CHAPTEE Y. 



DIGESTION PREHENSION AND MASTICATION. 



General arrangement of the digestive apparatus Prehension of solids and 

 liquids Mastication Physiological anatomy of the organs of mastication 

 Enamel of the teeth Dentine Cement Pulp-cavity Arrangement of the 

 teeth Anatomy of the maxillary bones Temporo-maxillary articulation 

 Muscles of mastication Muscles which depress the lower jaw Action of the 

 muscles which elevate the lower jaw and move it laterally and antero-pos- 

 teriorly Action of the tongue, lips, and cheeks in mastication Summary of 

 the process of mastication. 



General Arrangement of the Digestive Apparatus. 



THE inorganic alimentary principles are, with few excep- 

 tions, introduced in the form in which they exist in the 

 blood, and require no preparation or change before they are 

 absorbed; but the organic nitrogenized principles are al- 

 ways united with more 'or less matter possessing no nutritive 

 properties, from which they must be separated ; and even 

 when pure they always undergo certain changes before they 

 become part of the great nutritive fluid. The non-nitro- 

 genized principles also undergo changes in constitution or in 

 form preparatory to absorption. With the varied forms in 

 which food is presented to different animals, we find great 

 differences in the arrangements of the digestive apparatus; 

 from the simple pouch with a single orifice, which constitutes 

 the entire digestive system of many of the infusorial animal- 

 cules, to the immense length of intestine, with its numerous 

 glandular appendages, found in the mammalia. In the 

 higher classes of animals, great differences exist in the 



