342 MANUAL OF PHYSIOLOGY. 



gas is removed from the blood by breathing freely air which is 

 only deficient in oxygen, and, secondly, because an excess of car- 

 bonic acid gas in the air causes a drowsy condition and not an 

 active dyspnoea. 



Although the respiratory centre is in the strictest sense auto- 

 matic, yet it is profoundly affected by many influences coming 

 from other parts, which reflexly modify the respiratory move- 

 ments. Thus, mental emotions variously influence both the rate 

 and the depth of breathing, sometimes causing more rapid and 

 sometimes slower respiratory action. The application of stimulus 

 to almost any part of the air-passages completely changes the 

 respiratory rhythm. The ordinary sensory nerves passing from 

 the skin are also capable of exciting respiratory movement?. 

 This is well seen from the gasping that follows the sudden appli- 

 cation of cold to the body. It is along these sensory nerves that 

 one tries to transmit impulses by applying mechanical, thermal, 

 or other stimulus to the skin of a new-born infant, whose respira- 

 tory centre, having been kept long in the condition of apncea, is 

 slow to respond to an exciting influence caused by a deficiency of 

 oxygen. 



Experiment shows that most, if not all, afferent nerves can 

 affect the respiratory centre, either by increasing or reducing its 

 activity ; but there is one special nerve, namely, the pneumogastric 

 or vagus, and its branches, which have both these capabilities 

 developed to a much greater degree than any other. 



If the two vagi be cut, a marked change takes place in the 

 respiratory rhythm, though section of one vagus has little or no 

 effect on respiration. The rate of the inspiration is reduced to 

 less than half, while each breath becomes extremely deep and 

 prolonged, the respiratory function of the lungs goes on for some 

 time unimpaired, and the htemoglobin of the blood receives the 

 due amount of oxygen. Although the character of the breath- 

 ing is completely changed from the rapid gentle motion of natural 

 respiration to a series of slow deep, gasps, the air volume per 

 minute and the chemical changes remain the same. If the cen- 

 tral end of the cut vagus be now stimulated gently, the rate of 

 the respiratory movements may again be quickened to the nor- 



