34:0 THE CIRCULATION. 



needle, the inner extremity of which, somewhat flattened in shape, pro- 

 jected into the interior of the vessel, and received the impulse of the 

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

 marked upon a semicircular graduated 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 ascertained beforehand by attaching the apparatus to an 

 elastic tube and passing through it a stream of warm water of known 

 rapidity. 



Chauveau found, by these experiments, that the details of the circu- 

 latory movement differ somewhat in the larger arteries near the heart 

 from those in the smaller branches farther removed. 



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

 blood is suddenly put in motion with a high degree of rapidity, amount- 

 ing on the average to a little over fifty 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 aortic valves, the circulation receives 

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

 rather more than 20 centimetres per second. 



Subsequently, the rapidity of the current diminishes gradually during 

 the period of 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 centi- 

 metres per second. 



b. In the smaller arterial branches, such as the facial, the movement 

 of the arterial current is more uniform. It is less rapid at the moment 

 of the heart's systole; and on the other hand, it is always more active 

 during the period of ventricular repose. 



The secondary impulse, following the closure of the aortic valves, is 

 much less perceptible than in the larger arteries, and may even be alto- 

 gether absent. 



The Venous Circulation. 



The veins are composed, like the arteries, of three coats ; an inner, 

 middle, and exterior. They differ from the arteries in containing a 

 much smaller quantity of muscular and elastic fibres, and a larger pro- 

 portion of condensed connective tissue. They are consequently more 

 flaccid and compressible than the arteries, and less elastic and contrac- 

 tile. They are furthermore distinguished, throughout the limbs, neck, 

 and external portions of the head and trunk, by being provided with 

 valves, arranged in the form of festoons, and so placed as to allow the 

 blood to pass readily from the periphery toward the heart, while they 

 prevent its reflux in the opposite direction. 



Although the walls of the veins are thinner and less elastic than those 

 of the arteries, yet their capacity for resistance to pressure is equal, or 

 even superior, to that of the arteries. Milne Edwards has collected the 



