RESPIRATORY INTERCHANGE UNDER DIFFERENT CONDITIONS 523 



this inhibition lasting until the extra amount of oxygen has again been 

 used up. Head,^ however, has shown that this effect may also be 

 obtained by inflating the lungs with pure hydrogen, although it is true 

 that the apneic cessation of respiration is then briefer in its duration 

 and may, in fact, be abolished altogether. Besides, it should be men- 

 tioned that the contention of Ewald, that in apnea the blood is actually 

 oversaturated with oxygen, has been disproved by Hoppe-Seyler.^ 

 It seems, therefore, that some other factor, besides the oxygen, must 

 be responsible for this phenomenon. It has been suggested that the 

 repeated distention of the lungs acts as an excitaat to the receptors 

 of the vagi nerves, in consequence of which impulses are generated 

 which reflexly inhibit the inspiratory discharges from the respiratory 

 center.^ A more plausible explanation, however, is the one offered by 

 Mosso,^ which states that any excessive ventilation of the lungs induces 

 a scarcity of carbon dioxid (acapnia) which eventually leads to a con- 

 dition of subnormal stimulation of the respiratory center. The re- 

 spiratory actions then cease until the accumulation of carbon dioxid 

 in the blood has again been raised to normal. That this is so may be 

 gathered from the fact that augmentations of the respiratory move- 

 ments fail absolutely to produce the apneic standstill if the carbon 

 dioxid content of the inspired air is retained at 4.5 per cent. In order 

 to account for the different discrepancies just enumerated, it has been 

 suggested to recognize three types of apneas, namely: 



Apnea vera, which is due to the lowering of the CO2 content, 



Apnea vagi, which is caused by the stimulation of the inhibitor fibers of the 



vagi nerves, and 



Apnea spuria, which is dependent upon stimulations from other parts of the 



body. 



As an example of the first type might be mentioned the apnea fetalis, 

 i.e., the permanent inhibition of the respiratory activity of the young 

 while in the uterus. As an example of the second type may serve the 

 rather temporary inhibition following the distention of the lungs by 

 air or inert gases, and as an example of the third type, the cessation 

 of respiration exhibited by diving animals as soon as their nares or 

 beaks are brought in contact with water. 



A very peculiar type of respiration is frequently observed during 

 such pathological states as arteriosclerosis, uremic coma, anemia, 

 increased intracranial pressure and lesions of the central nervous 

 system. The respiratory movements then occur in groups which are 

 separated from one another by apneic pauses. This condition of 

 periodic breathing is commonly designated as Cheyne-Stokes respira- 

 tion (Fig. 261). The periodicity of these movements, however, is not 

 the same in all cases; but whether only ten or forty of them appear 



1 Jour, of Physiol., x, 1889, 1. 



2 Zeitschrift fur physiol. Chemie, iii, 1879, 105. 



' Miescher-Rtisch, Wiener Akad., Ixxxv, 1882, 101. 

 * Arch. ital. de biol., xl, 1903. 



