INNERVATION OF THE RESPIRATORY MOVEMENTS. 697 



labored breathing in which the expirations are active and forced. 

 When dyspnea is produced by 'withholding air (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 hes quietly, with feeble heart-beat and dilated pupils, in 

 a condition designated as asphyxia or complete 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 

 cannula 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 deep 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 produce 

 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. It 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 

 procedure, as usually carried out with a bellows, the rapid stimula- 

 tion of the inhibitory fibers of the vagus by the expansion of the 

 lungs facihtates the production of a true or chemical apnea. In 

 the preceding paragraphs evidence has been given to show that 

 the normal stimulus to the center is due to the hydrogen-ion con- 

 centration of the blood, and that this concentration varies with 



