638 PHYSIOLOGY OF RESPIRATION. 



the lungs the new inspiration is due to a normal discharge from the 

 inspiratory center. 



Loewy* has shown by an ingenious experiment that the expansion of 

 the lungs is the factor that actually stimulates the sensory fibers and quickens 

 the respiratory rate, as follows : An animal was made to breathe pure oxygen 

 for a while to displace the nitrogen in the alveoli. The chest on one side 

 say, the right side was then opened with the result that the lung collapsed, 

 and, owing to the rapid absorption of the oxygen, soon became practically 

 solid. The respirations (rabbit) showed their normal rate 66. The vagus 

 nerve on the left side was then cut and immediately the respirations took 

 on the character usually shown when both vagi are severed, respirations 

 = 34. Next the collapsed right lung was expanded by artificial respiration, 

 with the result that the respiratory rate at once returned to normal. 



Respiratory Reflexes from the Larynx, Pharynx, and Nose. 



The mucous membrane of the larynx receives its sensory fibers 

 from the superior laryngeal nerve. When this nerve is stimulated 

 artificially the respirations are always inhibited ; the chest comes to 

 rest in the position of passive expiration. The same effect may be 

 obtained from the sensory fibers of the glossopharyngeal supplying 

 the pharynx, and indeed a temporary inhibition of respirations 

 occurs through this nerve during every act of swallowing. The 

 sensory fibers of the nasal mucous membrane (trigeminal) cause a 

 similar reflex inhibition when stimulated by injurious or so called 

 irrespirable gases, such as HC1, Cl, NH 3 , SO 2 , etc. We may regard 

 this inhibitory influence exerted by the sensory fibers distributed 

 along the air passages as a protective reflex which guards the lungs 

 automatically from injurious gases. This protective action is 

 made more evident by the fact that, together with the cessation of 

 respirations, the glottis is reflexly closed by contraction of the ad- 

 ductor muscles and, if the stimulation is strong, even the bronchial 

 musculature may be contracted, so that in every way the passage 

 to the alveoli is made more difficult. The reflex is, of course, more 

 or less temporary, but it possesses the great advantage of being 

 automatic, and may enable the animal or individual to escape 

 unharmed from a dangerous locality before the increasing irritabil- 

 ity of the respiratory center breaks through the inhibition. In 

 special cases the inhibition may last for an unusually long time. 

 Thus, Fredericq states that in aquatic birds water allowed to flow 

 over the beak so as to penetrate slightly into the nostrils brings 

 about an inhibition of respirations for many minutes. There 

 would seem in this case to be a special adaptation of the reflex to 

 the needs of diving. We know also that irritating gases or foreign 

 bodies of any sort that enter the larynx may lead to a coughing 

 reflex, that is, to a series of expiratory blasts which have a pur- 

 poseful end in the expulsion of the stimulating object. In this case 



* "Archiv f. die gesammte Physiologic, " 42, 273. 



