176 THE HUMAN MECHANISM 



eight to twenty times a minute) discharge impulses to the 

 respiratory muscles independently of any stimulation either 

 by afferent nerves or by other means. Like the beat of the 

 heart, however, this automatic action is regulated in various 

 ways. A dash of cold water on the skin reflexly changes 

 the character of respiration ; coughing and sneezing are simi- 

 larly examples of reflex modification of the breathing move- 

 ments ; during vigorous muscular activity the change in 

 composition of the blood by the addition of waste products 

 deepens and quickens the breathing ; last, but not least, one 

 of the most important discoveries of recent years has shown 

 that the carbon dioxide of the arterial blood going to the 

 respiratory center is a most important agent in regulating 

 the automatic activity of the center. No sooner does the 

 carbon dioxide of the blood increase than the center dis- 

 charges more powerfully, thus deepening the breathing. An 

 increase of from 3 to 4 per cent in the carbon dioxide 

 of the arterial blood doubles the quantity of air breathed 

 per minute. From this it is evident that the high content 

 of this gas in arterial blood (see p. 166) serves the very 

 important function of adjusting the work of the center to 

 the needs of the body. Whenever, for any cause, the respira- 

 tory movements no longer adequately ventilate the lungs 

 so that carbon dioxide discharged upon the blood in its 

 course through the body is not completely removed in the 

 lungs the consequent increase of this gas in the arterial 

 blood excites the center to greater activity, with a resulting 

 increase of breathing and more efficient ventilation of the 

 lungs. We may recall, in this connection, the warning given 

 in Chapter VI against supposing that a " waste product " 

 of the activity of one organ is necessarily harmful, for 

 carbon dioxide is the chief waste of the body; yet it is 

 most important that the amount usually present in arterial 

 blood be maintained. Only the excess above this amount 

 is injurious. 



