ACTION OF THE VAGUS ON RESPIRATION 293 



been looked upon, and rightly, as the most important for the carrying 

 out of normal respiratory movements, since they must convey, during 

 the whole of life, impulses which affect and regulate these movements. 

 We have, therefore, to discuss the nature of the influence exerted by the 

 vagi on the respiratory centre and the mode in which these impulses are 

 initiated. 



As in all other cases, where we wish to study the functions of a 

 nerve trunk or nerve centre, our experiments can be divided into two 

 main classes : Those in which the normal function is abolished by 

 section or extirpation, and those in which this function is initiated or 

 exaggerated by artificial stimulation. 



The earlier experiments of this nature were inconclusive, conflicting 

 results having been obtained by different observers. It is to Rosen- 

 thal x that we owe the first accurate investigation of the subject, with a 

 clearly defined hypothesis of the normal working of the vagus and 

 respiratory centre founded on experiment. This observer showed in 

 the first place that the constant effect of section of both vagi was to 

 make the respiratory movements deeper and less frequent. The amount 

 of air taken in during a given time was, however, unaltered, the increase 

 in depth of the respirations exactly counteracting the slower rhythm. 

 Before Rosenthal's time, the effect of electrical excitation of the central 

 end of one vagus was found to produce sometimes expiration, sometimes 

 inspiration. Rosenthal showed that the trunk of the vagus contains 

 two definite sets of fibres. Excitation of one set produces reflex con- 

 traction of the inspiratory muscles, especially of the diaphragm, while 

 stimulation of the second set produces relaxation of the diaphragm, and 

 might therefore be looked upon as inhibitory of inspiration. The first 

 set of fibres is, according to Rosenthal, distributed solely to the lungs, 

 so that inspiration or a tetanic spasm of the diaphragm is an invariable 

 sequence of excitation of the nerve in the lower part of the neck. If 

 the exciting current be weak, the result may be not inspiratory spasm, 

 but simply a quickening of the respiratory rhythm to that which 

 obtained before the section of the vagi. On the other hand, a purely 

 expiratory effect is produced by stimulating the central end of the 

 superior laryngeal nerve, and Rosenthal ascribed the occasional ex- 

 piratory effects, observed by his predecessors on stimulation of the 

 vagus in the lower part of the neck, to an escape of current to the 

 superior laryngeal nerves. As a result of his investigations, Rosenthal 

 founded a theory of the normal working of the respiratory centre in 

 the medulla oblongata, based to some extent on a hypothesis conceived 

 about twelve years earlier by Ludwig and Hoffa' 2 to account for the 

 action of the vagus on the heart. These observers, on recording the 

 contractions of the mammalian heart by a lever resting on the surface 

 of the exposed ventricle, found that, during stimulation of the vagus, 

 the heart beats, though less frequent, appeared much more powerful, and 

 they concluded that the total amount of work performed in any given 

 time by the heart muscle was the same under all circumstances, the 

 only result of the vagus stimulation being to increase the resistance to 

 the motor discharge, so that the tendency to discharge had to rise to a 



1 "Die Athembewegungen, u. ihre Beziehung zum Nervus vagus," Berlin, 1865. See 

 also the section on the respiratory movements by the same author in Hermann's ' ' Hand- 

 buch," Bd. iv. (2) S. 165. 



2 Ztschr.f. rat. Med., 1850, Bd. ix. S. 129. 



