228 A MANUAL OF PHYSIOLOGY 



may occur. These facts undoubtedly suggest the existence in 

 the vagus of two kinds of afferent nerve-fibres that affect the 

 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 pulmonary 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. 159). 



FIG. 105. EFFECT OF STIMULATION OF CENTRAL END OF BRACHIAL NERVE ON 

 RESPIRATION (UPPER TRACING) AND BLOOD-PRESSURE (LOWER TRACING) 

 IN THE CAT. 



At the top of the figure are the time trace (seconds) and the signal line, showing 

 beginning and end of stimulation. 



The most probable conclusion, and the one which best recon- 

 ciles the conflicting hypotheses, is that two sets of fibres are 

 present : (i) Fibres which inhibit inspiration (and cause expira- 

 tion), and are excited in ordinary inspiration by the expansion 

 of the lungs. (2) Fibres which 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. 



