CHAP, iv.] THE VASCULAR MECHANISM. 



123 



In such a mercury manometer, the inertia of the mercury obscures 

 many of the features of the minor curves caused by the heart-beats. 

 When therefore these, rather than variations in the mean pressure, are 

 being studied, other methods have to be adopted. 



The average pressure of the blood in the same body is greatest 

 in the largest arteries, and diminishes as the arteries get less ; but 

 the fall is a very gradual one until the smallest arteries are reached, 

 in which it becomes very rapid. In the carotid of the horse, the 

 mean arterial pressure varies from 150 to 200 mm. of mercury ; of 

 the dog from 100 to 175 ; of the rabbit from 50 to 90. In the 

 carotid of man it probably amounts to 150 or 200. 



FIG. 19. LARGE KYMOGRAPH WITH CONTINUOUS ROLL OF PAPER. 



The clock-work machinery, some of the details of which are seen, unrolls the 

 paper from the roll C, carries it smoothly over the cylinder B, and then winds it up 

 into the roll A. 



Two electromagnetic markers are seen in the position in which they record their 

 movements on the paper as it - travels over B. The manometer, or any other 

 recording instrument used, can be fixed either in the notch immediately in front of 

 B or in any other position that may be desired. 



Since in all arteries the blood is pressing on the arterial walls 

 with some considerable force, all the arteries must be in a state of 

 permanent distension, so long as blood is flowing through them 

 from the heart. When the blood-current is cut off, as by a 

 ligature, this expansion or distension disappears. 



Not only is there a permanent expansion corresponding to the 

 mean pressure, but just as the mercury in the manometer rises 

 above the level of mean pressure at each systole of the heart, and 



