Pulse Wave and the Measurement of Arterial Pressure. 347 



connect it with the Hg manometer and force up the pressure till the writer 

 of the C-spring reaches the crest of the pulse curve (fig. 1). 



We obtain similar results on applying to the carotid artery {in situ) 

 of the cat the bag of the small Leonard Hill instrument instead of the 

 armlet. 



On the other hand, if we place a long length of the exposed carotid artery 

 on the convex surface of a watch-glass, we find that the pulse wave is 

 obliterated by a pressure in the bag much less than before. For example, in 

 a dog with an arterial pressure of 190 mm. Hg. it was found that with the 

 armlet 190 mm. pressure was required ; in the same animal with the artery 

 exposed but 60 mm. Hg. was required. In a cat we found — 



The systolic pressure was 65 mm. Hg. 



And with the artery unexposed 64 „ was necessary to damp 



down pulse. 



Artery lying exposed on muscles ... 26 „ „ „ 



Artery lying on scalpel handle (cf. 12 „ „ „ 



fig. 2b). 



To elucidate the cause of this marked difference in readings we devised 

 the following experiment : — A long length of cat's carotid was exposed, the 

 uppermost part ligated and divided. This end was first passed through a 

 T-piece and then an arterial cannula inserted into it, which in its turn was 

 connected with the C-spring manometer. As the artery passed in and out 

 of the T-tube through a piece of rubber tubing, the latter could be constricted 



Fig. 2a. 



VOL. LXXXVII. — B. 



Record of C spring 



Artery exposed on cover glass 



r \=2 Y mm . Hg. 



I V Zero 



