388 RESPIRATION. 



the lungs, the alveoli collapse, and the contractions of the 

 diaphragm increase, and finally the diaphragm becomes quiescent 

 in the inspiratory position. Analogous conditions occur in normal 

 breathing. When the lungs are distended, as in inspiration, the 

 expiratory fibers of the vagus are stimulated, and an expiratory 

 act follows ; when expiration is complete and the alveoli are in 

 the condition of diminished size, it can hardly be called collapse ; 

 the inspiratory fibers are stimulated and an act of inspiration 

 occurs. 



Other afferent respiratory nerves are the superior laryngeal, 

 glossopharyngeal, trigeminus, and sensory nerves of the skin. 



The superior laryngeal is the sensory nerve of the larynx, and 

 whenever any foreign body touches this sensitive organ, or when 

 food is inclined to go down the " wrong way " i. e., gets into the 

 larynx instead of the esophagus inspiration is at once stopped and 

 violent coughing ejects it. In this process the afferent impulses 

 are carried to the respiratory center, and not only is the inspiratory 

 center restrained or inhibited, but the expiratory center is stimu- 

 lated, and a pronounced expiratory effort, the cough, results. 



The glossopharyngeal is also an afferent respiratory nerve, and 

 carries to the inspiratory center inhibitory impulses that cause all 

 inspiratory movements to cease, as when food is swallowed. The 

 food stimulates the terminations of the nerve in the mucous mem- 

 brane of the pharynx, and the inhibition results. Were this not 

 so, there would be danger of food entering the larynx at the time 

 of inspiration. 



The trigeminus sends fibers to the mucous membrane of the nose, 

 and when these fibers are irritated by an irritant like ammonia, 

 respiration may be arrested. 



The nerves of the skin also act as afferent respiratory nerves, as 

 is well shown when cold water is dashed on the body. 



The efferent respiratory nerve's are the phrenics, which supply 

 the diaphragm ; the vagi, which supply the muscles concerned in 

 producing the respiratory movements of the glottis (p. 373) ; and 

 the spinal nerves, which supply the respiratory muscles of the 

 thorax. 



There are certain terms used in connection with respiration 

 which need to be understood. 



Eupnea. This term means easy respiration, and is applied to 

 the normal act. 



Apnea. This term as used by physiologists, physiologic 

 apnea, applies to a condition in which the respiratory movements 

 are suspended, as when the lungs are distended with air forced in 

 by a pair of bellows. It was formerly attributed to the hyper- 

 oxygenation of the blood, but this cannot be the only explanation, 

 because if hydrogen is the distending gas, apnea results. Disten- 

 tion with air is practically what has been described as positive 



