698 



PHYSIOLOGY OF RESPIRATION. 



tends to show that the hypothesis is essentially correct. Two 

 kinds of afferent fibers exist in the vagus, one of which is stimu- 

 lated by expansion of the lungs, the other by collapse. This 

 fact is shown most clearly by Einthoven's* experiments with 

 his string-galvanometer. When the vagus nerve is cut high in 

 the neck and is then connected in the usual way with the string- 

 galvanometer, the latter shows a marked action current through- 

 out each inspiration, indicating, therefore, the passage of a series 

 of nerve impulses during inspiration (Fig. 280). When by suction 

 the lungs were collapsed, .another electrical variation of a different 

 character was produced, indicating the existence of a separate 

 set of fibers brought into action by the diminution in volume of 

 the lungs. In quiet respirations the expiration consists in 

 merely a passive return to what may be called the neutral or 

 normal volume of the lungs, and in this movement it is probable 

 that the inspiratory fibers are not affected, being stimulated 

 only by an active expiration. We may assume, therefore, 

 with Gad that the normal rate of respirations is maintained 



Fig. 280. To show the electrical changes in the afferent fibers of the vagus nerve 

 caused by the respirations and the heart beats: V, The electrovagogram, the large waves 

 are electrical oscillations synchronous with the respiratory movements. The smaller ones 

 are electrical changes synchronous with the heart beats; P, Mechanical record of the 

 respiratory movements, ascent of curve, inspiration; C, Mechanical record of pulse beats. 

 (From Einthoven.) 



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 the lungs the new inspiration is due to a 

 normal discharge from the inspiratory center. 



Loewyf nas shown by an ingenious experiment that the expansion of 



the lungs is the factor that actually stimulates the sensory fibers and quickens 

 for a 



, "Quarterly Journal of Exp. Physiology," 1908, 1, 243; also 

 the Physiological Laboratory of the University of Leyden," 



* Einthoven, 

 "Researches of 

 VII., 1908. 



t "Archiv f. die gesammte Physiologic," 42, 273. 



