324 EXPERIMENTAL INVESTIGATION OF THE ACTION OF MEDICINE3. 



Although the excitement of the respiratory centre and the 

 amount of work done by the respiratory muscles depend on 

 the venosity of the blood in the medulla oblongata, yet this 

 work may be differently distributed by the respirations becom- 

 ing quicker but shallower, or slower but deeper, without the 

 quantity of air respired being at all altered. This is effected 

 by the action of various afferent nerves, of which the chief are 

 the vagus, the superior laryngeal, and the nasal nerves ; though 

 others, such as the cutaneous nerves generally, have consider- 

 able influence. 



Irritation of the vagus, or of its central end when divided, 

 lessens the resistance in the respiratory centre, and quickens 

 the respiration, but makes it shallower. Stronger irritation 

 causes prolongation of inspiration. A very strong excitation 

 annihilates the resistance in the centre, and causes inspiration 

 to be almost indefinitely prolonged, so that the breathing is 

 completely arrested. 



The ends of the vagi in the lung are normally in a state of 

 constant excitation, and therefore division of these nerves 

 renders the respiration slow. 



Irritation of the vagi, on the contrary, causes, as we have 

 said, considerable acceleration of the respiration; and the 

 quick breathing which we observe in pneumonia is probably 

 due to the irritation of the pulmonary branches of the vagus 

 which the inflammation produces. It can hardly be caused by 

 the venous condition of the blood alone, nor yet by increased 

 temperature ; for the blood may be very much more venous in 

 bad bronchitis, and the temperature higher in fever, without 

 the respiration becoming anything like so rapid as in a case of 

 pneumonia. But, although I thus speak of the vagus as an 

 inspiratory or accelerating nerv^e alone, I do this only for the 

 sake of simplicity, as this is its chief function. It really con- 

 tains, however, both inspiratory and expiratory fibres, although 

 the former predominate. (Hering and Breuer, Wiener Acad. 

 Sitzungs-her. Math.-Naturwiss. CL, vol. Ivii, Ab. 2, page 672.) 

 The inspiratory fibres are excited by collapse, and the expira- 

 tory fibres by distension, of the lung. Thus, these nerves form 

 a sort of regulating mechanism for the respiratoiy movements. 



