IV DYSPXCEA AND ASPHYXIA 161 



If on the other hand the branch of the vagus supplying 

 the larynx, the superior laryngeal nerve, be cut, and 

 its central end be stimulated, the result is that the 

 respiration may be sloived, even to a complete cessation of 

 all respirator}' movements. 



In the case of the vagus impulses seem to be ordinarily 

 always passing up it to facilitate the action of the respira- 

 tory centre, for if the vagus nerves be simply cut, the 

 respiration becomes at once extremely slow. 



These two nerves may be taken as typical of their kind, 

 the one quickening, the other slowing the respiration. 

 But similar nerves run to the respiratory centre from all 

 parts of the body, notably from the skin, also from the 

 brain, and by their varied action largely determine the 

 manifold changes which the respiratory movements from 

 time to time undergo. 



11. Influence of Blood-Supply on the Respiratory 

 Centre. Dyspnoea and Asphyxia. — The function of 

 respiration has for its one great object the conversion of 

 venous into arterial blood. Hence we might expect that 

 the mechanism which controls it should be adjusted so as 

 to be extremelj' sensitive to the varying condition or 

 quality of the fluid (blood), whose gaseous composition it 

 has to regulate. This expectation is justified by facts, for 

 although the respiratory centre is keenly responsive to 

 impulses brought to bear upon it along various nerves, it 

 is even more so to the influence exerted by the varying 

 quality of the blood circulating in the capillai'ies of the 

 spinal bulb. Thus, when by any means the blood becomes 

 less arterialised than it should be, the respiratory centre 

 feels this change, and is at once stimulated to greater 

 activity in the endeavour, by an increased force and 

 frequency of the respiratory movements, to restore the 

 blood to its proper condition. In other words, venous 

 blood makes the respiratory centre work faster and more 

 vigorously. 



The blood becomes more venous whenever the free 



H 



