110 PHYSIOLOGY FOR NURSES 



tion produced by a marked increase of carbon dioxide 

 or a similar decrease in oxygen. It could be produced 

 by diminishing the amount of oxygen in the respired 

 air, by blocking up the air passages, as by choking, or 

 decreasing available lung space. When carried to ex- 

 cess as by hanging, a condition of aspliyxia results. 



The vagus nerve carries two sets of fibers to the mus- 

 cles of the bronchioles those which excite contraction 

 when stimulated, and those which cause dilatation. They 

 are called broncho constrict or and bronclwdilator fibers. 



Cardiac Center. This center is found in the neighbor- 

 hood of the roots of the vagus nerve, through which it 

 exerts a slowing and regulating influence on the heart- 

 beats, and is, therefore, called the cardioinhibitonj cen- 

 ter. Whether there be a distinct accelerating center in 

 the medulla is disputed. It is certain that nerve fibers 

 sent to the heart by the sympathetic system increase the 

 rate of the heartbeat. Other nerves affect the action of the 

 heart reflexly. Painful sensations, particularly from the 

 viscera, slow the heart, These sensations are carried 

 into the central nervous system and act reflexly on the 

 inhibitory center stimulating it to greater activity. On 

 the other hand, emotion may stimulate the accelerator 

 center and cause a more rapid beat, sometimes attended 

 by less power. Hence it has been inferred that acceler- 

 ator nerves carry two sets of fibers, one simply to in- 

 crease the rate of action of the heart and the other to 

 augment its power. Fear, therefore, might increase the 

 rapidity with loss of strength, while anger could aug- 

 ment both. 



Vasomotor Center. The action of the blood vessels 

 consists in dilating to increase and constricting to di- 

 minish their capacity. 



A vasoconstrictor center in the medulla is demon- 



