296 HUMAN PHYSIOLOGY 



Carbon Dioxid and the Respiratory Center. A brief 

 statement was made in Chapter VIII respecting the 

 control exercised by the 'medulla over the breathing 

 movements. Attention was there drawn to the fact that 

 the center from which the impulses issue to command 

 the respiratory muscles is regulated in its action largely 

 by the concentration of carbon dioxid in the circulating 

 blood. The changes in the quantity of this gas are 

 very positive sources of influence upon the neurons of 

 the center. To make this clear it will be well, first of 

 all, to show that the composition of the air in the lungs 

 has little if any effect on the character of the breathing. 

 The center is responsive to the chemical state of its im- 

 mediate environment rather than to that which pre- 

 vails in the air-sacs. A striking experiment shows this 

 decisively. 



Two rabbits are anesthetized and placed side by side. 

 By an operation of some delicacy connections are made 

 which lead the blood from the arterial system of rabbit 

 A to the head of rabbit B. Similarly the arteries of 

 rabbit B are made to supply the head of rabbit A. If 

 the trachea of rabbit A is now obstructed it is the second 

 animal which shows the labored breathing movements 

 of asphyxia. The air in the lungs of rabbit A is falling 

 off in oxygen and gaining in carbon dioxid but this does 

 not stimulate the nervous mechanisms so long as the 

 standard blood of rabbit B is flowing through the vessels 

 of the brain. 



When the composition of the blood tends to be altered 

 either because of restricted breathing or excessive 

 muscular activity we may expect a simultaneous rise 

 in the carbon dioxid of the blood and a fall in its oxygen. 

 How do we know which of these changes stirs the re- 

 spiratory center to compensatory action? It is possi- 

 ble for purposes of experiment to separate the two. A 

 trial may be made upon the human subject without un- 

 due hardship. A man may breathe in and out of a 

 silk bag, his breath passing back and forth through a 



