DIGESTION OP FOOD. 331 



favorable to rumination, extreme distention tends to paralysis 

 of the muscular coat of the organ, allowing of the accumulation 

 of the gases of fermentation which may lead, if not artificially 

 relieved; to rupture of the organ. 



THE ALIMENTARY CANAL OF THZ! VERTEBRATE. 



Amid all variations in this great group, the alimentary canal 

 has common features, both of structure and function. Through- 

 out the entire tract muscle cells of the unstriped (involuntary) 

 kind, arranged in two layers, constitute the motor mechanism 

 for the transportation of food from one part to another. Out- 

 side of these is the serous coat, consisting of fibrous and elastic 

 tissue, and admirably adapted to preserve organs from undue 

 distention, at the same time providing a smooth external cover- 

 ing which lessens the friction of one organ against another in 

 the abdominal cavity ; while folds of such tissue constitute the 

 omentum for supporting the various organs. 



Between the muscular and mucous coats of the organs that 

 constitute the alimentary canal there is .a submucous coat of 

 loose connective tissue in which ramify blood-vessels, nerves, etc. 



It is the mucous coat, however, that is of paramount impor- 

 tance, and for which all other parts may in some sense be con- 

 sidered to exist ; for it is from the glands with which it is sup- 

 plied that the digestive juices are derived, as well as that mucus 

 which keeps the tract moist and its delicate structures shielded 

 under all circumstances. The amount of surface provided by 

 the mucous membrane is increased by its various foldings 

 (rugce, valvulce conniventes, etc.), so generally present, and 

 which also allow of distention ; and if the secreting glands 

 are regarded as minute induplications of this coat, it will 

 be evident that its total area is much greater than at first ap- 

 pears. 



While each part has glands with structure pecuhar to them- 

 selves, it may be noticed that all the essential epithelium has a 

 tendency to assume a somewhat cubical form. 



The secreting glands of the stomach and intestines are tubu- 

 lar ; while the salivary glands, the pancreas, and the liver are 

 masses of cells so packed together as to form great colonies of 

 cells with lesser subdivisions (lobules), the whole being bound 

 together by some form of connective tissue, and well supplied 

 with blood-vessels and nerves, thus constituting organs with a 



