288 THE CIRCULATION. 



but rises and falls with the pulsations of the heart. Thus, at every 

 contraction of the ventricle, the mercury rises for about half an 

 inch, 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 reaction of the arteries by about one-twelfth ; and these 

 vessels are kept filled by a succession of cardiac pulsations, and 

 discharge their contents in turn into the capillaries, by their own 

 elastic reaction. 



The rapidity with which the blood circulates through the arterial 

 system is very great. Its velocity is greatest in the immediate 

 neighborhood of the heart, and diminishes somewhat as the blood 

 recedes farther and farther from the centre of the circulation. This 

 diminution in the rapidity of the arterial current is due to the suc- 

 cessive division of the aorta and its primary branches into smaller 

 and smaller ramifications, by which the total calibre of the arterial 

 system, as we have already mentioned, is somewhat increased. The 

 blood, therefore, flowing through a larger space as it passes outward, 

 necessarily moves more slowly. At the same time the increased 

 extent of the arterial parietes with which the blood comes in con- 

 tact, as well as the mechanical obstacle arising from the division of 

 the vessels and the separation of the streams, undoubtedly contri- 

 butes more or less to retard the currents. The mechanical obstacle, 

 however, arising from the friction of the blood against the walls of 

 the vessels, which would be very serious in the case of water or any 

 similar fluid flowing through glass or metallic tubes, has compara- 

 tively little effect on the rapidity of the arterial circulation. This 

 can readily be seen by microscopic examination of any transparent 

 and vascular tissue. The internal surface of the arteries is so smooth 

 and yielding, and the consistency of the circulating fluid so accu- 

 rately adapted to that of the vessels which contain it, that the 

 retarding effects of friction are reduced to a minimum, and the 

 blood in flowing through the vessels meets with the least possible 

 resistance. 



It is owing to this fact that the arterial circulation, though some- 

 what slower toward the periphery than near the heart, yet retains 

 a very remarkable velocity throughout ; and even in arteries of the 

 minutest size it is so rapid that the shape of the blood-globules can- 

 not be distinguished in it on microscopic examination, but only a 



