MECHANICS OF THE CIRCULATION IN THE VESSELS 121 



from the artery rushes in and displaces the oil into the other limb, the 

 defibrinated blood passing on into the circulation. As soon as the blood 

 has reached a certain height, indicated by a mark, the instrument is 

 reversed and the oil is again displaced into the limb it originally 

 occupied. This process is repeated again and again, the time from 



beginning to end of an experiment being 

 carefully noted. The number of times 

 the blood has filled a bulb in that 

 period, the capacity of the bulb and the 

 cross-section of the vessel being known, 

 all the data required for calculating the 

 velocity of the blood in the vessel have 

 been obtained. 



Suppose, for example, that the cap- 

 acity of the bulb up to the mark is 5 c.c., 

 and that it is filled twelve times in a 

 minute, the quantity flowing through 

 the cross-section of the artery is I c.c., 

 or 1,000 cub. mm., per second. Let the 

 diameter of the vessel be 3 mm., then its 



sectional area is re x (- \ = -=7-06 



sq. mm. The velocity is 



-- ^ 



= 11 mm 



Fig. 47. StromuhrofLudwigand 

 Dogiel. A, B, glass bulbs; a, a 

 metal disc, to which A and G 

 are attached, and which can be 

 rotated on the disc 6 ; E, F, can- 

 nulae attached to b, and con- 

 nected with the peripheral and 

 central ends of a divided blood- 

 vessel. At the beginning of the 

 experiment, A and the junction 

 between A and B are filled with 

 oil; B is filled with physiological 

 salt solution or defibrinated 

 blood: a being turned into the 

 position shown in the figure, the 

 blood passes through F and D 

 into A , and the oil is forced into 

 JB. As soon as the blood has 

 reached the mark m, the disc a, 

 with the bulbs, is rapidly ro- 

 tated, so that C is now opposite 

 F. The blood now passes into 

 B, and the oil is again driven 

 into A. When the oil has 

 reached D, reversal is again 

 made, and so on. 



per second. 



Various improvements in this method 

 have been made, such as a graphic regis- 

 tration of the reversals of the stromuhr. 



3. A tube or box, in which swings a 

 small pendulum, is inserted in the course 



Fig. 48. Pitot's Tubes. 



of the vessel. The pendulum is deflected 

 by the blood, and the amount of the 

 deflection bears a relation to the ve- 

 locity of the stream (Vierordt's hczmatachometer ; Chauveau and Lortet's 

 much more perfect dromograph) (Fig. 49). 



4. Pilot's Tubes. If two vertical tubes, a and b, of the form shown in 

 Fig. 48, be inserted into a horizontal tube in which liquid is flowing hi 

 the direction of the arrow, the level will be higher in a than would be 

 the case in an ordinary side-tube without an elbow ; in b it will be lower. 

 For the moving liquid will exert a push on the column in a, and a pull 



