. Elements of human physiology. Physiology. RESPIRATION 415 of both vagi is deepening and slowing of respiration. In the normal state we must imagine that the function of the vagus is to increase the excitability of the respiratory centre, and so make it more susceptible to slight changes in the COo tension of the blood. Although Gad has rendered great service in emphasising the importance of the inhibitory or expiratory impulses which ascend the vagi, there is no doubt that he went too far in denying the existence of inspiratory fibres in the vagus. This is shown by the following experiment o
. Elements of human physiology. Physiology. RESPIRATION 415 of both vagi is deepening and slowing of respiration. In the normal state we must imagine that the function of the vagus is to increase the excitability of the respiratory centre, and so make it more susceptible to slight changes in the COo tension of the blood. Although Gad has rendered great service in emphasising the importance of the inhibitory or expiratory impulses which ascend the vagi, there is no doubt that he went too far in denying the existence of inspiratory fibres in the vagus. This is shown by the following experiment of Head. According to Gad's view, collapse of both lungs implies simply a removal of the normal inhibitory impulses ascending the vagi, and is therefore equivalent to division of these two nerves. If in the rabbit the left vagus be divided, a tube can be introduced into the left bronchus and artificial respiration can be performed by alter- nate inflation and collapse of the left lung, without in any way affecting the respiratory centre, all connections with the latter being destroyed {v. Fig. 199). Fig. 199. R' artif resp app. L^ Lun Diagram to illustrate Head's experiment on the effect of collapse of the lung. , respiratory centre ; , , right and left vagi. Meanwhile the animal carries out normal respiratory movements, which can be recorded by the diaphragm slip method. While the slip is contracting regularly, the right pleura is opened and the right lung allowed to collapse. The effect of this collapse carried up by the right vagus to the centre is an extreme contrac- tion of the diaphragm, and since the onset of asphyxia is prevented by the artificial respiration carried out on the left lung, the tonic standstill of the diaphragm may last over a minute. In this case therefore, the effect of collapse of one lung is enormously greater than that produced by section of both vagi, showing that the effect is due, not to abolition of the ordinary tonic inhibitory
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