RESPIRATION. 371 



carbonic acid. Hence the hyperpncea seen in exercise can be explained 

 by a response of the center of respiration to the rise of pressure of 

 carbon dioxide in the blood. The gaseous composition of the blood 

 going to the respiration center regulates in great part the respiratory 

 movements. The regulation of the rate of alveolar ventilation, 

 normally depends upon the pressure of carbon dioxide in the center 

 of respiration and gives a satisfactory explanation of the normal 

 breathing (eupncea), hyperpnoea and apncea. 



Hering and Breuer put animals in a state of apncea by repeatedly 

 filling the lung with air from a bellows. Then when the chest was 

 greatly distended, the tracheal cannula was closed and the thorax kept 

 in that position. The first movement with a distended chest was one 

 of expiration. Then after the animal was again made apnceic by re- 

 peated insufflations, the air was sucked out of the chest, the tracheal 

 cannula closed, and the chest kept in that position. The first move- 

 ment to be made was one of inspiration. The act of inspiration, 

 inhibits the inspiration and calls out expiration, whilst the act of 

 expiration inhibits expiration and calls out an inspiration. 



Hering, Breuer and Head held that the vagus contained two kinds 

 of afferent fibers to the center of respiration, one concerned in calling 

 out expiration and the other in calling out inspiration. 



Gad believes that only one kind of fiber is necessary to explain the 

 results of Hering-Breuer. It may be assumed that the center of res- 

 piration steadily tends to discharge inspiratory impulses and this dis- 

 charge of impulses is inhibited by an impulse in the vagus produced 

 by the filling up of the lungs in the act of inspiration. Then, in the 

 expiratory collapse of the lungs, this inhibition is removed and the 

 proneness to inspiratory impulse by the respiration center again pro- 

 duces an inspiration. This view was also supported by the experiments 

 of Lewandowsky, who found that inflation of the lungs is accompanied 

 by an action current in the vagus, whilst in the sucking out of the 

 lungs, or collapse in expiration, no action current was present. Alcock 

 and Seemann by using the more sensitive capillary electrometer, found 

 that inflation of the lungs produced a negative variation in the vagus. 

 They also found in contradiction to Lewandowsky that sucking out of 

 the lungs also produced in the vagus of the rabbit a negative variation. 

 In the cat there was either a positive variation, or a short negative 

 followed by a prolonged positive variation, lasting as long as the 

 diminished pressure. On letting the air into the lungs, so as to restore 

 the normal distension, a negative variation followed. They explain 

 the difference by supposing that these results are the algebraic sum of 



