644 PHYSIOLOGY OF RESPIRATION. 



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 desig- 

 nated 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, 



Fig. 259. To show the recovery from apnea. The animal (rabbit) had been venti- 

 lated with a bellows and thrown into a condition of apnea shown at the beginning 

 of the record. The respirations returned first as feeble movements which gradually in- 

 creased to the normal. (Dawson.) 



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 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 



