INNERVATION OF THE RESPIRATORY MOVEMENTS. 697 



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

 gators.* The two main effects described above are usually inter- 

 preted to mean that the vagus contains two kinds of sensory fibers 

 which are distributed to the lungs and act normally on the respira- 

 tory 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 inspira- 

 tory 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 difference in the nature of the stimulus or 



Fig. 279. To illustrate the inspiratory effect from stimulation of the central end of 

 the vagus. The downstroke represents inspiration; the upstroke, expiration. During 

 the period of stimulation the respirations are increased in frequency and the chest remains 

 in a condition of inspiration. (Lewandowsky.) 



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. This hypothesis has been much discussed 

 and many efforts have been made to prove or disprove it by 

 means of experiments. The result of this work on the whole 



