424 TEXT-BOOK OF PHYSIOLOGY. 



uted to the production and discharge into the blood of metabolic products 

 which act as stimuli to the respiratory center and thus increase its activity, 

 Of these metabolic products CO 2 is undoubtedly one of the most efficient, 

 as stated in foregoing paragraphs. Emotional states temporarily increase 

 respiratory activity. With their disappearance the normal condition returns. 



Apnea. Apnea may be denned as a temporary cessation of the respira- 

 tory movements. It may be developed by rapid and deep inspirations due 

 to volitional efforts, by rapid mechanic inflation of the lungs, and by stimu- 

 lation of various afferent nerves. If one volitionally breathes rapidly and 

 deeply for a period varying from two to ten minutes, it will be found on 

 cessation of the effort that a condition of apnea is established which may 

 last for from thirty seconds to several minutes. One experimenter succeeded 

 after forcible inspiration for two and a half minutes, in establishing in 

 himself an apnea that lasted for several minutes before there was the slightest 

 desire to breathe. Before the cessation of the apnea, the face became pale 

 and corpse- like, indicative of a marked condition of anoxhemia. If the 

 lungs of an animal be rapidly inflated through a cannula inserted in the 

 trachea, a similar condition is developed. Whether the apnea be estab- 

 lished by volitional efforts or by mechanic inflation, the respiratory move- 

 ments gradually return. At first they are feeble but soon increase in 

 amplitude and frequency until the normal is reached. At one time the 

 apnea that results from rapid ventilation of the lungs, whether volitional or 

 mechanical, was attributed, on the assumption that a deficiency of oxygen 

 in the arterial blood is the physiologic stimulus to the activity of the inspira- 

 tory center, to an excess of oxygen in the blood, the result of the forced 

 ventilation, complete saturation of the plasma and the hemoglobin, in 

 consequence of which the inspiratory center remained inactive. The 

 apneic state is at present attributed, on the assumption that carbon dioxid 

 in the arterial blood is the physiologic stimulus to the inspiratory center, to a 

 diminution in the percentage of the carbon dioxid in the alveolar air (to 4 per 

 cent, or less), in the blood, and therefore in the center, the result of the forced 

 ventilation. The increased ventilation eliminates the carbon dioxid to such 

 an extent that the percentage and pressure in the blood is insufficient to 

 arouse the center to activity. To the condition of the blood that results 

 from this rapid ventilation, viz., a diminished percentage of CO 2 , the term 

 acapnia has been given. An apnea which is thus developed is termed apnea 

 chemica or apnea vera. As previously stated, stimulation of certain afferent 

 nerves, especially the vagus, will induce a similar cessation of the respiratory 

 movements. Thus if the central end of the divided vagus be stimulated, 

 the thorax will come to rest in the state characteristic of deep expiration 

 from inhibition of the inspiratory center. Inasmuch as stimulation of the 

 vagus causes an apnea resembling that caused by rapid inflation of the lungs, 

 it has been suggested that in the development of apnea the inspiratory center 

 is inhibited in its activity simultaneously with the elimination of the CO 2 , 

 from the mechanic stimulation of the pulmonary terminations of the vagus. 

 An apnea caused by stimulation of the vagus is termed apnea vagi or apnea 

 inhibitoria. 



In the apnea that results from voluntary or mechanic inflation of the 

 lungs it is difficult to state in how far the condition is due to a diminution 



