278 A Manual of Veterinary Physiology. 



the inspiratory efforts of the nostril and mouth when the 

 medulla is divided, or in the sudden gasping produced by 

 throwing cold water on the body. The chief efferent nerve 

 is the phrenic, the diaphragm being the principal muscle of 

 inspiration. The vagus has a twofold action on respiration, 

 being capable through certain fibres of increasing and 

 through others of decreasing respiration. It is believed 

 that impulses conveyed through the vagus to the medulla 

 excite the inspiratory portion of the centre, whilst others 

 passing into it through the superior laryngeal excite the 

 expiratory portion. This view of the matter must not be 

 too rigidly adopted. Another view is that during inspira- 

 tion dilatation of the lungs mechanically stimulates certain 

 nerves, which convey to the medulla through the vagus 

 expiratory stimulation, and conversely during expiration 

 mechanical stimulation of the inspiratory centre occurs. 

 This is termed the self-adjusting mechanism. 



An inhibitory influence is exercised over the respiratory 

 centre, through influences conveyed to it by either the 

 superior or inferior laryngeal or both, for stimulation of 

 these divided nerves, from that end still in contact with the 

 brain, produces a considerable fall in the number of respira- 

 tions, and even arrest of them during expiration ; on the other 

 hand, stimulation of the cut end of the vagus (from that end 

 still in contact with the brain) produces rapid inspiratory 

 efforts, and therefore may possibly inhibit the expiratory 

 centre. The respiratory centre is stimulated by blood contain- 

 ing a reduced proportion of oxygen, and it is this which largely 

 influences the automatic action of the medulla (see p. 91). 



Division of the cord above the origin of the phrenic, it 

 is said, produces death by suffocation, though judging from 

 experiments on the division of the phrenic in the horse, the 

 suffocation is not produced by paralysis of the diaphragm 

 alone, but by paralysis of diaphragm and chest walls (see 

 p. 92); fracture of any vertebra above the phrenic does 

 not necessarily mean immediate' death, unless the cord be 

 extensively damaged. 



Above the respiratory centre is the coughing centre. 



