NEW ELEMENTS 



OF 



PHYSIOLOGY 



CHAPTER I. 



OF DIGESTION. 



I. Digestion is a function common to all animals, by which substances 

 extraneous to them, are introduced into their body, and subjected to the 

 action of a peculiar system of organs, their qualities altered, and a new 

 compound formed, fitted to their nourishment and growth. 



II. General considerations on the Digestive organs. Animals alone are 

 provided with organs of digestion ; all of them, from man down to the 

 polypus, contain an alimentary cavity, variously shaped. The existence 

 of a digestive apparatus may, therefore, be taken as the essential charac- 

 teristic of the animal kind. In man, this apparatus consists of a long tube 

 extending from the mouth to the anus ; within this canal, there empty 

 themselves the excretory ducts of several neighbouring glands, that secrete 

 fluids fit for changing, for liquefying, and animalizing the alimentary sub- 

 stance. The different parts of this digestive tube are not of equal capaci- 

 ty; at first, enlarged in the part whjch forms the mouth and pharynx, it 

 becomes narrower in the oesophagus; this last, dilating considerably, 

 forms the stomach, which again contracting, is continued down under the 

 name of intestine. The tube itself varies in size in different parts of its 

 extent: and it is by the consideration of these differences of size, that 

 anatomists have principally been guided in their divisions. 



The length of the digestive tube is from five to six times the length of 

 the whole body, in an adult: it is greater in proportion in a child. At 

 this age, likewise, digestion is more active, and proportioned to the ne- 

 cessities of growth in the individual. The digestive cavity is in man open 

 <it both extremities ; in some animals, in the zoophyte for example, one 

 opening serves the purpose of mouth and of anus, receives the food, and 

 ejects the excrementitious remains. 



The extent of the digestive canal is according to the nature of the ali- 

 ments on which the animals feed : the less those aliments are analogous 

 in their nature to the substance of the animal they are to nourish, 

 the longer must they remain in his body to undergo the necessary changes. 

 Therefore, it is observed, that the intestine of graminivorous animals is 

 very long, their stomach very capacious, and often complex, while carni- 

 vorous animals have their intestinal canal short and strait, and so arranged, 



