INNERVATION OF THE RESPIRATORY MOVEMENTS. 689 



altogether or become slower and more feeble (Figs. 276 and 277). 

 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 



Fig. 277. To show the inhibition of the respiratory movements in a rabbit due to 

 stimulation of the central end of the vagus. The respiratory movements in this case, 

 before and after stimulation, were forced, owing to the fact that both vagi were cut. 



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. 278). 

 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 observed 

 44 



