CHAP, ii.] RESPIRATION. 605 



by a kind of summation of their effects in the medulla stop for 

 a while the generation of respiratory impulses in the respiratory 

 centre. This conclusion moreover is strongly supported by the 

 fact that an apnoea may be produced, so long as the vagus 

 nerves are intact, by forcible artificial respiration with hydrogen 

 instead of atmospheric air; in other words, the inhibitory im- 

 pulses generated in the vagus nerves by the inflation are 

 sufficient wholly to neutralize the development of respiratory 

 impulses which the deficient arterialization of the blood would 

 otherwise have produced. The exact nature and development 

 of such a summation of inhibitory impulses, especially in the 

 presence of correlative augmentative impulses called forth by the 

 corresponding successive collapses of the lungs, is too complex a 

 matter to be dwelt on here. Moreover an apncea may be produced 

 though, as we have said, with difficulty after section of both vagus 

 nerves; but in this case air and not hydrogen must be used for 

 inflation, the use of the latter, in contrast to the result when the 

 nerves are intact, leading to dyspnoea. The subject cannot as 

 yet be considered as fully cleared up. That apncea as ordinarily 

 produced is in some way the result of inhibitory impulses gene- 

 rated by the inflations can however hardly be doubted. 



375. Secondary Respiratory Rhythm. Cheyne-Stokes Re- 

 spiration. A remarkable abnormal rhythm of respiration, first 

 observed by Cheyne but afterwards more fully studied by Stokes, 

 and hence called by their combined names, occurs in certain 

 pathological cases. The respiratory movements gradually decrease 

 both in extent and rapidity until they cease altogether, and a 

 condition of apncea, lasting it may be for several seconds, ensues. 

 This is followed by a feeble respiration, succeeded in turn by a 

 somewhat stronger one, and thus the respiration returns gradually 

 to the normal, or may even rise to hyperpncea or slight dyspnoea, 

 after which it again declines in a similar manner. A secondary 

 rhythm of respiration is thus developed, periods of normal or 

 slightly dyspnceic respiration alternating by gradual transitions 

 with periods of apncea. The cause of the phenomena is not 

 thoroughly understood. Whether the waning and waxing of the 

 respiratory movements be due to corresponding rhythmic changes 

 in the nutrition of the respiratory centre itself, or to a rhythmic 

 increase and decrease of inhibitory impulses playing upon that 

 centre from other parts of the body, for instance from higher 

 regions of brain, has not yet been settled. It frequently appears 

 in connection with a fatty condition of the heart, but has been 

 met with in various maladies. Closely similar phenomena have 

 been observed during sleep, under perfectly normal conditions; 

 and this fact is rather in favour of the latter of the two expla- 

 nations just given. The phenomena present a striking analogy 

 with the 'groups' of heart-beats so frequently seen in the frog's 

 ventricle placed under abnormal circumstances. 



