378 RESPIRATION [CH. XXVI. 



inherent rhythm, which was quickened by the vagus impulses ; but 

 it is claimed that when all impulses, both from the brain above and 

 the sensory nerves below, are cut off from the respiratory centre, 

 the respiratory rhythm ceases. The operation is, however, a very 

 severe one, and therefore inconclusive. 



Leaving the question of normal respiration, we may proceed to 

 consider the impulses passing up the vagus during forced respiration. 

 The presence of impulses in these nerves can again be best detected 

 by their action-currents, and in forced inspiration the same action- 

 current is shown by the galvanometer as we have just mentioned 

 occurs during normal breathing; it can also be induced by artificial 

 inflation of the lung. When the lung is alternately and deeply 

 inflated and deflated, a small electrical variation frequently appears 

 also in the vagus nerve during each deflation. We have, therefore, 

 evidence that a nervous impulse is passing up the vagus during this 

 period, but whether this impulse of the expiratory period is inhibitory 

 to an expiratory centre, or a stimulus to an inspiratory centre, is very 

 difficult to decide. The following experiments of Head, however, 

 suggest the existence of a double centre. 



His method of recording the movements was by means of that con- 

 venient slip of the diaphragm which is found in rabbits (see p. 357). 



His method of dividing the vagus was by freezing it ; he laid it 

 across a copper wire, the end of which was placed in a freezing 

 mixture. This method is free from the disadvantage which a cut 

 with a knife or scissors possesses, namely, a stimulation at the 

 moment of section. On dividing one vagus, respiration became 

 slightly slower and deeper ; on dividing the second nerve, this effect 

 was much more marked. 



On exciting the central end of the divided nerve, inspiratory 

 efforts increased until at last the diaphragm came to a standstill in 

 the inspiratory position. But if a weak stimulus was employed, the 

 reverse was the case ; the expiratory efforts increased, inspiration 

 becoming weaker and weaker, until at last the diaphragm stopped in 

 the position of expiration. 



These facts were known previously, but the interpretation of them, 

 in the light of further experiments now to be described, is the 

 following : 



There are in the vagus two sets of fibres, one of which produces 

 an increased activity of the inspiratory part of the respiratory 

 centre, and the other an increased activity of the expiratory part of 

 that centre. Stimulation of the first stops expiration and produces 

 inspiration ; stimulation of the second does the reverse. 



The question now is, What is it that normally produces this 

 alternate stimulation of the two sets of fibres ? If we discover this 

 we shall discover the prime moving cause in the alternation of the 



