646 PHYSIOLOGY OF RESPIRATION. 



assumed that the apnea is due to an overoxygenation of the blood, 

 but since the vigorous respirations lower especially the contents of 

 the blood in CO 2 it is probable, as insisted upon by Traube, that this 

 latter factor is the more important. In the preceding paragraphs 

 some evidence has been given to show that the normal stimulus to 

 the center is due to the presence of CO 2 , and it follows logically that 

 the more complete removal of this gas by ventilation of the lungs 

 should be considered as the chief cause of true apnea. Experi- 

 mentally this view is well borne out by an old observation of Berns, 

 according to which a condition of apnea in a rabbit may be cut 

 short instantly at any moment by a blast of C0 2 sent into the lungs, 

 a blast of air having no such effect. This observation is further 

 supported by recent experiments by Mosso * upon men, in which 

 he shows that apnea cannot be produced by inflation with carbon 

 dioxid. This author designates the condition of diminished CO 2 

 in the blood as acapnia. According to this terminology, true apnea 

 is due to a condition of acapnia. 



In the intact animal, therefore, we may say that apnea is 

 due to two causes: first, the removal of C0 2 from the blood by 

 better ventilation, whereby the center is stimulated less strongly 

 or not at all; and, second, the rhythmical inhibition of the center 

 through the vagus fibers ending in the lungs. The two causes work 

 together and, as it were, aid each other, for the less the irritability 

 of the center, the more easily it is inhibited, and the more it is 

 inhibited, the less the internal stimulus affects it. It is known 

 that a cessation of respirations may be brought about in still a 

 third way, namely, by a condition of more or less complete anemia 

 produced by shutting off the blood-supply to the center. The 

 lack of activity in this case is not a true apnea in the physiological 

 sense of the term, since we may suppose that under these conditions 

 the tension of the carbon dioxid increases rather than decreases. 

 It is due to a loss of irritability in the center, as a direct result 

 of the deprivation of the blood-supply, rather than to a diminution 

 in the normal stimulus. 



Innervation of the Bronchial Musculature. Numerous 

 investigators, using different methods, have demonstrated that the 

 bronchial musculature is supplied through the vagus with motor 

 and inhibitory fibers, bronchoconstrictor and bronchodilator fibers, 

 as they are usually called. f Stimulation of the constrictors causes 

 a narrowing of the bronchi, and therefore increases the resistance to 

 the inflow and outflow of air. Some observers state that these fibers 

 are normally in a condition of tonic activity (Roy and Brown), 

 but others find little evidence for this belief. An artificial tonus 



* Mosso, "Archives italiennes de biologie," 40, 1, 1903. 

 t For a recent paper with references to literature see Dixon and Brodie, 

 "Journal of Physiology," 29, 97, 1903. 



