296 A TEXTBOOK OF PHYSIOLOGY 



Exposure to a cold wind induces a man to breathe more to 

 keep himself warm. 



A yawn is a long, deep inspiration through the widely-opened 

 mouth, and at the same time the body may be stretched in a charac- 

 teristic way. This furthers the circulation, and increases the oxygen 

 supply. 



A sigh is a long-drawn inspiration, followed by a deep expiration. 



The Influence of the Vagi. The division of one vagus alone has^ 

 little effect upon the respiratory movements. After section of both 

 vagi, the breathing in animals, such as the dog, cat, or rabbit, becomes 

 slower and deeper; inspiration, normally the shorter, becomes longer 

 than expiration (Fig. 160). The same effect is obtained if the two 



Normal respiration 



After section of one 

 vagus the frequency 

 of respiration is some- 

 what diminished 



After section of botli 

 vagi the frequency rf 

 respiration is much 

 diminished 



Excitation of cential 

 end of left vagus 

 cceelerates respiration 



Time in seconds 



FIG. 160. INFLUENCE oz THE VAGTTS urox RESPIRATORY MOVEMENTS. (Waller.) 



vagi be cooled to 3 C., a process which eliminates any irritative 

 effect of a current of injury such as might be established by dropping 

 a cut nerve into a wound. 



When non-polarizable electrodes are placed upon the vagus nerve, 

 and connected with a string galvanometer, it may be observed that 

 inflation of the lungs induces a marked current of action, deflation a 

 less marked one. In the animal breathing normally an electrical varia- 

 tion in the vagus nerve has been recorded synchronously with each 

 inspiration, indicating the passage of a nerve impulse (Fig. 163) . A for- 

 cible collapse of the lung also excites a negative variation in the vagus. 

 It has been shown that positive inflation (blowing up) of the lungs 

 causes the diaphragm to come to a standstill in the expiratory position ; 

 negative ventilation (sucking air out of the lungs), induces inspiratory 



