616 PHYSIOLOGY OF RESPIRATION. 



stimuli, and has led to much difference of opinion among investi- 

 gators.* The two main effects described above and obtained 

 with stimuli not too strong are usually interpreted to mean that the 

 vagus contains two kinds of sensory fibers which are distributed to 

 the lungs and act normally on the respiratory center. These are: 

 (I) The inspiratory fibers, whose effect is to increase the rate of 

 inspiratory discharge from the respiratory center; therefore to 

 quicken the rate. (II) The expiratory (or inspiratory inhibiting) 

 fibers, whose effect is to inhibit the inspiratory discharges, partially 

 or completely. Some authors find it simpler to assume only one 

 kind of sensory fiber and to explain the different results by a dif- 

 ference in the nature of the stimulus or in the condition of the center; 

 but it seems advisable at present, in accordance with the doctrine 

 of specific nerve energies, to hold to the view of two varieties. 



Influence of the Inspiratory and the Inhibitory Fibers of 

 the Vagus on the Normal Respirations. It is assumed that 

 these two sets of fibers are in constant activity and keep the re- 

 spiratory rate more rapid than it would be otherwise. Hence the 

 slowing and deepening of the respirations when the vagi are cut. 

 The way in which these sensory fibers are stimulated normally was 

 referred by Hering and Breuer to the alternate expansion and 

 collapse of the lungs. Each inspiration stimulates the inhibitory 

 fibers in consequence of the expansion of the lungs, and thus cuts 

 short the inspiration, prematurely, as it were. So at each expira- 

 tion the collapse of the lungs stimulates the inspiratory fibers and 

 brings on an inspiration sooner than would otherwise occur. In 

 this way the respiratory rate is kept automatically at an accel- 

 erated rhythm. A stimulation of the sensory fibers as a result of 

 expansion of the lungs is easy to comprehend and, indeed, it has 

 been demonstrated by showing that with each expansion an action 

 current may be obtained in the vagus by means of the galvanometer 

 or capillary electrometer. But that the normal collapse of the 

 lungs also acts as a mechanical stimulus to a different set of nerve 

 endings is not such a probable hypothesis, and most physiologists 

 believe that it is not necessary to adopt it, at least for normal 

 respirations. Head and also Schenck have shown that with a 

 certain extreme extent of collapse evidence may be obtained of a 

 stimulation of the inspiratory fibers. We may assume, with Gad, 

 that the normal rate of respirations is maintained by the action 

 of the inhibitory fibers alone. Each inspiration is cut short by 

 the mechanical stimulation of these fibers, but on the collapse of 



* For discussion and literature see Meltzer, " Archiv f . Physiologie, " 

 1892, p. 340; also "New York Medical Journal, " January 18, 1890. Le- 

 wandowsky, " Archiv f. Physiologie," 1896, pp. 195 and 483. 



