RELATION OF RESPIRATION TO THE NERVOUS SYSTEM 273 



respiratory centre in opposite ways inspiratory fibres, which 

 stimulate it to greater activity of discharge, and expiratory fibres, 

 which inhibit its action. The latter variety we may suppose to be 

 more numerous in the superior laryngeal, the former in the pul- 

 monary branches of the vagus. And there is nothing forced in the 

 hypothesis that certain kinds of stimuli act particularly on the one 

 set of fibres, and certain kinds on the other, for we have already 

 seen an instance of this in studying the differences between the vaso- 

 constrictor and the vaso-dilator nerves (p. 173). 



The most probable conclusion, and the one which best reconciles the 

 conflicting hypotheses, is that two sets of fibres are present : (i) Fibres 

 which inhibit inspiration (and cause expiration), and are excited in 

 ordinary inspiration by the expansion of the lungs. (2) Fibres which 



Fig. 125. Effect of Stimulation of Central End of Vagus in a Cat. Upper Trace, 

 Respiration; Lower Trace, Blood- Pressure. At the top are the time-trace 

 (seconds), and below it the signal line, the depression in which indicates the 

 duration of the excitation. Practically no effect was produced on the respira- 

 tion, but a fall of blood -pressure with slowing of the heart. 



cause inspiration (and inhibit expiration), and are excited in strong 

 expiration, as in dyspnoea, by the collapse of the lungs, but are not 

 active in ordinary expiration. 



However this may be, the facts we have been discussing have an 

 importance of their own, apart from any hypothetical explanations 

 of them. Some of them have been more than once unintentionally 

 illustrated on man. In one case the left vagus trunk was included 

 in a ligature with the common carotid. The respiratory move- 

 ments immediately stopped, the pulse was slowed, and death 

 occurred in thirty minutes (Rouse). The superior laryngeal fibres, 

 unlike those of the vagus proper, are not constantly in action, as 

 section of both nerves has no effect on respiration. Any source of 

 irritation in the larynx may stimulate these fibres and produce a 



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