258 PHYSIOLOGY CHAP, 



this occurs, the cylinder is reversed by a mechanism resembling 

 that of Ludwig's Stromuhr, and the ball moves in the opposite 

 direction, driving the blood before it. 



We also invented a haeraodromometer, which, with the utmost 

 simplicity of construction, presents the advantage of being able 

 to vary the capacity of the two receivers, which correspond to the 

 bulbs of Ludwig's apparatus, by employing two elastic bags in 

 a receiver full of water. The h'rst two editions of this text-book 

 gave the description and figure omitted here in favour of the new 

 model constructed by Hiirthle, which has the further advantage of 

 automatically registering its movements. 



Hiirthle's haemodromorneter (as shown in Fig. 100) consists 

 essentially of an inverted U-tube, in one branch of which there is 

 a cylindrical receiver containing a piston, which is easily movable 

 from the top to the bottom and the bottom to the top. The blood- 

 stream, which issues from the central end of the artery (carotid in 

 the dog), ascends by a branch of the said tube, penetrates the 

 cylindrical receiver, and, by lowering the piston, empties out the 

 fluid (artificial serum with which it was filled at the outset) into 

 the distal end of the artery. 



When the ball reaches the extreme end of its course, the 

 experimenter at once reverses the blood current through the 

 cylinder by giving a half-turn to a disc beneath it by means of a 

 screw. The blood current will then flow into the cylinder from 

 below, driving the piston up, and turning the blood into the distal 

 end of the artery. 



The reversal of the current is repeated each time the ball 

 reaches the top or bottom. The interval between one reversal and 

 the other expresses the duration of each filling and emptying of 

 the cylinder that measures the current. The excursions upward 

 and downward of the ball are transmitted by a system of pulleys 

 to a lever writing on the smoked paper of a rotating drum. A 

 Deprez signal simultaneously records the time on the same drum, 

 while an elastic manometer (Hiirthle) applied to the artery shows 

 the pulsatory oscillations of the arterial pressure. 



The tracings in Fig. 101 are reproduced from those obtained 

 by Hiirthle with his ingenious haemodromometer (reduced by one- 

 third), which serves at the same time for a spring manometer 

 recording the oscillatory pulsations in pressure and for an electric 

 time-marker. 



These haemodromometric methods are certainly not free from 

 defects, and they give, not the normal absolute values of current 

 velocity in any given artery, but values as much lower than the 

 normal as the resistances artificially opposed to the passage of the 

 blood through the measuring apparatus are greater. Since, how- 

 ever, these new resistances are a fixed and constant coefficient, 

 they do not interfere with the value of the comparative results. 



