462 



PHYSIOLOGY 



CHAP. 



of the stimuli, so that the action now of the inspiratory, now of the 

 expiratory fibres preponderates. 



As early as 1847 it was remarked by Traube, and later on by 

 Kosenthal, that electrical excitation of the central end of a 

 vagus cut in the cervical region caused excitation of the inspiratory 

 centres, as shown by acceleration of rhythm, and with stronger 

 stimuli by an inspiratory tetanus (Fig. 213). This proves that 

 the vagus trunk contains centripetal fibres which on excitation 

 act on the inspiratory centres by acceleration of their rhythmical 



Fio. 213. Inspiratory effects of electrical excitation of central trunk of vagus in rabbit (Fredericq). 

 The period of excitation is marked on the abscissa. The tracing shows the respiratory 

 oscillations of pulmonary pressure. 



impulses. This result, however, is not constant ; it is only 

 necessary to alter the strength of the exciting current in order to 

 produce a diametrically opposite effect, i.e. slowing of rhythm, 

 with preponderating expirations, and also expiratory tetanus. 

 This contrary effect proves that the vagus contains other afferent 

 fibres which act on the expiratory centres. If these are rarely 

 manifested with electrical excitation of the central end of the 



FIG. 214. Expiratory effects of electrical excitation of central trunk of vagus in chloralised 

 rabbit (Fredericq). The period of excitation is marked on the abscissa. Each stimulation is 

 followed by a respiratory arrest. 



vagus, it is because the antagonistically working fibres pre- 

 ponderate. Chemical excitation of the central end of the vagus, 

 however, causes reflexes of a predominating expiratory nature 

 (Gad). When the animal is poisoned with strong doses of chloral 

 hydrate, which, as we have seen, weakens the activity of the 

 inspiratory centres, the centripetal expiratory fibres of the vagus 

 come into play. For under these conditions electrical stimulation 

 of the central end of the vagus nerve is invariably followed by an 

 expiratory tetanus, as appears from the researches of L. Fredericq 

 and Wagner (Fig. 214). 



