INNERVATION OF THE RESPIRATORY MOVEMENTS. 707 



(suffocation) the respiratory movements become more and more 

 violent until they take on a convulsive character. This stage 

 is succeeded by one of apparent calm, indicative of exhaustion of 

 the centers. Deep, long-drawn inspirations follow at intervals 

 and finally cease. The animal lies quietly, with feeble heart beat 

 and dilated pupils, in a condition designated as asphyxia or com- 

 plete asphyxia. 



The term apnea means literally a condition of no breathing, and 

 since this condition may occur from several causes some confusion in 

 nomenclature has resulted. In medical literature the term is some- 

 times employed as a synonym for asphyxia or suffocation. In 

 physiological literature it is restricted to a very interesting con- 

 dition which is of great importance with reference to the theories 

 of respiration. This condition is one of cessation of breathing 

 movements due to lack of stimulation of the respiratory center. 

 It is brought about by rapid and prolonged ventilation of the 

 lungs. If, for instance, in a rabbit or other animal, a tracheal 

 cannuia is inserted and connected with a bellows or respiration 

 apparatus, the lungs may be inflated artificially at a rapid rate 

 for any given period of time. If such an experiment is per- 

 formed it will be found that when the blasts are stopped the 

 animal makes no breathing movements at all, sometimes for a con- 

 siderable interval. When the respirations start again they begin 

 with feeble movements, which gradually increase to the normal am- 

 plitude (Fig. 281). One may produce a similar condition upon him- 

 self, approximately at least, by a series of rapid, forced inspirations. 

 The question of importance is: Why does the respiratory center 

 cease to act? The numerous researches made upon this condition 

 seem to show very clearly that in the ordinary method used to pro- 

 duce it two factors co-operate, namely, a change in the condition of 

 the gases of the blood and a stimulation of sensory fibers in the 

 lungs, the latter factor bringing about a reflex inhibition of the 

 respiratory center. Since either one of these factors alone may 

 cause a cessation of breathing, some authors have distinguished 

 two kinds of apnea, apnea vera or chemical apnea, and apnea vagi 

 or inhibitory apnea. 1 1 is generally stated* that after section of the 

 vagi it is more difficult than in the normal animal to produce apnea 

 by vigorous artificial respiration, so doubtless in this last proce- 

 dure, as usually carried out with a bellows, the rapid stimulation 

 of the inhibitory fibers of the vagus by the expansion of the lungs 

 facilitates the production of a true or chemical apnea. In the pre- 

 ceding paragraphs evidence has been given to show that the normal 

 stimulus to the center is due to the presence of CO 2 , and it fol- 

 lows logically that the more complete removal of this gas by venti- 



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



