346 MANUAL OF PHYSIOLOGY. 



and thus its activity is intimately related to its nutrition and 

 supply of oxygen. 



So long as the amount of oxygen flowing through the centre 

 keeps up to a certain standard, the normal excitability of the 

 centre continues, and we have natural, quiet breathing, called 

 Eupnwa. When the oxygen falls below the normal standard the 

 respiratory centre becomes more excitable, and labored breathing 

 is produced, commonly called Dyspnoea. 



If the theory that a deficiency of oxygen is the normal stimulus 

 to action of the respiratory centre be correct, a superabundant 

 quantity should diminish the activity of the centre, and a con- 

 dition the opposite of dyspnoea would be produced. This is dif- 

 ficult to show in natural breathing, though every one knows the 

 efficiency of the few deep breaths one takes before a dive into 

 water ; but with artificial breathing, if the movements be carried 

 on very energetically for some time, and then be stopped, the 

 animal will not at first attempt to breathe, but after a short time, 

 somewhat less than a minute, gentle and slow respiratory move- 

 ments commence. This cessation of breathing, called apncea, 

 depends upon the blood being so charged with oxygen that it no 

 longer acts as a stimulus to the centre. 



We find that dyspnoea is produced by a deficiency in the 

 amount of oxygen rather than by an excess of carbonic acid gas. 

 This is proved by the fact that it occurs when the carbonic acid 

 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 movements. 



