528 



PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



To avoid this loss of blood, it is therefore advisable to inject the sodium carbo- 

 nate solution into the tube of "the manometer until the column of mercury has 

 been elevated to the height which will probably correspond to that of the mean 

 arterial pressure. 



Comparative experiments made in the above-described manner will 

 show that the blood pressure is considerably higher in the arteries than 

 in the veins, and greater in the large arteries than in the arterial 

 branches (Fig. 221) ; so, also, the pressure may be demonstrated to be 

 higher in the small veins than in the large veins at their opening into 

 the heart. Experiment will further show that in the veins the pressure is 

 almost constant, overlooking the insignificant variations which are due to 

 respiration ; so, also, the pressure will be found to be almost constant in 



Fig. 219.— Ludwio's Kymographion, after Hermann. (Yeo.) 



The ordinary form of rotating, blackened cylinder (R), which is moved by the clock-work in the box 

 (A) by means of the disk (D) pressing on the wheel (n), which can be raised or lowered by the screw (L) 

 so as to rub on a part of the disk more or less near the centre, and thus rotate at dinerent rates. The 

 cylinder may be raised by the screw (o) which is turned by the handle (U). 



the small arteries, while in the large arteries considerable variations, 

 corresponding in their increase to the systole and their decrease to the 

 diastole, are invariably found. It is evident that the mean between the 

 maximum and minimum pressures in the arteries, as indicated by these 

 oscillations, will represent the force which is concerned in the propul- 

 sion of the blood. Although theoretically and practically the pressure 

 decreases as the distance increases from the heart, yet in any artery 

 which is not too small to be subjected to such manometrical experiments 

 it will be found that the pressure will be slightly affected, not more than 

 one-tenth lower than the mean pressure in the aorta. This fact indicates 

 that the blood in moving through the arteries has to overcome but slight 



