HORMONES 59 



easier than the regular filling and emptying 

 of our lungs with air, and yet the operation 

 is carried on in a way that illustrates prac- 

 tically all the forms of nervous and hormone 

 action of which I have spoken. 



The muscles concerned with the respiratory 

 movements are those of the lower neck, the 

 chest, and the upper abdomen. They are con- 

 trolled from a pair of centers closely connected 

 and lying in the midst of the medulla oblon- 

 gata. Although these centers can be influ- 

 enced from almost any part of the central 

 nervous system, they will continue to give 

 out impulses to respiratory movements even 

 after they have been separated from practi- 

 cally all afferent tracts. This independence of 

 afferent control has led to the conclusion, well 

 supported from other directions, that the res- 

 piratory centers are stimulated by the quality 

 of the blood that passes through them. It has 

 been shown that when the blood that reaches 

 them is very impure, their action results in 

 vigorous breathing, and that when it is more 

 nearly pure, they subside in their activity or 

 even cease altogether. The constituent of 

 the blood which is effective in this respect 

 seems to be the carbon dioxid, and, as this is 



