MECHANICS OF THE CIRCULATION IN THE VESSELS 129 



'mouth may be. connected with the recording apparatus, the nostrils 

 being closed. One factor in the production of these movements may 

 be the change of blood- volume in the soft tissues of the mouth, naso- 

 pharynx, and perhaps also in the lower respiratory passages accom- 

 panying the heart-beat. Another factor, and a more influential one, 

 is the rhythmical alteration of pressure caused directly by the alter- 

 nate systole and diastole of the heart in the air contained in the 

 lung-tissue surrounding it, which acts as a kind of air plethysmo- 

 graph. One interesting way in which the cardio-pneumatic move- 

 ments may reveal themselves is by a variation with each beat of the 

 heart in the intensity of a note prolonged in singing, especially after 

 fatigue has set in. Upon the whole, the air-pressure falls during 

 systole, owing to the expulsion of blood from the chest, and rises 

 during diastole. The main cardio-pneumatic movement is, there- 

 fore, a systolic inspiration and a diastolic expiration (Practical 

 Exercises, p. 303). 



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

 ing to the beat of the heart, and so would the temperature^-at least, 

 of the superficial parts. For the amount of heat given off by the blood 



Fig- 57- Plethysmograph Tracing from Arm. The tracing was taken by means 01 

 a tambour connected with the plethysmograph. The dicrotic wave is distinctly 

 marked. 



to the skin increases with its mean velocity, and, therefore, although 

 the difference may not in general be measurable, more heat is pre- 

 sumably given off during the systolic increase of velocity than during 

 the diastolic slackening. And this, along with other considerations, 

 suggests that, at any rate in certain situations and under certain con- 

 ditions, there may even be a pulse of chemical change ; that is, a slight 

 and as yet doubtless inappreciable ebb and flow of metabolism corre- 

 sponding 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 capil- 

 laries, 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 the presence of circularly-arranged muscular 

 fibres in the arterioles, their absence in the capillaries has its 



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