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THE MAINTENANCE OF THE INDIVIDUAL 



Circular folds or 

 plicae circulares in 

 the intestine of 

 man. These occur 

 from the duodenum 

 to the anus. 



Other devices such as throwing the surface into transverse ridges 

 are quite common, for example, in man they occur in the intestine 

 and colon as plicae circulares. 



Parts of the Digestive System 



The Oral Cavity. The various mouth cavities 

 of vertebrates are all developed for one fundamental 

 purpose, namely, the ingestion of food. The 

 mouth cavity is specialized in many different ways 

 and is further complicated in air-breathing forms 

 by the necessity for completely separating the air- 

 intake apparatus from the digestive tract. This 

 is accomplished quite readily in water-inhabiting 

 species through the use of gill-slits. In land forms, 

 however, the external nares (nostrils) and associated 

 nasal passages are dorsal and the lungs ventral to 

 the opening of the digestive tract. It is neces- 

 sary therefore to arrange in the pharyngeal region 

 for the crossing of the air passageways over the 

 food tube. This separation is facilitated in most 

 forms by the presence of a hard bony plate known as the hard palate 

 that lines the roof of the mouth. At the posterior end of the hard 

 palate is attached a flap of soft tissue, the soft palate, which further 

 expedites the separation of respiratory and digestive tracts. 



The oral cavity is lined throughout by a mucous membrarie the cells 

 of which secrete mucus that serves as a lubricant facilitating the 

 passage of food. This same tissue is found throughout the entire 

 surface of the food tube. In various parts of the alimentary canal are 

 found openings of various glands which add their digestive ferments 

 to the mucus. These glands will be considered in detail under the 

 digestive processes of man. 



Usually the surface of the palate, especially the posterior part, 

 known as the soft palate and uvula, contains numerous mucus-secreting 

 glands called the palatine glands. The secretions of these glands help 

 to keep the cavity of the mouth moist. In many animals, especially 

 carnivores, there appear a number of washboardlike palatine ridges 

 that appear to be an adaptation to enable its owner to secure a surer 

 grip upon the unfortimate victim that has been seized in its jaws. 

 The large, bulky tongue, which occupies practically the entire floor of 

 the buccal cavity, likewise plays an important role in eating. 



