Human Physiology. 195 



and ox get all the matter of their bodies from grass; the lion 

 and tiger get the same elements at second-hand from the car- 

 cases they devour. 



The alimentary canal begins at the lips, and passes a length 

 of about thirty feet through the body. This great length is 

 required to give sufficient surface for the immense number of 

 glands, absorbents, excreting ducts, arteries, veins, nerves, &c., 

 which line the intestinal canal; and this surface is perhaps 

 quadrupled by the numerous folds of the intestines. The 

 functions of the alimentary canal are thus evidently of- great 

 importance. The extent of the alimentary canal is much 

 greater in man than in carnivorous animals. 



In the lowest forms of life the alimentary canal is very sim- 

 ple. There is a mere sack or tube. A polype can be turned 

 inside out, and seems to digest as well with what was his skin 

 as with what was his stomach. The alimentary canal of the 

 insect is considerably more complex, with 

 elaborate glandular apparatus. We shall see 

 that the alimentary system in man is compli- 

 cated and perfect in proportion to the dignity 

 and importance of the life it is made to nour- 

 ish. 



Within the lips we find the skin changed 

 to a soft mucous membrane, lined with cells 

 which are perpetually renewed, and kept 

 moist by the exudation of a clear, and slightly 

 slimy fluid. This membrane lines all por- 

 tions of the interior of the body which have 

 open communication with the external world. , 

 The interior cavities, as around the lungs and 

 intestines, are lined with serous membranes. Fig. 37. ALIMENT- 



Entering the mouth we first encounter the ARY CANAL OF AN 

 teeth, hard bone covered with a harder 

 enamel ; a set for childhood, and a larger and stronger set foi 



