700 PHYSIOLOGY OF RESPIRATION. 



be caused either by stimulation of sensory nerves, particularly 

 the pain nerves, or by an increased venosity of the blood that 

 is, by an increase in the CO 2 or by a marked decrease in the oxygen. 

 Changes of other kinds in the composition of the blood, some of 

 which are considered in the next chapter, may also stimulate the 

 respiratory center and cause dyspnea. The dyspneic movements 

 naturally show many degrees of intensity corresponding with the 

 strength of the stimulus, and sometimes the initial stages are 

 designated as hyperpnea, while the term dyspnea is reserved for 

 the more 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 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 

 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 ifc 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 



