3G2 NERVOUS MECHANISM. [BOOK n. 



plasm of the nerve-cells in that region. If the blood in the carotid 

 artery in an animal be warmed above the normal, dyspnoea is at 

 once produced. The overwarm blood hurries on the activity of 

 the nerve-cells of the respiratory centre, so that the supply of 

 blood, even though greater than normal owing to the blood-vessels 

 of the medulla becoming dilated by the increased temperature, is 

 yet insufficient for their needs. The condition of the blood then 

 affects respiration by acting directly on the respiratory centre itself. 

 Deficient aeration produces two effects in blood : it diminishes 

 the oxygen, and increases the carbonic acid. Do both of these 

 changes affect the respiratory centre, or only one, and if so, which ? 

 When an animal is made to breathe an atmosphere containing 

 nitrogen only, the exit of carbonic acid by diffusion is not affected, 

 and the blood, as is proved by actual analysis, contains no excess 

 of carbonic acid. Yet all the phenomena of dyspnoea are present. 

 In this case these can only be attributed to the deficiency of 

 oxygen. On the other hand, if an animal be made to breathe an 

 atmosphere rich in carbonic acid, but at the same time containing 

 abundance of oxygen, though the breathing becomes markedly 

 deeper and also somewhat more frequent, there is no culmination 

 in a convulsive asphyxia, even when the quantity of carbonic acid 

 in the blood, as shewn by direct analysis, is very largely increased. 

 On the contrary the increase in the respiratory movements after a 

 while passes off, the animal becoming unconscious, and appearing 

 to be suffering rather from a narcotic poison than from simple 

 dyspnoea. It does not seem certain that the increased respiratory 

 movements seen at first are the direct result of the action of the 

 carbonic acid on the respiratory centre; it is possible that the 

 carbonic acid may affect the respiratory centre in an indirect way, 

 by stimulating the respiratory passages, or by its action on higher 

 parts of the brain; and in all cases there is a marked contrast 

 between the slow development and evanescent character of the 

 hyperpncea of carbonic acid poisoning, and the rapid onset and 

 speedy culmination in convulsions and death of the dyspnoea due 

 to the absence of oxygen. There can in fact be no doubt that the 

 action of deficiently arterialized blood on the respiratory centre, as 

 manifested in an augmentation of the respiratory explosions, is due 

 primarily to a want of oxygen, and in a secondary manner only to 

 an excess of carbonic acid. 



Cheyne-Stokes Respiration. 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 



