342 Dr. Poiseuille, on the 



rent velocities, even in the same vessel. Some of them have 

 two simultaneous motions — one of rotation, the other of 

 translation ; while others remain motionless for a time. 

 Two globules, presenting at first the same velocity, only 

 preserve by accident the distance which separates them, 

 and, if the motion be such as to permit us to follow the 

 same globule, we can observe it sometimes in the same 

 capillary vessel presenting these different phases of motion. 

 The velocity of the globules in the capillaries is less than in 

 the arteries and veins ; it is seldom greater. This remark 

 extends also to a capillary vessel which rises immediately 

 from an artery, or which proceeds directly into a venous 

 trunk. These different phenomena lead to the conclusion 

 that the globules are endued with a spontaneous motion, 

 or, rather, that the cause of the flow of blood through the 

 capillaries is different from the cause which regulates the 

 motion of the blood in the large vessels. 



Dr. Poiseuille has endeavoured to examine with great 

 attention the causes of the motion of the blood in those 

 parts of vessels which have been isolated from the action of 

 the heart by means of ligatures, or separated from the body 

 by cutting instruments, and then to study the influence of 

 the heart and arteries upon the capillary circulation. 



He has established, by a great number of experiments, 

 that the calibre, which the arteries and veins present, pro- 

 ceeds from the pressure of the liquid which they contain ; 

 that their coats are constantly distended by the blood which 

 they receive ; that these vessels tend to collapse suddenly, 

 in consequence of the elasticity of their coats, as soon as 

 the cause of their dilatation is removed. 



The large arteries and veins, as well as the small ones, 

 possess this property ; but, besides, the diameter of the last 

 gradually diminishes when they cease to receive blood. This 

 retraction is sometimes so great, that the mysenteric vessels 

 of the frog, salamander, young rats and mice, are reduced 

 to two-thirds of their original diameter. He has also ascer- 

 tained that, cceteris paribus, this retraction is more decided 

 in the arteries than in the veins. These facts being known, 

 it is easy to determine the motions of the blood in parts 

 which have been separated from the trunk either by ligature 



