334 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



sphincter may be relaxed, the stomach cannot entirely empty itself. 

 This indicates that the intrinsic contractions of the muscular walls of 

 the stomach are of little importance in ejecting the contents of the 

 stomach. This was demonstrated by Majendie, who showed that vom- 

 iting might take place in an animal from whom the stomach had been 

 excised and a bladder substituted for it. When such an operation was 

 performed on a dog, and the bladder connected with the oesophagus and 

 small intestine inserted in the abdominal cavity 'and the wound in the 

 abdominal walls closed, injections of tartar emetic were perfectly capable 

 of producing vomiting, thus showing that the contractions of the 

 walls of the stomach are by no means essential to the act of vomiting. 



Schiff, however, found that if the cardiac sphincter was not removed, 

 or if the longitudinal fibres of the lower extremities of the oesophagus 

 were damaged, as by crushing, this experiment of Majendie would then 

 be ineffectual, thus showing that, while the contractions of the muscular 

 walls of the stomach are of no importance in the mechanism of vomiting, 

 the action of the longitudinal oesophageal fibres in overcoming the contrac- 

 tion of the cardiac sphincter is essential. This contraction of the longi- 

 tudinal fibres always precedes by a few seconds the act of vomiting, and 

 may be recognized by the insertion of a finger through a gastric fistula. 

 As a consequence of this opening of the sphincteric muscle, the pressure 

 within the stomach falls, as may be recognized by the connection of a 

 manometer with the interior of this organ. 



In the normal process of vomiting the contraction of these fibres 

 is enabled to open the oesophageal sphincter, as pointed out by Foster, 

 through the support which descent of the diaphragm has given to the 

 stomach : consequently, in the horse the impossibility of the stomach 

 being so supported by the diaphragm will largely explain the difficulty 

 of vomiting in these animals ; for the longer the portion of gullet be- 

 tween the diaphragm and the stomach, the greater will be the effect of 

 the radiating fibres in pulling down the oesophagus and the less their 

 capability of dilating its orifice. 



The nervous mechanism which governs this process of vomiting is 

 complicated, and, as is well known, it is a reflex action, and the afferent 

 impulses which excite this process may reach the vomiting centre in the 

 medulla oblongata through the most diverse paths. The vomiting centre 

 lies in the medulla close to the respiratory centre, and this connection is 

 probably partly functional as well as anatomical, since, as is well known, 

 nausea may to a certain extent be overcome by rapid and deep respira- 

 tions. Mechanical stimulation of the fauces, irritation of the stomach, 

 obstruction of the alimentary canal, may all serve as stimuli which in- 

 augurate the action of vomiting. 



Again, vomiting may take place by direct stimulation of the reflex 



