RESPIRATION 53 



its action, or that its alternate inspiratory and expiratory dis- 

 charges are, under normal resting conditions, determined simply 

 by alternating stimuli transmitted through the vagus nerves. On 

 the other hand a too exclusive consideration of the chemical side 

 leads to the erroneous impression that the discharges of the center 

 are, apart from occasional voluntary or other interferences, de- 

 termined in strength and duration solely by chemical stimuli. If, 

 finally, we attempt to determine, one by one, the "factors'* in the 

 regulation of breathing, the sum of the supposed factors turns 

 out to be illusory, since no one of them is a constant quantity. The 

 evaluation of each factor depends on its varying relation to the 

 others. 



The "respiratory center" is a small area situated in the medulla 

 oblongata. It has been found that when this area is destroyed, all 

 rhythmical respiratory movements cease, and that so long as this 

 area is intact and in connection with any efferent nerves supply- 

 ing respiratory muscles, discharges of the center through these 

 nerves continue, as shown by the rhythmical contractions of the 

 muscles, although all the other nervous connections upwards and 

 downwards have been severed. 



It is also now clear that the activity of the center depends upon 

 the composition of the blood circulating through it, and not on 

 chemical stimuli acting elsewhere. If the circulation to the medulla 

 is interrupted by closure of all the four arteries supplying it, so 

 that its blood has time to become venous, violent hyperpnoea re- 

 sults, as Kiissmaul and Tenner showed about the middle of last 

 century; and the crossed circulation experiments of Fredericq, 

 already referred to, prove that either apnoea or hyperpnoea is 

 produced, according as the blopd supplied to the central nervous 

 system is more aerated or less aerated in the lungs. 



It has been suspected that although the stimuli dependent on 

 the composition of the blood act directly within the brain, nervous 

 end-organs situated elsewhere are also sensitive to these stimuli, 

 so that the corresponding nerves convey impulses which play an 

 important part in the regulation of breathing. It was, for instance, 

 believed by Traube that chemical stimuli are conveyed directly 

 from the lungs by the vagus nerve, and others have supposed 

 that stimuli to increased breathing are conveyed by direct nervous 

 paths from the muscles. This hypothesis was investigated with 

 great care by Geppert and Zuntz,^ who severed all the nervous 

 connections between actively working muscles and the medulla, 



'Geppert and Zuntz, Pfiiiger's Archiv, XLII, pp. 195, 209, 1888. 



