RESPIRATORY REFLEXES 329 



We may conceive, therefore, that the peripheral endings of the afferent pul- 

 monary fibers of the vagi are excited by the variations in the volume of the 

 lungs, and this is confirmed by the fact that if the vagus be cut on the one side 

 and the lung on the other side be caused to collapse by puncture of the pleural 

 cavity, we get the same result as after section of both vagi (Loewy). 



Any more precise explanation of these facts is very difficult to give. It has 

 been supposed that there are two kinds of fibers in the vagus, one of which 

 serves to mediate an inspiratory effort, the other an expiratory effort. But it 

 is also possible to suppose that there is but a single kind of fibers and that the 

 effect produced depends upon the momentary condition of the respiratory center. 

 Thus, if the center already roused to inspiratory action were affected by a 

 stimulus arriving over the vagus, it might be inhibited, and the result of stop- 

 ping the inspiratory movements might be to inaugurate the relatively passive 

 movements of expiration. Then after expiration proceeds to a certain stage, 

 the collapsing lungs might send up a stimulus by the vagus and the inspiratory 

 phase would be started because the respiratory center at that particular instant 

 was in a condition to discharge such an impulse. We have as a matter of fact 

 some data which can be harmonized very well with this conception of the way 

 in which the respiratory center works (cf. Chapter XXII). But there is still 

 another explanation possible, namely, that the respiratory center constantly tends 

 to discharge inspiratory impulses, but this tendency is inhibited by an impulse 

 resulting from the inflation of the lungs ; that after the lungs have collapsed to 

 a certain extent, the inhibition is then removed and the tendency to inspiration 

 once more asserts itself. This explanation is strongly supported by the observa- 

 tion made by Lewandowsky, that inflation of the lungs is accompanied by an 

 action current in the vagus, but that collapse is not. 



Artificial stimulation of the central cut end of the vagus ought, one would 

 think, to give a definite answer to this question as to the mode of action of the 

 nerve. But it does not. For in the many experiments of this kind which have 

 been made, both inspiratory and expiratory effects of stimulation have been 

 observed, and the statements of authors differ so much that it is impossible as 

 yet to draw any definite conclusion from them. Still less is it possible to decide 

 from these experiments whether one or two kinds of nerve fibers are concerned. 



B. FIBERS FROM ANTERIOR PARTS OF THE BRAIN TO THE MEDULLA 



We have already seen that the brain can be sectioned above the medulla 

 without affecting respiration to any considerable extent. If, however, the 

 vagi be sectioned in such an animal, or if the brain be sectioned above the 

 medulla in an animal whose vagi have already been cut, noteworthy alterations 

 of the respiratory movements ensue. Respiration is greatly diminished in 

 frequency, since the inspiratory pauses are now very much prolonged. In- 

 spiration becomes spasmodic, expiration begins very suddenly and not infre- 

 quently is aided by contraction of the abdominal muscles. The expiratory 

 pause is of short duration, being soon interrupted by a new, long-drawn in- 

 spiration. The breath volume is very much diminished and the animal dies 

 for want of sufficient respiratory exchange in the lungs (Marckwald). These 

 phenomena may appear in varying degree, and it is even stated that the 

 inspiratory spasms may be at times entirely wanting after this operation. 



The respiratory center isolated from the higher parts of the brain may there- 

 fore maintain respiration in an essentially normal fashion even after section 



