n8 A MANUAL OF PHYSIOLOGY 



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 car dio -pneumatic movements 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, therefore, a systolic 

 inspiration and a diastolic expiration (Practical Exercises, p. 291). 



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

 sponding 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 to the skin increases with its mean velocity, and, there- 

 fore, although the difference may not in general be measurable, more 

 heat is presumably given off during the systolic increase of velocity 

 than during the diastolic slackening. And this, along with other 



FIG. 48. PLETHYSMOGRAPH TRACING FROM ARM. 



The tracing was taken by means of a tambour connected with the plethysmo- 

 graph. The dicrotic wave is distinctly marked. 



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 the presence of circularly- 

 arranged muscular fibres in the arterioles, their absence in the 

 capillaries has its physiological correlative. The calibre of the 

 arterioles can be altered by contraction of these fibres under 

 nervous influences ; the calibre of the capillaries, although it 

 varies passively with the blood-pressure, and is possibly to some 

 extent affected by active contraction of the endothelial cells, 

 cannot be under the control of vaso-motor nerves acting on 

 muscular fibres (but see p. 157). 



