RESPIRATION 313 



fact that the contracting ventricles draw on and expand the 

 lungs. (3) Succeeding this is a slower outrush of air corre- 

 sponding to the active filling of the ventricles during the 

 beginning of ventricular diastole. (4) Lastly, during the 

 period of passive diastole, the cardio-pneumatic movements of 

 air are in abeyance (fig. 142). 



These cardio-pneumatic movements are of great importance 

 in animals which hibernate. During their winter sleep the 

 ordinary respirations almost stop, but a sufficient gaseous 

 interchange is kept up by these cardio-pneumatic movements. 



In the examination of the heart sounds they must be borne 

 in mind, because, if there is any constriction in a small bronchus 

 near the heart, the rush of air through it may give rise to a 

 murmuring sound, in character very like a cardiac murmur 

 and synchronous with the heart's action. On making the 

 patient cough, such a murmur at once disappears. 



B. Interchange between the Air breathed and the Blood in 

 the Lung Capillaries 



I. Effect of Respiration upon the Air breathed. To de- 

 termine this, some method of analysing the air exhaled must 

 be employed. For this purpose the expired air is caught in 

 a graduated vessel in which its amount can be measured; 

 then a measured amount is forced into a chamber containing 

 caustic potash, by which the C0 2 is absorbed and the volume of 

 air is again measured. It is now forced into a chamber contain- 

 ing pyrogallate of soda, which absorbs the oxygen and is again 

 measured. The residue is nitrogen. In this way the amount 

 of the gases present is determined. (Practical Physiology.) 



The following table shows the average percentage compo- 

 sition of the air inspired and the air expired (fig. 143) : 



Per Cent, of N. 0. C0 2 



Inspired air .... 79 21 



Expired air .... 79 17 4 



i.e. about 4 per cent, of oxygen is taken from the air, and 

 about 4 per cent, of carbon dioxide is added to it. In man 

 the amount of carbon dioxide given off is smaller than the 



