294 FUNCTIONS OF NUTRITION. 



ment of the blood has only a partial retarding influence ; and even in 

 the smallest arteries its flow is so rapid that the separate blood-globules 

 cannot be distinguished, but only a mingled current shooting forward 

 with increased velocity at each pulsation. 



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

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

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

 of Chauveau,* who introduced into the carotid artery of the horse a 

 thin brass tube, about five centimetres long and eight or nine millime- 

 tres in diameter. The tube was introduced through a longitudinal 

 incision in the walls of the vessel, and secured by a ligature near each 

 extremity ; so that the arterial current might pass, without serious ob- 

 struction, through the 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, 

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

 the membrane was passed a light metallic needle, the inner extremity 

 of which, somewhat flattened in shape, received the impulse of the 

 blood ; while the outer portion, prolonged into a slender index, marked 

 upon a semicircular scale the oscillations of the inner extremity, and 

 consequently the varying rapidity of the arterial current. The actual 

 velocity, indicated by any given oscillation of the needle, was ascer- 

 tained beforehand by attaching the apparatus to an elastic tube and 

 passing through it a stream of warm water of known rapidity. 



The details of the circulatory movement, as indicated by these exper- 

 iments, differ somewhat in the larger and the smaller arteries. 



a. In the carotid artery, at the instant of ventricular systole, the 

 blood is suddenly put in motion with a high velocity, amounting on 

 the average to a lit.tle over 50 centimetres per second. 



At the termination of the systole, and immediately before the closure 

 of the aortic valves, the movement of the blood decreases considerably, 

 and may even, for the time, be completely arrested. 



At the instant of closure of the valves, the circulation receives a 

 new impulse, and the blood again moves forward with a velocity of 

 rather more than 20 centimetres per second. 



Afterward, the rapidity of the current gradually diminishes during 

 the heart's inaction, until, at the end of this period and just before 

 a new systole, it is reduced, on the average, to 15 centimetres per 

 second. 



b. In the smaller arteries, such as the facial, the movement of the 

 blood is more uniform. At the moment of the heart's systole it is less 

 rapid than in the carotid; and on the other hand, it has a greater 

 velocity during the period of ventricular repose. The secondary im- 

 pulse, following the closure of the aortic valves, is less perceptible 

 than in the larger arteries, and may even be altogether absent. 



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



