274 



VETERINARY PHYSIOLOGY 



account of the depression in the proximal limb which accom- 

 panies the elevation in the distal limb, gives the blood pressure 

 in mm. of mercury. 



On the record made with such an instrument the rhythmic 

 variations in the arterial blood pressure already considered 

 on p. 262 and p. 271 are clearly visible. (Practical Physiology.} 



The arterial pressure may be measured by various methods 

 without operative interference. Some of these give the systolic 

 pressure, i.e. the pressure at the maximum of the pulse wave ; 

 while others give the diastolic pressure the pressure between 

 the pulse waves. As shown in p. 262, the difference between 

 these is most marked in the great arteries, and falls to zero 

 before the capillaries are reached (fig. 128). 



To measure the systolic pressure it is necessary to find the 



FIG. 128. To show the difference between systolic, diastolic and mean blood 

 pressure throughout the arterial system. S, systolic pressure ; D, 

 diastolic pressure ; M, mean pressure. 



pressure which must be applied to an artery in order to 

 prevent the pulse from passing. 



This may be done with Riva Rocci's apparatus, by applying 

 a bag round a limb so that it rests upon an artery. The 

 bag is firmly strapped on by means of a broad supporting 

 belt, and it is connected with a pump by which the pressure 

 within it may be raised, and with a mercurial manometer 

 by which the pressure applied may be measured in mm. of 

 mercury. The pressure is then raised till the pulse beyond 

 is no longer felt, and the column of mercury indicates the 

 systolic pressure in the artery. (Practical Physiology.) 



Instead of using the arterial pulse as the index, the passage 

 of blood into a region rendered bloodless may be used. In 

 Gaertner's tonometer the bag and band are applied round 

 the finger rendered bloodless at a pressure sufficient to prevent 



