372 THE RESPIRATION 



periodic breathing is nearly always aggravated during sleep. Many of 

 these cases are greatly benefited by administration of caffeine. 



In both the physiological and the pathological groups, the breathing may 

 develop a periodic character only when the person is asleep, and even 

 normal people, particularly infants or very old people, may exhibit it to 

 a certain degree. 



Fig. 132. Various types of periodic breathing. (From Mosso's "Life of Man in the High Alps.") 



Causes of Periodic Breathing 



Great interest attaches to an investigation of the causes of periodic 

 breathing, but it can not be claimed that any perfectly satisfactory ex- 

 planation has as yet been offered. Pembrey 31 attributes it to a diminished 

 excitability (a raised threshold) of the respiratory center due to faulty 

 blood supply, the supposition being that, when thus suppressed, the 

 normal C H of the blood is unable to excite the center, so that breathing 

 stops. During the resulting apnea, C0 2 again accumulates until it has 



