294 MECHANISM OF THE RESPIRATORY MOVEMENTS. 



greater extent before it could be effective. Although this hypothesis 

 was shown to be untenable by Coats, 1 working under Ludwig's guidance, 

 Rosenthal made use of a very similar conception to explain the activity 

 of the respiratory centre. I have already mentioned that (according to 

 Rosenthal) the cause of the respiratory rhythm is to be sought in 

 the existence of a resistance to the passage of impulses from the centre 

 to the inspiratory muscles. The centre itself is kept in a condition of 

 constant stimulation, in consequence of the unsaturated condition of the 

 blood circulating through it. So long as the normal conditions for the 

 conveying of oxgyen to the blood are maintained, the activity — and 

 total work done by the centre — could be regarded as constant, an in- 

 crease of the resistance to discharge serving only to make each discharge 

 more forcible, but the interval between consecutive discharges longer. 

 In the normal animal this resistance is diminished by tonic influences 

 ascending both vagi from the lungs. If both vagi be cut, these in- 

 fluences are abolished, with the result of an increased resistance to 

 discharge of the centre. We thus get the familiar picture of slow deep 

 inspirations following section of both vagi. Stimulation of the central 

 end of the vagus fibres coming from the lungs diminishes this resistance, 

 so that the respiration may be quickened, or, if the resistance be entirely 



To be read from left to right 



Fig. 170. — Effect of closing trachea at height of inspiratory period. The trachea was 

 suddenly closed at + and again opened at o. — Head. 



abolished, there may be a continuous flow of energy from the respiratory 

 centre to the inspiratory muscles, so producing a tetanic spasm of the 

 diaphragm. Stimulation of the superior laryngeal has the reverse effect. 

 It increases the resistance to discharge, so that inspiration is abolished. 

 If stimulation be prolonged, the store of energy in the respiratory centre 

 rises to such an extent that, being unable to burst through the normal 

 resistance to inspiratory impulses, it escapes through the resistance be- 

 tween it and the expiratory muscles, thus producing an active expiration. 

 A complete revolution in our idea of the relationship of the vagus 

 to the respiratory movements was effected by the experiments of Hering 

 and Breuer 2 in 1868. These observers considered that, in order to 

 arrive at some idea of the normal functions of the vagus, it was im- 

 portant to employ a stimulus for these nerves resembling as far as 

 possible the normal stimulus. Proceeding on the assumption that the 

 normal stimulus for the vagus was to be sought in the alternate disten- 

 sion and contraction of the lungs, they attempted to exaggerate these 

 influences by closing the trachea at the end sometimes of an expiratory, 

 sometimes of an inspiratory movement. They found that, if the trachea 



1 Arb. a. d. physiol. And. zu Leipzig, 1869. 



- Sitzungsb. d. k. Akad. d. JFissensch., Wien, 1868, Bd. lviii. S. 909. 



