434 TEXT-BOOK OF PHYSIOLOGY 



diaphragm (Fig. 197) and by the state of the thorax which is that charac- 

 teristic of extreme inspiration, a result similar in all respects to that produced 

 by moderate stimulation of the central end of the divided vagus. 



Theories of the Mode of Action of the Respiratory-nerve Mechanism. 

 A satisfactory explanation of the action of the respiratory-nerve mechanism 

 is very difficult to present. Theories vary in accordance with the estimate 

 of an investigator as to the degree of automaticity of the inspiratory center, 

 of the effects of vagus stimulation and as to the extent to which the expira- 

 tory center is involved with the activity of the inspiratory center either 

 simultaneously or successively. 



The following theories have each found adherents. 



i. If it is assumed that the inspiratory center is automatic and in a state of 

 continuous excitation the result of the action of carbon dioxid in the lymph 

 circulating around it, then it is only necessary to assume the existence, m 

 the trunk of the vagus of one set of nerve-fibers, viz.: inhibitor fibers, 

 the central terminations of which arborize around the inspiratory center, 

 and the function of which is to check or inhibit the discharge of the inspiratory 

 center and thus permit of an expiratory movement. The inhibitor fibers 



Negative 

 ventilation. 



Diaphragm. 



Seconds. 



FIG. 197. NEGATIVE VENTILATION. (Head). At a negative ventilation was commenced. 

 The expiratory relaxation of the diaphragm is seen to become more and more incomplete, until it 

 finally enters into continued contraction. 



are supposed to be stimulated peripherally by the expansion of the lungs. 

 With the recoil of the lungs the inhibitor effect gradually dies away, while the 

 inherent excitation of the inspiratory center again returns, to be followed 

 by another discharge of nerve impulses and a new inspiratory movement, 

 which will in turn be again inhibited as the inhibitor fibers are stimulated 

 by the expanding lung. This explanation is in accordance with the results 

 which follow stimulation of the superior laryngeal nerve or the trunk of 

 the vagus with induced electric currents of slight intensity. 



2. If it is assumed, on the contrary, that the inspiratory center is not in a 

 state of constant excitation leading to a frequent periodic discharge of nerve 

 impulses, but requires the arrival of a stimulus to call forth its normal 

 activity, then this theory does not suffice, inasmuch as it leaves out of con- 

 sideration the presence of nerve-fibers in the vagus which increase or aug- 

 ment the activity of the inspiratory center; and that such fibers are present is 

 apparently indicated by the effects of stimulation of the central end of the 

 vagus nerve with moderately strong induced electric currents, and from the 

 experiments of Hering and Breuer, and later of Head. These observers 

 assume, therefore, that in addition to the inhibitor fibers there are also present in 



