THE RESPIRATORY SYSTEM. 



381 



If, in the tracing, the small waves on the upper part of the 

 expiratory curve really represent the efPect of the heart-beat, 

 it makes it easier to understand how such might assist in venti- 

 lating the blood when the respirations occur only once in a 

 considerable interval and very feebly then, as in hibernating 

 animals or individuals that have faiuted ; though it must be 

 remembered that diffusion is a ceaseless • process in all living 

 vertebrates. 



Fig. 306. — Tracings of respiration of horse when at rest and after exercise (after Than- 

 hoffer). /, inspiration; E, expiration. Spaces between vertical lines indicate 

 time periods of one second each. 1, animal standing at rest; 3, after wivlk of few 

 minutes; 7 and 8, after trotting; 9, after a brief rest; 11, after trotting and run- 

 ning for some minutes; 17, after resting from last for a short time; 51, tracing at 

 end of experiment. 



It is scarcely necessary to point out that the respiratory 

 movements are increased by exercise, emotions, position, sea- 

 son, hour of the day, taking meals, etc. 



Bespiratory Sounds, — The entrance and exit of air are ac- 

 companied by certain sounds, which vary with each part of the 



