VOMITING. 335 



centre in the medulla oblongata. In this way certain emetics, such as 

 tartar emetic, probably act, for they may produce vomiting even when 

 injected into the blood, and without reaching the stomach at all. 



Again, vomiting may be produced by sensations coming from the 

 central nervous system higher up than the medulla ; thus, offensive smells 

 or tastes or disturbed cerebral circulation, as in sea-sickness, may pro- 

 duce vomiting. The efferent path of this action seems to lie mainly in 

 the pneumogastric nerve, for when this nerve is divided the cardiac 

 sphincter remains tightly closed, and vomiting is then impossible. Sec- 

 tion, therefore, of these nerves will, as a rule, prevent vomiting. The 

 efferent paths of the nervous impulses passing to the muscles of vomit- 

 ing are, of course, through the motor nerves supplying the gullet, the 

 larynx, and abdominal muscles. 



In the horse, as is well known, vomiting occurs only under very ex- 

 ceptional circumstances. The explanation of this fact, as pointed out by 

 Colin, is to be found in the anatomical relations and physical conforma- 

 tion of the stomach. 



In the first place, in the horse the stomach is never in contact with 

 the abdominal muscles; hence, it is not readily subjected to pressure 

 when the abdominal muscles contract. Again, as already mentioned, the 

 portion of the oesophagus between the diaphragm and the stomach is 

 longer than in carnivorous animals, and, as the stomach cannot be sup- 

 ported by close contact with the diaphragm, the longitudinal fibres are 

 unable to overcome the permanent contraction of the cardiac sphincter. 



In carnivorous animals which readily vomit the oesophageal orifice 

 is at the left extremity, far from the pylorus. An antiperistaltic action 

 tends to force food into the opening of the oesophagus. The gullet has 

 dilatable walls and an infundibular insertion into the stomach and 

 marked radiating fibres. The pylorus is narrow and nearly always 

 closed, while the stomach is large and directly in contact with the 

 diaphragm and abdominal walls, and is thus in the best possible condition 

 for being subjected to pressure through the contraction of these muscles, 

 while the oesophageal fibres are supported through the contractions of 

 the diaphragm. 



In the horse, on the other hand, or the hare or rabbit, the oesophageal 

 orifice is in the middle of the lesser curvature of the stomach and near 

 to the pylorus. Its orifice is always closed by a powerful sphincter 

 muscle. It passes obliquely through the walls of the stomach, and is 

 further obstructed by folds of mucous membrane, and the pylorus is 

 large and nearly always patulous (Fig. 144). 



Subjection of the stomach to pressure in the horse will, therefore, 

 still more tightly close the oesophageal orifice, and will force the contents 

 of the stomach into the small intestine. Occasionally, however, vomiting 



