RESPIRATION. 549 



quiet, the quantities of O and CO 2 in the blood being within the normal mean 

 limits. 



Apnoea may be produced by rapidly repeated respirations of atmospheric 

 air, under which circumstances the respiratory movements may be arrested for 

 a period varying from a few seconds to a minute or more. This condition is 

 produced most easily upon animals which have been tracheotomized and con- 

 nected with an artificial respiration apparatus. If under these conditions the 

 lungs are repeatedly inflated with sufficient frequency, and the blasts are then 

 suspended, the animal will lie quietly for a certain period in a condition of 

 apnoea. The respirations after a time begin, usually with very feeble move- 

 ments which quickly increase in strength and depth to the normal type. The 

 ultimate cause of apnoea is still a mooted question, and the heretofore prevalent 

 belief that it is due to hyperoxygenation of the blood is almost entirely dis- 

 carded. The connection between the quantity of O in the blood and apnoea 

 is, however, suggested by several facts : thus, apncea is more marked after the 

 respiration of pure O than after that of atmospheric air, and less marked if the 

 air is deficient in O ; moreover, Ewald states that the arterial blood of apnoeic 

 animals is saturated with O. These facts naturally lead to the inference that the 

 blood is surcharged with O, and that the respiratory movements are arrested 

 until the excess of O is consumed or until sufficient CO 2 accumulates in the 

 blood to excite respiratory movements. But Head l has shown that apnoea can 

 be caused by the inflation of the lungs with pure hydrogen as well as by infla- 

 tion with air or with pure O, although the apnoeic pause after the cessation of 

 the inflations is not so long or may be absent altogether ; while Ewald's asser- 

 tion as to the saturation of the blood with O is contradicted by Hoppe-Seyler, 

 Gad, and others. The fact that the apnoeic pause exists for a longer period 

 when O is respired lends confirmation to Gad's theory that it is due in part to 

 the large amount of O carried into and stored up, as it were, in the alveoli 

 an amount sufficient to supply the blood for a certain period and thus to dis- 

 pense with respiratory movements. Gad found that even when apnoea follows 

 the inflation of the lungs with air, the air in the lungs contains enough O to 

 supply the blood during the period occupied by the blood in making a com- 

 plete circuit of the system. The fact, however, that apnoea can be caused 

 by the inflation of the lungs by an indifferent gas such as hydrogen, by 

 which every particle of O may be driven from the lungs, certainly shows 

 that there exists some important factor apart from the O ; and this assump- 

 tion receives support in the observation that after section of the pneumo- 

 gastric nerves (the channels for the conveyance of sensory impulses from 

 the lungs to the respiratory centre) it is very difficult to cause apnoea by in- 

 flation of the lungs with air, while if pure hydrogen is used violent dyspnoea 

 results. It seems, then, that apnoea cannot be produced after division of the 

 vagi unless there be an accumulation of O in the lungs. These facts suggest 

 that the frequent forced inflations of the lungs excite the pulmonic peripheries 

 of the pneumogastric nerves, thus generating impulses which inhibit the inspi- 

 1 Journ. Physiology, 1889, vol. 10, pp. 1, 279. 



