614 PHYSIOLOGY OF RESPIRATION. 



may be affected more markedly than the other. In general, how- 

 ever, experimental stimulation of a sensory nerve trunk which con- 

 tains cutaneous fibers gives one of two effects : either a stimulating 

 action, manifested by quicker, stronger inspirations and active ex- 

 pirations, or an inhibitory effect, in which the respirations cease 

 altogether or become slower and more feeble (Figs. 247 and 248). 

 If in this, as in other similar cases, we assume that the two oppo- 

 site effects are produced by different nerve fibers we may speak of 

 sensory fibers which have a stimulating or augmenting effect, and 

 of those that have an inhibiting influence on the center, or following 

 the terminology used in the case of the vasomotor center, we may 

 speak of respiratory pressor and respiratory depressor fibers. It is 

 quite probable that these fibers have other functions, that is, they 

 are not distributed exclusively to the respiratory center. A cuta- 

 neous fiber, which through its central chain of neurons eventually 

 ends in the cortex cerebri and gives us a sensation of pain, may 

 by collateral connections affect also the medullary center and pro- 

 duce effects upon the heart, blood-vessels, and respirations. 



The Special Relations of the Afferent Fibers of the Vagus 

 to the Center. Although the sensory nerves in general exert a 

 reflex effect upon the respiratory center, experimental work has 

 shown that the sensory fibers distributed along the respiratory 

 passages from the anterior nares to the alveoli have a specially 

 important relation to this center. This fact is most clearly shown 

 in the case of the sensory fibers of the vagus, which are distributed 

 to the lungs themselves. If the two vagi are cut in the neck the 

 respiratory movements are at once altered in character; they 

 show a much slower rhythm and greater amplitude (Fig. 249). 

 The inspirations especially are deeper and longer, with something 

 of a pause at the end. When only one vagus is cut an intermediate 

 effect may be obtained, the respiratory movements may be slowed 

 somewhat and slightly deepened ; but the striking effect is obtained 

 only after section of both nerves. This result is not a temporary 

 one due to the stimulation of cutting, but is permanent, and there- 

 fore leads to the conclusion that some influence has been cut off 

 which normally keeps the respiratory movements at a more rapid 

 rate. Experiment has shown that this influence consists in the 

 tonic action of sensory fibers contained in the vagus and distributed 

 to the lungs. It is the constant effect of these fibers on the respira- 

 tory center which maintains the normal rhythm; when they are 

 severed the center drops into a slower, unregulated rhythm. Ex- 

 periment has shown, also, that when the central stump of the 

 divided vagus is stimulated artificially the respiratory center is 

 affected, as indicated by the respiratory movements, in a variety 

 of ways which depend upon the strength of the stimulus and the 



