40 PHYSIOLOGY OF FARM ANIMALS [CH. 



chloric, but as already mentioned, it is a matter of great doubt 

 as to whether lactic acid is ever actually produced by the 

 stomach itself, the suggestion being that its presence is due to 

 the fermenting food. Hydrochloric acid is unquestionably pro- 

 duced by the gastric glands ; nevertheless lactic acid appears to 

 predominate in the horse's stomach during digestion. The 

 secretor}' part of the stomach produces also a quantity of mucin 

 which frequently forms a gelatinous coat over the surface. 



The pyloric aperture unlike the opening of the oesophagus is 

 usually wide, and beyond this the small intestine is dilated. The 

 passage of food out of the stomach is regulated by a U-shajied 

 curve (sometimes called the 'syphon-trap' of the duodenum). 

 When the large bowels become distended they press against this 

 portion of the duodenum and close the pyloric outlet from the 

 stomach, and if fermentation is taking place in the stomach, its 

 contents may be unable to escaj)e since the oesophageal opening 

 is also contracted as already described. In such cases rupture of 

 the stomach may supervene. 



The stomach of the pig as already mentioned represents a 

 transition stage between the simple stomach of the dog and the 

 complex stomach of ruminants. There is an oesophageal patch 

 without glands, which correponds to the proventriculus of the 

 horse, and according to some authorities the rest of the stomach 

 is divisible into three or four zones in which the number of glands 

 and the secretory activity are somewhat different. Ellenberger 

 and Hof meister have found that the swallowed saliva has a strong 

 amylolytic action on the carbohydrates in the pig's stomach and 

 that conversion of starch into dextrin and sugar takes place to 

 a marked extent, the sugar so formed becoming further converted 

 into lactic acid. The cardiac end of the stomach is utilised for 

 digesting starch while the pyloric half digests albumen, but the 

 latter process begins at a later stage than the starch digestion and 

 continues after it is over. 



Rumination. In the ox and sheep and all ruminating animals 

 the food after being swallowed for the'first time does not ordinarily 

 reach the true or digestive stomach until it has been first re- 

 gurgitated to the mouth where it undergoes a second and more 

 complete mastication. This process is called rumination or the 

 chewing of the cud. It involves a complicated system of 

 oesophageal enlargements which will now be described. 



