384 THE RESPIRATION 



In agreement with this explanation it has been found that, if the 

 last two or three forced respirations preceding the apnea are made in 

 an atmosphere of 2 instead of air, so as to fill the alveoli with 2 , the 

 apnea can be maintained for a very much longer period; and when the 

 natural desire to breathe returns, the C0 2 tension of the alveolar air, 

 instead of being below the normal, is above it. The effect of 2 in pro- 

 longing apnea, must, therefore, be dependent on the fact that it prevents 

 anoxemia. By this means the period during which the breath can be 

 held after breathing 2 is sometimes phenomenal; in one individual, for 

 example, after breathing forcibly for a few minutes and then filling the 

 lungs with 2 , apnea lasted for eight minutes and seventeen seconds. 

 The excess of 2 also serves to drive out considerably more C0 2 from 

 the blood in the alveolar capillaries (cf. page 399). Collip, 79 and Grant 

 and Goldman 80 have called attention to the peculiar "tetany"-like symp- 

 toms that develop in many individuals after the forced breathing. The 

 condition of alkalosis induced by the forced breathing is believed to be 

 responsible for the tetany (cf. page 381). 



THE SUPPOSED NERVOUS ELEMENT IN APNEA 



The apnea following forced breathing has been attributed to a sort 

 of inhibition of the respiratory center brought about by its repeated 

 stimulation by afferent impulses transmitted to it along the vagus nerves, 

 as a result of the frequent collapse and distention of the alveoli. In 

 justification of this nervous interpretation, it was claimed that apnea 

 could not readily be produced in animals after severing both vagus 

 nerves. More recent work has shown that this is not an accurate obser- 

 vation, for if the severing of the vagi is accomplished not by cutting 

 but by freezing, then apnea is as readily produced as in an intact animal 

 (Milroy 28 ). 



That chemical and not nervous factors cause the apnea is further 

 demonstrated by the well-known experiment of Fredericq, who, after 

 ligating the vertebral and one of the carotid arteries in two dogs, anas- 

 tomosed the central end of the remaining carotid of the one to the 

 peripheral end of the carotid of the other animal, thus establishing a 

 crossed circulation. He then found by applying forced artificial respira- 

 tion to the one animal, that the apnea which supervened affected the 

 other animal and not that to which the artificial respiration had 

 actually been applied. Another proof of the chemical theory of 

 apnea is furnished by the observation that if forced breathing is per- 

 formed in an atmosphere containing C0 2 in about the same partial pres- 

 sure as in the alveolar air, no apnea supervenes, and if the experiment 

 is repeated several times with progressively declining percentages of 



