240 ANATOMY AND PHYSIOLOGY 



Normal respiration is, as we have seen, a rhythmic process; 

 that is, the demand for oxygen is met by the act of inspiration; 

 this demand satisfied, the passive chest walls sink back into posi- 

 tion and the lungs retract, but only for an instant; another 

 demand is followed by another inspiration and the passive 

 expiration, regularly repeated ad infinitum. 



What causes the respiratory center to make this demand? It 

 is believed that the COz in the blood, which is flowing to the lungs 

 from the heart, is the normal direct excitant of the center. Many 

 facts in our experience are explained by this theory; e.g., after 

 several deep breaths the blood is rich in oxygen and one is content 

 to suspend respiration, but after a few seconds the oxygen is con- 

 sumed, the blood is charged with COz and the act of inspiration is 

 at once stimulated. 



The -same theory is advanced to explain the cause of the new- 

 born infant's first inspiration. With the cessation of the blood 

 stream from the mother, oxygen is lost, COz accumulates and 

 inspiration follows. 



In injancy and youth the rapid tissue changes of the growing 

 body cause, in the same manner, a higher rate of respiration than 

 in adult life. So with muscle exercise, which sends the blood 

 coursing through the body to gather its load of CO2. 



Whatever causes rapid circulation causes rapid breathing. This 

 is the explanation of the increased respirations of fever. 



Reflex stimuli of the respiratory center are without number; 

 the whole sensory apparatus affects it. Exposure to cold (low 

 temperature of the air) or sudden contact with a cold body, for 

 example the chill of cold water, causes a gasp or forced inspiration, 

 which is soon followed by rapid breathing. (The sudden contact 

 of the baby's body with the surrounding air is probably a power- 

 ful excitant to the first respiration, perhaps the principal one, and 

 often has to be aided by cold sprinkling.) Sharp recurring pain, 

 emotions of pleasure, anger, surprise, etc., all have a similar effect. 



Slowing of respiration follows an accumulation of 0; after a 

 few deep breaths one may refrain from inspiration for a time 

 (for from 40 seconds upward) according to training, the blood 

 using the residual air after the O of the tidal and complemental 

 supply is exhausted. 



Warmth disposes to moderate breathing; a sense of physical 



