II. DIGESTION 

 I. Structure of Alimentary Canal 



THE anatomy and histology of the alimentary tract must 

 be studied practically. We shall here merely give such an 

 outline of the various structures as will assist in the com- 

 prehension of their physiology. 



The Alimentary Canal (Fig. 144) may be divided into the 

 mouth, the oesophagus or gullet, the stomach, the small and 

 large intestines, and three sets of supplementary structures 

 the salivary glands, the liver, and the pancreas. 



The Mouth, provided with its teeth, and surrounded by 

 its mobile muscular wall, with the muscular tongue lying in 

 its floor, is the part of the canal in which the food is broken 

 up and prepared for digestion. Into the mouth three pairs 

 of compound glands the Salivary Glands open. The 

 parotid, lined entirely by enzyme -secreting epithelium, 

 opens on the side of the cheek, while the submaxillary 

 gland, composed partly of acini with enzyme-secreting, and 

 partly of acini with mucin-secreting epithelium, and the sub- 

 lingual, composed entirely of mucin-secreting acini, open 

 under the tongue (S.C.). 



The tongue is covered with a fine fur of processes, the fili- 

 form papilla?, which are of use in passing the food backwards 

 along its surface in the act of swallowing. (For Organs of 

 Taste see p. 137.) 



Posteriorly, the mouth opens into the pharynx (Ph.) or 

 upper part of the gullet. On each side, between the mouth 

 and the pharynx, is the tonsil (T.), an almond-like mass of 

 lymphoid tissue. The pharynx is a cavity which can be shut 

 off above from the posterior nares by raising the soft palate, 

 and by pulling forward the posterior pharyngeal wall. It is 

 surrounded by three constrictor muscles, which, by contracting 



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