260 PHYSIOLOGY CHAP. 



of blood which escapes in the unit of time from a divided artery. 

 In this case all the peripheral resistances which the blood has to 

 overcome under physiological conditions are artificially excluded. 

 Even when the lumen of outflow is artificially constricted (e.g. by 

 introducing a glass cannula of narrow bore) in order to build up a 

 resistance similar to that which the blood normally encounters, it 

 is impossible to obtain any correct values of current velocity, both 

 because we do not know if the resistance added is of the same 

 value as that subtracted, and because in any case the animal is 

 losing the blood that flows out, which sets up quite abnormal 

 conditions. 



FIG. 101. Curve of velocity and pressure of blood in left carotid of a dog of 13 kgrms. (Hurtlile and 

 Tschuewsky.) The upper tracing is divided along the abscissa into six periods (Pi, Pu, etc.), 

 at which the current in the measuring cylinder was reversed. The spaces comprised between 

 the horizontal lines of the tracing correspond with c.c. of blood. The curves of the second 

 level represent the pulsatory oscillations of blood pressure in the central cannulae of the 

 haemodromometer, and were recorded with a spring manometer. The bottom line gives the 

 time in seconds. The vertical line C.d.e. of the third period gives the moment at which the 

 right carotid was compressed, which produced an augmentation both of velocity and of pressure 

 in the left carotid, as shown in the tracing. 



The experimental results obtained by Dogiel and Nicolaides 

 with Ludwig's stromuhr are somewhat meagre owing, no doubt, to 

 the very variable conditions that affect the velocity of current in 

 any given artery. They show that both in the carotid and the 

 femoral of the dog or rabbit the velocity may alter greatly from 

 one moment to another. Since the first determinations usually 

 show a higher value than the subsequent, it was conjectured that 

 this was due to a commencement of clotting at the insertion point 

 of the cannula. This supposition was, however, excluded, inasmuch 

 as the same fact was observed with blood that had been rendered 

 incoagulable by peptone injection, viz. that velocity diminished as 

 the experiment proceeded, more often in the carotid, less frequently 

 in the crural artery. 



