386 THE RESPIRATION 



physiological and pathological groups. Of the physiological conditions the 

 following may be taken as examples: (1) Breathing in an atmosphere 

 containing a deficiency of 2 ; thus, periodic breathing is very readily pro- 

 duced in persons living in rarefied air. It is often more particularly after 

 returning to air at normal barometric pressure, that the periodic breathing 

 sets in. (2) Breathing through a long tube having a small vessel contain- 

 ing soda lime inserted between the tube and the mouth, the whole capacity 

 of this vessel and tubing being about a liter. This will cause periodic 

 breathing in persons that are susceptible to oxygen deficiency. Even 

 breathing through the tube without soda lime will sometimes cause a periodic 

 type of breathing in such individuals. (3) The initial breathing follow- 

 ing an apnea induced by forced ventilation of the lungs. In this post- 

 apneic periodicity, the apneic periods may at first be quite marked, but as 

 breathing returns they become gradually shorter and the breathing in- 

 tervals gradually longer, until normal respiration is restored (Fig. 131). 

 (4) Restricted breathing, brought about either by limiting the quantity 

 of inspired air or by restricting the respiratory movements by corsets. 



The pathological conditions in which periodic breathing becomes devel- 

 oped are particularly those associated with renaj^^disej^^njLiie^ebral 

 hemorrhage. In many of these cases, the periodic breathing does not 

 appear to depend on the same factors as are concerned in the experi- 

 mental types. The symptoms would rather appear to depend on some 

 influence of the higher cerebral (supranuclear) centers on the respiratory 

 center. At least some other evidence of disturbance of the cerebral func- 

 tions is always forthcoming, such as a slight paralytic stroke, and the 

 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. Infants or 

 very old people may indeed exhibit a certain degree of periodic breathing 

 apart from any of the above mentioned causative factors. 



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 depressed, 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 

 raised the C H sufficiently to excite the depressed center. Hyperpnea 

 follows, causing a washing out of the C0 2 and a resulting diminution of 



