404 THE HUMAN BODY 



it, and continues its discharge until the aeration of the blood, re- 

 sulting from the inspiration, lowers the carbon dioxid below the 

 point of stimulation. There follows a period of expiration and 

 rest which continues until sufficient carbon dioxid has again ac- 

 cumulated to start the action anew. 



Since with the vagus nerves cut the respirations follow the car- 

 bon dioxid concentration of the blood, but with the nerves intact 

 do not, being much more shallow and rapid, we must determine 

 the influence of the vagus nerves upon the center in order to un- 

 derstand ordinary breathing. It has been shown that the influence 

 of the vagus nerves is a simple reflex one. These nerves contain 

 sensory fibers arising in the lung tissue and so situated as to be 

 stimulated mechanically every time the lung is inflated. The im- 

 pulses conveyed over these fibers to the central nervous system 

 are inhibitory to the respiratory center. Bearing this action of the 

 vagus fibers in mind we may account for normal breathing thus; 

 the blood contains enough carbon dioxid all the time, under ordi- 

 nary circumstances, to stimulate the respiratory center; when- 

 ever the center discharges under this stimulus it brings about 

 the movements of inspiration which result in expansion of the 

 lungs; whenever the lungs expand the sensory fibers contained in 

 their walls are stimulated and so inhibitory influences are sent to 

 the respiratory center. Inspiration proceeds, then, until the in- 

 hibitory impulses from the lungs overcome the stimulus of carbon 

 dioxid, when it comes to an end and the thorax falls back to the 

 position of rest. This falling back, which constitutes normal ex- 

 piration, collapses the lungs somewhat; the inhibitory impulses 

 diminish or disappear; and the stimulating action of the carbon 

 dioxid again becomes effective. Thus in normal breathing in- 

 spiration and expiration follow one another without any pause 

 between, and the respirations are shallow because the inhibition 

 cuts them off almost as soon as started. 



The entire purpose of breathing is to ventilate the lungs. It is 

 relatively a minor matter whether 'a system of rapid shallow breaths 

 or of slow deep ones is used so long as the result is secured. The 

 necessary amount of air would be taken into and discharged from 

 the lungs every minute by either arrangement. There may pos- 

 sibly be some advantage to the Body in the rapid shallow type in 

 avoiding such wide fluctuations in the concentrations of the respira- 



