252 THE DIGESTIVE SYSTEM 



stomach is present, a "focus of irritation" (p. 195) may be 

 established in the medulla oblongata as well as in the spinal 

 medulla. On the ingestion of food afferent impulses ascend to 

 the medulla oblongata, and the resulting response probably 

 governs the gastric peristalsis. If a " focus of irritation " is 

 present in the medulla oblongata, these normal afferent stimuli 

 become exaggerated as they ascend to the cortex and they 

 cause an exaggerated response, e.g., emesis. It may be noted 

 that such a focus may be caused by a lesion in any part of the 

 stomach, and that, on this account, the rapidity with which 

 emesis follows the ingestion of food merely indicates the 

 presence of a "focus of irritation " and is no guide to the site 

 of the lesion. 



Abnormal afferent impulses from the gastric branches of the 

 vagus may "overflow" in the medulla obiongata and affect 

 the neighbouring nerve-cells. This "overflow" stimulus may 

 affect the cells which exert a depressor influence on the heart, 

 and in this way, without any cardiac lesion, bradycardia may 

 be associated with lesions of the stomach. Similarly, the 

 irritable, uncontrollable cough which sometimes accompanies 

 gastric disturbances is caused by the exaggeration of normal 

 afferent impulses from the larynx, as they pass through a 

 "focus of irritation" in the medulla oblongata. 



At the same time it must be remembered that, just as gastric 

 lesions may give rise to disturbances in other viscera either by 

 " overflow " of impulses in the medulla oblongata or by the 

 establishment of a "focus of irritation" in the spinal medulla, 

 so the stomach may be affected reflexly in lesions of other 

 viscera innervated by the vagi. Thus, stimulation of the 

 auricular branch of the vagus in the external acoustic meatus 

 may give rise to symptoms of serious gastric disorder (p. 96) ; 

 a severe fit of coughing may culminate in vomiting; affections 

 of the biliary passages may lead to the vomiting of food as 

 soon as it is ingested. In the latter case, however, it is doubt- 

 ful whether we have to deal with a pure vagus reflex or 

 with a sympathetic reflex. 



