250 A MANUAL OF PHYSIOLOGY 



Quite a number of observers have supposed that rhyth- 

 mical discharges from the vaso- motor centre, either 

 automatic or due to stimulation of the centre by the venous 

 blood, and causing a periodic increase and diminution in the 

 peripheral resistance, are responsible for the respiratory 

 oscillations. Such rhythmical variations in the blood- 

 pressure (Traube-Hering curves) may, under certain condi- 

 tions, appear in the absence of respiratory movements, e.g., 

 when in a curarized animal the artificial respiration is 

 stopped. But this is hardly an argument in favour of the 

 central origin of the normal respiratory waves, since the 

 Traube-Hering curves have a much longer period. This is 

 well seen when, as sometimes happens, Traube-Hering 

 oscillations appear while respiration is still going on. Their 

 long sweeping curves then show the ordinary respiratory 

 waves superposed on them. 



Mechanical Theory. A more satisfactory explanation is 

 afforded by a consideration of the mechanical changes pro- 

 duced in the thorax by the respiratory movements. Of 

 these two are of special importance : (i) the changes of 

 intra-thoracic pressure, (2) the changes of vascular resist- 

 ance in the lungs. 



The intra-thoracic pressure, which, as we have seen, is 

 always less than that of the atmosphere, unless during a 

 forced expiration when the free escape of air from the lungs 

 is obstructed, diminishes in inspiration and increases in 

 expiration. The great veins outside the chest, the jugular 

 veins in the neck, for example, are under the atmospheric 

 pressure, which is readily transmitted through their thin 

 walls, while the heart and thoracic veins are under a smaller 

 pressure. The venous blood both in inspiration and ex- 

 piration will, therefore, tend to be drawn into the right 

 auricle. In inspiration the venous flow will be increased, 

 since the pressure in the thorax is diminished ; and upon 

 the whole more venous blood will pass into the right heart 

 during inspiration than during expiration. But all the blood 

 which reaches the right heart during an inspiration is at 

 once sent into the lungs, although not even the first of it 

 can have passed through to the left side of the heart at the 



