An active expiratory muscle in the chicken which is inhibited by stimulation of the central end of the vagus. A demonstration

In mammals expiration is passive under ordinary conditions. It is only under abnormal conditions that certain muscles become active during the expiratory phase of respiration. At the last meeting of the Federation of American Societies for Experimental Biology 1 we made the statement that in the fowl normal expiration is active. We wish to demonstrate the truth of this statement by a graphic method. We have found that the innermost of the abdominal muscles in the chicken when carefully isolated contracts regularly with each expiration. When the contractions of this muscle are recorded simultaneously with the movements of the thorax it will be observed that the muscle contracts during expiration and suddenly relaxes during the onset of inspiration.

The literature concerning the effect upon the respiration of stimulation of the central end of the vagus in mammals is very extensive and full of conflicting opinion as to the nature of this effect. In fowls stimulation of the central end of the vagus causes an unmistakable inhibition of the contractions of this muscle. When the movements of the thorax and the contractions of the expiratory muscle are registered simultaneously, stimulation of the central end of the vagus brings out an instructive picture. Throughout the period of stimulation the thorax remains quiescent in an inspiratory position, while the expiratory muscle remains completely relaxed.

This phenomenon is another instance of the general law of “ contrary innervation ” (Meltzer), or I “reciprocal innervation” (Sherrington). Inhibition of the expiratory group of muscles during inspiration was suggested by one of us over thirty years ago. 1