60 BIOLOGY AND SOCIAL PROBLEMS 



produced in many parts of the body, it may be 

 regarded as the hormone for the respiratory 

 centers. Thus a somewhat regulated interaction 

 between the amount of carbon dioxid in the 

 blood and the activity of the respiratory mech- 

 anism takes place, in that with an increase of 

 carbon dioxid comes an increase in the activity 

 of the respiratory mechanism whereby the or- 

 ganism is relieved of this excess of waste. Thus 

 an equilibrium between the amount of carbon 

 dioxid and the efficiency of respiratory move- 

 ment is quickly established. 



But the respiratory rhythm is not one de- 

 pendent simply upon carbon dioxid. The lungs 

 are supplied with afferent nerve fibers which 

 make their way to the central organs through 

 the vagus nerves, and when these nerves are 

 cut the respiratory movements are consider- 

 ably changed, becoming slower and deeper. 

 Under normal conditions a given respiratory 

 movement apparently stimulates these afferent 

 fibers in such a way that they bring about in 

 a purely reflex fashion a hastening of the next 

 response. Thus this reflex mechanism has a 

 marked influence on the respiratory rate. 



Although the respiratory movements ordi- 

 narily proceed without relation to voluntary 



