NERVOUS MECHANISM OF RESPIRATION. 347 



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 apnoaa, 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 pneumogas- 

 tric 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 haemoglobin 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- 

 mal. If the stimulus be very strong, respiratory spasm can be 

 produced. On the other hand, if the central end of the superior 

 laryngeal branch of the vagus be stimulated, breathing becomes 

 slow, and can be made to cease while the thorax is in the position 

 of ordinary expiration, a spasm of the laryngeal and expiratory 

 muscles is caused. 



So that in the pneumogastric nerve, fibras exist which convey 

 impulses of two kinds to the respiratory centre, the one increas- 

 ing its excitability and causing more rapid discharges of iuspira- 

 tory impulses, the other decreasing its irritability and causing a 

 slowing of the respiratory movements. The marked change 

 which has just been described as occurring when the two pneu- 

 mogastrics are cut proves that these afferent influences are con- 



