624 PHYSIOLOGY OF RESPIRATION. 



himself, approximately at least, by a series of rapid, forced inspira- 

 tions. The question of importance is: Why does the respiratory 

 center cease to act? Rosenthal explained the phenomenon in terms 

 of his theory that the normal stimulus to the center results from a 

 lack of oxygen. With vigorous artificial respiration he imagined 

 that the blood takes up more oxygen and thus fails to act upon 

 the center. The apnea is due to overoxygenation of the blood, 

 and indeed this is the definition he gave to the word.* The 

 numerous researches made upon this condition seem to show 

 very clearly that in the method used to produce it two factors 

 co-operate, and that it is necessary, in reality, to distinguish two 

 different kinds of apnea, the apncea vera or chemical apnea, and the 

 apncea vagi or inhibitory apnea. When the lungs are vigorously 

 inflated by artificial blasts the alveoli are better ventilated; and 

 consequently the blood takes up somewhat more of oxygen and gives 

 off more carbon dioxid. It reaches the center in what may be 

 called a more arterialized or less venous condition. At the same 

 time the repeated expansions of the lungs cause repeated stimula- 

 tions of the inhibitory fibers in the vagus, and this tends to bring the 

 center to rest by inhibition. Either of these factors alone may 

 cause a condition of apnea and in the method by which the phenom- 

 enon is usually produced the two co-operate, as may be inferred 

 from the following facts: If the vagi are cut it is much more difficult 

 to produce apnea by artificial respirations. If in an animal with 

 vagi intact the artificial respirations are made with hydrogen in- 

 stead of air an apneic pause may be obtained, but this is no longer 

 possible if the vagi are cut.f These two facts indicate the impor- 

 tance of the inhibitory factor. That chemical apnea in Rosen- 

 thal's sense may exist is shown by the fact that after section of 

 both vagi apnea may still be produced by artificial respiration, 

 and indeed several observers^ find that after section of both vagi 

 and of the medulla above the center the animal may still be 

 made apneic. In such cases it is difficult to see any other cause for 

 the apnea than a change in the gases of the blood. Rosenthal 

 assumed that this latter condition is due to an overoxygenation of 

 the blood, but since the vigorous respirations lower the contents of 

 the blood in C0 2 it is possible, as insisted upon by Traube, that this 

 factor may be the more important. At present there are no facts 

 which will enable us to decide definitively between these views; 

 but, since, in the preceding paragraphs, some evidence has been 

 given to show that the normal stimulus to the center is due to the 



* See Rosenthal, in vol. iv, p. 264, of Hermann's "Handbuch der Physi- 

 ologic " 



t See Head, "Journal of Physiology," 10, 1, and 279, 1889. 



j Loewy, " Archiv f. die gesammte Physiologie, " 42, 245, 1888; and 

 Langendorff, "Archiv f. Physiologie," 1888, p. 286. 



