520 SPECIAL PHYSIOLOGY. 



motor vomiting. Emotional causes may likewise excite this act. Emetic 

 medicines, which operate just as readily when injected into the veins, 

 as when introduced into the stomach, probably act directly on the re- 

 flex nervous centres concerned in vomiting; but they may -operate on 

 the extremities of the afferent nerves of the stomach. These are the 

 pneumogastric nerves, irritation of which causes, amongst other results, 

 contraction of the muscles of the abdomen, and vomiting. In the 

 vomiting named cerebral vomiting, which occurs after concussion of 

 the brain, and in certain diseases of that organ, the cause of irritation 

 is central. In some individuals, vomiting can be performed voluntarily, 

 this power being either natural, or else acquired by practice. 



It is said that the act of vomiting but seldom occurs in the horse ; and it 

 has been attempted to explain this, by reference to the structure of the cardiac 

 end of the stomach ; but it would seem rather to be due to the very slight 

 susceptibility of that animal to the action of emetic medicines. 



THE BIGESTIVE FLUIDS. 



The chemical processes concerned in the function of digestion, con- 

 sist of peculiar reactions between the food and the various secretions 

 of the alimentary canal. 



The digestive fluids, which are added to, and act chemically on, the 

 food in its progress through the alimentary canal, are as follow : first, 

 the fluids of the mouth, consisting of the mucus secreted by the mu- 

 cous membrane and glands of that cavitity, and the saliva, the product 

 of the three pairs of salivary glands, named the parotid, submaxillary, 

 and sublingual glands ; secondly, the secretion of the stomach, named 

 the gastric juice, formed by minute gastric glands, or follicles, embedded 

 in the mucous membrane of that organ ; thirdly, the bile secreted by the 

 liver, and poured into the duodenum ; fourthly, the pancreatic juice se- 

 creted by the pancreas, and also added to the food in the duodenum ; 

 and lastly, the mucus and the intestinal juices, secreted by the mucous 

 ;glands and by the so-called tubuli, which exist in vast numbers in the 

 mucous membrane of every part of the small and large intestines. 

 Each of these fluids exercises a special transmutation on one or more 

 of the proximate constituents of the food, the tendency of such 

 changes being to convert those constituents, from an insoluble and 

 unabsorbable condition, into a state of solution, or into a state in which 

 they can be absorbed, that being the ultimate object of the digestive 

 process, 



Sources and Composition of the Buccal Mucus and Saliva. 



The mucous glands of the mouth are named, according to their posi- 

 tion, labial, buccal, molar, palatal, and lingual. These are chiefly 

 compound racemose glands, forming rounded masses beneath the mu- 

 cous membrane, and opening into the mouth by their proper ducts. 

 At the base of the tongue are a few simple follicles, and some fol- 

 licular depressions, having little closed sacs in their walls, like the 

 follicles of the tonsils. The tonsils themselves probably also furnish 

 some mucous secretion. Beyond the mouth, the pharynx possesses 



