366 THE HUMAN BODY 



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. 



Eupnea, Dyspnosa, Apnrea. Ordinary quiet breathing is known 

 as eupnea. When the breathing is forced, and especially when 

 forced expiration enters, we have the condition called dyspnoea. 

 This results from abnormal excitement of the respiratory center 

 either reflexly, as from stimulation of pain nerves, or by an increase 

 in the carbon dioxid content of the blood. The dyspnoea of the 

 early stages of suffocation arises from this latter cause. Apnoea, or 

 absence of breathing, may result from one of two conditions or 

 from both acting together. The first of these is a deficiency of 

 carbon dioxid in the blood, so that the respiratory center is not 

 stimulated. The second is inhibition of the center through vigor- 

 ous and repeated inflation of the lungs. Since inflation of the lungs 

 with ordinary air brings about both conditions the apnoea which 

 results from this treatment is partly chemical and partly inhibi- 

 tory. That inhibition enters in the production of apncea in this 

 way is shown by the greater difficulty of producing the condition 

 in animals with both vagi cut. 



