198 HANDBOOK OF PHYSIOLOGY. 



respiratory centre, although automatic in its action, may, however, be 

 reflexly excited. The chief channel of this reflex influence is the vagus 

 nerve, for when the nerve of one side is divided, respiration is slowed, 

 and if both vagi are cut it becomes still slower. 



The influence of the vagus trunk upon the centre may be twofold, 

 for if the nerve is divided below the origin of the superior laryngeal 

 branch and the central end is stimulated, respiratory movements are in- 

 creased in rapidity, and indeed follow one another so quickly if the 

 stimuli be increased in number, that after a time cessation of respiration 

 in inspiration takes place in consequence of a tetanus of the respiratory 

 muscles (diaphragm). Whereas if the superior laryngeal branch is di- 

 vided, although no effect, or scarcely any, follows the mere division, on 

 stimulation of the central end respiration is slowed, and after a time, if 

 the stimulus is sufficiently increased, stops, not in inspiration as in the 

 other case, but in expiration. Thus the vagus trunk contains fibres 

 which are capable of slowing and fibres which are capable of accelerating 

 respiration. The theory that the respiratory centre in the floor of the 

 medulla consists of two parts, one of which tends to produce inspiration 

 and the other to produce expiration, is very plausible. The inspiratory 

 part of the centre is complementary to the expiratory, and the two parts: 

 send out impulses alternately. If we adopt this th^y, we must look 

 upon the main trunk of the vagus as aiding the inlpjj^ory, and upon the 

 superior laryngeal as aiding the expiratory part of the centre, the first 

 nerve possibly inhibiting the action of the expiratory centre, whilst it 

 aids the inspiratory, and the latter nerve having the very opposite effect. 

 But inasmuch as the respiration is slowed on division of the vagi, and 

 not quickened or manifestly aifected at all on simple division of the su- 

 perior laryngeal, it must be supposed that the vagi fibres are always in 

 action, but that the superior laryngeal fibres are not. 



It appears that there are, in some animals at all events, subordinate 

 centres in the spinal cord which are able, under certain conditions, to 

 discharge the function of the chief respiratory centre in the medulla. 



The centre in the medulla may be influenced not only by afferent im- 

 pulses proceeding along the vagus and laryngeal nerves but also by im- 

 pulses passing downward from the cerebrum ; by impressions made upon 

 the nerves of the skin, or upon part of the fifth nerve distributed to the 

 nasal mucous membrane ; or upon other sensory nerves. Such afferent 

 influences are exemplified in the deep inspiration excited by the applica- 

 tion of cold to the surface of the skin, and by the production of sneezing 

 on the slightest irritation of the nasal mucous membrane. 



At the time of birth, the separation of the placenta, and the conse- 

 quent non-oxygenation of the foetal blood, are the circumstances which 

 immediately lead to the issue of automatic impulses from the respiratory 

 centre in the medulla oblongata. 



