The Nervous System. 277 



this part generally means instantaneous death. The whole 

 of the brain may gradually be removed without destroying 

 life, but the medulla will not tolerate such interference. 

 The centres found in the medulla are those for mastication, 

 swallowing, vomiting, respiration, coughing, movements of 

 the heart, bloodvessels, and iris, secretion of saliva, the 

 diabetic centre, and a centre for the sweat glands of the 

 head. 



The mastication and swallowing centre has for its afferent 

 nerves the inferior division of the fifth, glossopharyngeal, 

 and the superior laryngeal of the pneumogastric ; whilst its 

 motor branches are in the motor parts of the fifth for 

 swallowing, and in the facial for mastication. It would 

 appear that the reflex act of swallowing may be excited not 

 only by the presence of food in the pharynx, but even by 

 irritating portions of the respiratory apparatus; touching 

 the rima of the glottis excites the act, and so does touching 

 the interior of the trachea, even as low down as the bronchi. 

 A vomiting centre exists in the medulla, which in the horse 

 and ruminants is certainly most imperfectly developed. 

 We have previously, p. 137, drawn attention to the fact 

 that there is no drug which has the power of exciting 

 vomiting in the horse ; tartar emetic has not the slightest 

 action, and the effect of apomorphia is only to produce the 

 most alarming symptoms of cerebral excitement, but no 

 attempt at vomiting. Secretion of saliva is influenced by 

 the chorda tympani, which has its origin in the medulla 

 (see p. 105). A centre for dilating the pupil also exists, 

 the fibres for which are contained in the third nerve, and 

 through the sympathetic with the cilio-spinal centre in the 

 cervical cord. 



Other centres in the medulla depend upon automatic 

 action, for example, the respiratory and cardiac centres. 



The respiratory centre is situated close to the origin of 

 the pneumogastric nerves, and is divided into an expiratory 

 and inspiratory portion. The afferent nerve is chiefly the 

 pneumogastric ; but no doubt a large number of other nerves 

 may indirectly take part in the action, as is witnessed in 



