CHAPTER III. 



DIGESTION. 



Anatomy and Structure of the Mouth, Pharynx, and (Esophagus, 

 together with the Digestive Processes Occurring in Them. 



DIGESTION has for its aim the separating of the principles of 

 growth and repair from the aliments and the fitting of them for 

 absorption into the circulation. The process is both mechanical and 

 chemical, accomplished mainly through the action of certain soluble 

 ferments called digestive enzymes. 



Some form of digestion is found to take place in all animal organ- 

 isms no matter how low we proceed in the zoological scale. It is 

 essential to every one of them that they be able to take from their 

 environments those elements that are necessary to maintain their 

 economy and to give off those substances termed waste-products that 

 are no longer fit for use, for only by this exchange of the elements 

 outside of their own organisms are they able to live, grow, and pro- 

 duce others of their kind. 



In the higher grades of animal life, as the articulata and verte- 

 brata, the number of organs concerned in digestion is increased, and, 

 of course, in direct ratio the various stages and acts in the whole 

 process are multiplied. In their bodies it is a long tube, in some parts 

 much folded on itself ; in and along the outside of this tube there are 

 numerous glands which empty their products, called secretions, into 

 the long tube; at the beginning of which there is an apparatus for 

 crushing and grinding the solid parts of the food. Intimately con- 

 nected with this apparatus is the system of blood- and chyle- vessels 

 for absorbing the digested products, thus allowing them to circulate 

 through the entire body and come into contact with every part of the 

 organism. 



In the vertebrata there are modifications and forms of develop- 

 ment dependent upon the class, and even in mammalia there are 

 differences, as the animal may be insectivorous, carnivorous, herbivor- 

 ous, or omnivorous. 



Man, the highest of the mammalia, is the real and intimate study 

 upon which all our physiological researches bear. He is omnivorous, 

 (46) 



