THE ARTERIAL CIRCULATION. 



339 



vascular system. Its exact rate varies somewhat according to the 

 situation of the vessel and the period of the pulsation. Its velocity is 

 greatest in the immediate neighborhood of the heart, and diminishes as 

 the blood recedes from the centre of the circulation. The successive 

 division of the aorta and its primary branches into smaller and smaller 

 ramifications increases the extent of surface of the arterial walls with 

 which the blood comes in contact. The adhesion produced by this con- 

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

 of the vessels and the separation of the streams, contributes to retard 

 the current, which accordingly becomes perceptibly slower in the small 

 arteries than in those of larger or medium size. In the smallest arte- 

 ries, as examined by the microscope in the transparent tissues, the par- 

 tial adhesion of the blood to the vascular wall, and the greater rapidity 

 of its flow in the axis of the vessel are readily perceptible. The con- 

 sistency of the circulating fluid, however, and the smoothness of the 

 internal surface of the arteries, are such that this obstacle to the move- 

 ment of the blood has only a very partial influence in retarding its flow; 

 and even in the smallest arteries it is so rapid, when seen under the 

 microscope, that the shape of the separate blood-globules cannot be dis- 

 tinguished, but only a mingled current shootin'g forward with increased 

 velocity at each cardiac pulsation. 



The average rapidity of the blood stream in the larger arteries, in 

 dogs, horses, and calves, was determined by Yolkmann, as 30 centi- 

 metres per second. The most exact experiments on this point are 

 those of Chauveau. 1 He experimented by introducing into the carotid 

 artery of the horse a brass 

 tube with thin walls, about five 

 centimetres long and eight 

 or nine millimetres in diame- 

 ter. The tube was introduced 

 through a longitudinal incision 

 in the walls of the exposed 

 vessel, and secured in position 

 by a ligature near each ex- 

 tremity ; so that the arterial 

 current would pass, without 

 serious obstruction, through 

 the brass tube forming, for the 

 time, a part of the arterial 

 walls. In the side of the tube 

 was a small opening, three 

 millimetres long by one and 

 a half millimetre wide, closed 

 by an elastic membrane pro- 

 perly secured so as to prevent the escape of the blood. Through the 

 centre of the elastic membrane there was passed a very light metallic 



1 Journal de la Physiologic, Paris, Octobre, 1860, p. 695. 



w~^ - v% - A--" 



V 



CHAUVEAU'S INSTRUMENT, for measuring 

 the rapidity of the arterial current. a. Brass tube, 

 introduced into the calibre of the artery, b. Index- 

 needle passing through the elastic membrane in 

 the side of the brass tube, and moving by the im- 

 pulse of the blood-current, c. Graduated scale, 

 for measuring the extent of the oscillations of 

 the needle. 



