DIGESTIVE ORGANS. 



513 



1. The mouth is the first cavity 

 of the digestive tube, and that into 

 which the food is immediately re- 

 ceived, and subjected to the action 

 of the organs of mastication and in- 

 salivation. Above and below, it is 

 circumscribed by the jaws, and la- 

 terally by the cheeks ; anteriorly 

 by the lips and their aperture, con- 

 stituting the mouth proper; and, 

 posteriorly, it communicates with 

 the next portion of the tube, the 

 pharynx. It is invested by a mu- 

 cous exhalant membrane, which is 

 largely supplied with follicles; and 

 into it the ducts from the different 

 salivary glands pour their secre- 

 tion. 



In all animals furnished with 

 distinct digestive organs, means 

 exist for comminuting the food, and 

 enabling the stomach to act with 

 greater facility upon it. These 

 consist, for the most part, as in 

 man, of the jaws, the teeth fixed 

 into the jaws, and muscles by which 

 the jaws are moved. 



The jaws chiefly determine the 

 shape and dimensions of the 

 mouth; the upper forming an es- 

 sential part of the face, and mov- 

 ing only with the head ; the lower, 

 on the contrary, possessing great 

 mobility. Each of the jaws has a 

 prominent edge, forming a semi- 



Fig. 210. 



Diagram of the Stomach and Intestines to show 

 their course. 



1. Stomach'. 2. (Esophagus. 3. Left, and 4. Right 



circle, in which the teeth are im- e . nd f stomach. 5, G. Duodenum. 7. convoiu- 



mi . 1 . .. - . tions of jejunum. 8. Those of ileum. 9. Csecum. 



ThlS edge IS Called, the 10. Vermiform appendix. 11. Ascending; 12. Trans- 

 verse; and -13. Descending colon. 14. Commence- 

 ment of sigmoid flexure. 15. Rectum. 



planted. 

 alveolar arch. 



The teeth are small organs, of a 

 density superior to bone; and covered externally by a hard substance 

 called enamel. By many, they have been regarded as bone; but they 

 differ from it in many essential respects, although they resemble it in 

 hardness and chemical composition. At another opportunity we shall 

 inquire into their origin, structure, and developement. We may merely 

 remark, at present, that by many they are looked upon as analogous 

 to the corneous substances, which develope themselves in the tissue of 

 the skin. De Blainville assimilates them to the hair; and believes, that 

 they are primarily developed in the substance of the membrane lining 

 the mouth ; and that their enclosure in the substance of the alveolar 

 arches of the jaws occurs subsequently. 

 VOL. I. 33 



