RESPIRATION. 421 



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, 



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 blood 

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

 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 action of the inspiratory 

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

 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 feeble intensity. 



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 

 the vagus excitator fibers, the central terminations of which are in relation 

 with the inspiratory center also (Fig. 199); and just as the inhibitor fibers 

 are stimulated by the expansion of the lungs so the excitator fibers are stimu- 

 lated in turn by the recoil of the lungs. The nerve impulses thus developed 

 ascend to the inspiratory center, excite it, and call forth a new inspiration 

 sooner than it would otherwise take place. According to this view the 

 respiratory mechanism is self-regulative and maintained by the alternate 

 expansion and recoil of the lungs. 



Many experimenters, however, find difficulty in accepting the view that 

 the recoil of the lungs stimulates nerve endings and hence this theory has 

 not received general acceptance. 



Another explanation which is satisfactory in many respects has been 

 presented by Meltzer. This investigator asserts also the existence in the 

 trunk of the vagus the two classes of nerve-fibers, the inhibitor and the 

 excitator; but that for some reason they do not respond to stimulation at 

 the same time as shown by the effects which follow; the inhibitor fibers 

 respond first and the excitator fibers somewhat later. Therefore when 

 they are stimulated simultaneously the primary effect is an inhibition of 

 the inspiratory center followed by an expiratory movement. The secondary 



