PERIPI1ERIC ORGANS OF THE CIRCULATION. 147 



liquids through tubes having a small diameter. They may 

 be summed up in the two following laws: the quantities 

 flowing are to each other as the fourth power of the diameters 

 of the tubes / they are in inverse ratio to the length of the 

 tubes. Now the capillary vessels, in addition to their net- 

 like arrangement, form very long tubes, and thus unite all 

 the conditions necessary to slacken the flow of the blood and 

 prolong its contact with the tissues. 



In order to estimate the velocity of the blood in the large 

 vessels, special instruments are employed ; or else a glass 

 tube filled with an alkaline liquid is 

 substituted, and placed at a certain 

 point in an artery of large diameter, 

 the time being then determined neces- 

 sary for the blood to drive the liquid 

 in question from the tube and after- 

 wards traverse the known length of 

 this artificial channel. This appara- 

 tus is the hemodromometer (of Volk- 

 mann) (Fig. 45), and is composed of 

 a glass tube (A), bent like a horse- 

 shoe, furnished at each end with a 

 metal spout having a cock, and com- 

 municating with a straight metallic 

 tube inserted in the two ends of the 

 artery (,'). The tube being filled with the alkaline solu- 

 tion, and all communication with the artery (Fig. 45, No. 1) 

 is shut oif by means of the cocks (having three outlets) ; this 

 causes the blood to follow the metallic tube ; the two cocks 

 are suddenly turned, and the blood is thereby forced to devi- 

 ate from its course and enters the glass tube (Fig. 45, No. 

 2), which it traverses to gain the other 

 end of the artery, driving before it the 

 column of colorless liquid. An apparatus 

 which is quite as ingenious, called the 

 hemotachometer (of Vierordt), consists 

 of a small transparent box (Fig. 46), 

 placed in a portion of the artery. In this 

 box swings a pendulum, which the cur- 

 rent of blood causes to swerve to one side : M T 6- 46. yierordt's 



,,..,.. . ., T Hemotachometer. 



this deviation increases with the rapidity 

 of the current, and by its degree we can calculate the 

 velocity of the blood* These experiments show that the 

 velocity of the blood is Om. 33 (thirty-three centimetres) 



Fig 45. Volkmann's 

 Heinodromometer. 



