204 A MANUAL OF VETERINARY PHYSIOLOGY 



It is important to notice in connection with the subject of 

 vomiting that agents such as tartar emetic, ipecacuanha, and 

 apomorphia, which excite vomiting by their action on the cere- 

 bral centre, have no effect on the horse or ruminants, nor does 

 the horse vomit as the result of sea-sickness, though he suffers 

 extremely from it. Why he should vomit more often with a 

 ruptured stomach than a sound one is a fact we cannot explain. 



In those animals where vomiting is a natural process, the three 

 important factors are — The dilatation of the cardia by active 

 contraction of the longitudinal fibres of the oesophagus, pressure 

 on the walls of the stomach by a contraction of the diaphragm 

 and abdominal muscles, and closure of the pylorus. But there 

 is some evidence to show that the stomach itself is not passive ; 

 it is true Majendie produced vomiting after he had replaced the 

 stomach by a bladder, but under normal conditions there appears 

 no reason why the stomach wall should remain quiescent, and 

 in the cat it has been observed that during vomiting a strong 

 contraction of the pyloric end of the stomach occurred, shutting 

 it off from the cardiac portion. We may here have one explana- 

 tion of ruptured stomach in the horse, which supports the view 

 put forward on p. 176 of the influence of the duodenal trap. 



Rumination. — The physiology of rumination has been princi- 

 pally worked out in France by Flourens and Colin, and our 

 knowledge of this singular process is based almost entirely on 

 their observations. 



The oesophagus in ruminants has its inner layer of fibres 

 spirally arranged in double obliquity; the tube is wider at its 

 termination than in the part which precedes it. On entering 

 the gastric reservoirs, it forms a groove, previously described 

 (p. 194), which brings it into connection with all the sacs com- 

 prising the stomach. In this way material coming down the 

 oesophagus may enter either or any of the reservoirs, the choice 

 being determined by the condition in which it is swallowed. 



The lips or pillars of the oesophageal groove are composed of 

 involuntary muscular fibres, arranged longitudinally and trans- 

 versely (Fig. 71), by which means the groove can be shortened 

 and constricted. By a contraction of the pillars, the omasum 

 may be drawn forward and brought nearly in apposition with 

 the oesophagus. By relaxation of the pillars the oesophagus is 

 made to communicate with either the first or second reservoir. 

 Fig. 72 shows the rumen and reticulum in position. The 

 oesophageal groove is represented as open, though normally its 

 lips cover it so perfectly that it requires looking for. The close 

 proximity of the reticulum to the heart will be observed, and in 

 this connection see the remarks on p. 197. At one time it 



