RESPIRATION 217 



for example, the other muscular nerves or the pulmonary 

 branches of the vagus. 



That the respiratory centre is greatly affected by the 

 [uality of the blood which circulates through it is well 

 :nown. And it is generally acknowledged that it may be 

 ccited both by blood that is rich in carbon dioxide and by 

 >lood that is poor in oxygen, the actual stimulating sub- 

 stance in the latter case being, perhaps, an easily oxidizable 

 body which rapidly disappears from properly oxygenated 

 blood (Pfluger). 



But it has been the subject of long-continued discussion 

 whether excess of carbon dioxide or deficiency of oxygen is 

 the more potent stimulus. The truth appears to be that 

 much depends upon the conditions of the experiment, upon 

 the size of the chamber, for instance, in which an animal 

 or a man is made to breathe. The best evidence points 

 to the conclusion that comparatively small alterations in 

 the amount of carbon dioxide in the inspired air cause a 

 relatively great increase in the respiration, while in the case 

 of the oxygen the departure from the normal proportion 

 must be much more decided to bring about any notable 

 effect (Zuntz and Loewy). Nor is it at all out of harmony 

 with this that, when very large quantities of carbon dioxide 

 (30 per cent, and upwards in rabbits) are inhaled, a condi- 

 tion of narcosis comes on without any previous respiratory 

 distress (Benedicenti). For many substances act differently 

 in large and in small doses. 



Be this as it may, when the gaseous interchange from any 

 cause becomes insufficient, the respiratory movements are 

 exaggerated, and ultimately every muscle which can directly 

 or indirectly act upon the chest-walls is called into play in 

 the struggle to pass more air into and out of the lungs. To 

 a lesser and greater degree of this exaggeration of breathing 

 the terms Hyperpn&a and Dyspnoea have been respectively 

 applied. If the gaseous interchange remains insufficient, 

 or is altogether prevented, asphyxia or suffocation sets in. 

 Sometimes in man impending asphyxia from loss of function 

 by a part of the lungs, as in pneumonia, may be warded off 

 by inhalations of oxygen. Increase in the temperature of 



