one and is clue to 

 the cardiac pulsations. It consists of comparatively rapid undulations 

 of the mercurial column, simultaneous with the movements of the heart. 

 At every contraction of the ventricle, the mercury rises from 12 to 15 

 millimetres, and at every relaxation it falls to its previous level. Thus 

 the instrument becomes a measure, not only for the constant pressure 

 of the arteries, but also for the intermitting pressure of the heart ; and 

 on that account it has received the name of the cardiometer. It is seen, 

 accordingly, that each contraction of the heart is superior in force to 

 the resistance of the arteries by nearly one-tenth ; and the arterial system 

 is, therefore, kept filled by successive cardiac pulsations, and the arterial 

 tension maintained, notwithstanding that the blood is constantly being 

 discharged from the arteries into the capillary circulation. 



Velocity of the Arterial Current. The rapidity with which the blood 

 moves in the arterial tubes is much greater than in any other part of the 



