THE CIRCULATION OF THE BLOOD AND LYMPH 117 



pharynx, too, a volume-pulse (the so-called cardio-pneumatic 

 movement) can be detected by changes in the pressure of 

 the air in the respiratory passages, which may even reveal 

 themselves by a variation with each beat of the heart in the 

 intensity of a note prolonged in singing, especially after 

 fatigue has set in (Practical Exercises, p. 183). 



Doubtless the weight of an organ would also show a pulse cor- 

 responding to the beat of the heart, if it could be isolated from the 

 surrounding tissues (except for its vascular connections), and attached 

 to a recording balance, as could probably be done with a kidney. 



Further, it is possible that the temperature, at least of the super- 

 ficial parts, is altered with every beat of the heart. For the amount 

 of heat given off by the blood to the skin increases with its mean 

 velocity, and, therefore, although the difference may not in general 

 be measureable, more heat is presumably given off during the 



FIG. 40. PLETHYSMOGRAPH TRACING FROM ARM. 



The tracing was taken by means of a tambour connected with the plethysmograph. 

 The dicrotic wave is distinctly marked. 



systolic increase of velocity than during the diastolic slackening. 

 In fact, with a very sensitive instrument (bolometer, or resistance 

 thermometer, p. 479) applied directly to an exposed artery, indi- 

 cations of a change of temperature of the vessel-wall with each 

 beat of the heart have been observed. And this, along with other 

 considerations, suggests that, at any rate in certain situations and 

 under certain conditions, there may even be a pulse of chemical 

 change ; that is, a slight and as yet doubtless inappreciable ebb and 

 flow of metabolism corresponding to the rhythm of the heart. 



The Circulation in the Capillaries. From the arteries the 

 blood passes into a network of narrow and thin-walled 

 vessels, the capillaries, which in their turn are connected 

 with the finest rootlets of the veins. Physiologically, the 

 arterioles and venules must for many purposes be included 

 in the capillary tract, but the great anatomical difference 



