384 THE RESPIRATION 



In agreement Avith 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 CO 2 from 

 the blood in the alveolar capillaries. 



THE SUPPOSED NERVOUS ELEMENT IN APNEA 



It is necessary to point out that, prior to the elaboration of accurate 

 methods for the investigation of the chemistry of respiration, many 

 physiologists interpreted the apnea following forced breathing as the 

 result of a sort of inhibition of the respiratory center brought about by 

 its repeated stimulation by afferent nervous impulses transmitted to it 

 along the vagus nerves, these impulses being set up by the frequent col- 

 lapse and distention of the alveoli acting on the terminations of the 

 nerve. In justification for the nervous interpretation of apnea, it was 

 claimed by the earlier observers that it could not readily be produced 

 in animals after severing both vagus nerves. More recent work has 

 shown that this is not an accurate observation, 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 that by applying forced artificial 

 respiration to the one animal, 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 percentage of 



