514: THE TISSUES. 



CHAPTER XIII. 



STRUCTURE OF THE ALIMENTARY CANAL AND ITS 

 APPENDAGES. 



THE alimentary canal is a tube commencing with the mouth, or 

 oral cavity, and ending at the anus. It is divided into 1st, the 

 supra-diaphragmatic portion, consisting of the oral cavity, the pha- 

 rynx, and the oesophagus ; and, 2c%, the infra-diaphragmatic por- 

 tion, including the stomach, small intestine, and large intestine. 

 Underneath the mucous membrane is, in most parts, a layer of sub- 

 mucous areolar tissue; and externally to this, two layers of smooth 

 muscular fibres; except in the oral cavity, the pharynx, and the 

 upper portion of the oesophagus, where striated fibres are found 

 instead of the former. The external layers (areolar and muscular) 

 present no peculiarities of structure, except such as will be men- 

 tioned in connection with that of the mucous membrane of the 

 several portions just mentioned. But the latter presents marked 

 variations in its different portions, and will be particularly described. 

 The appendages to be described in this chapter are the liver and the 

 pancreas. The spleen will be described in the chapter upon the 

 blood-vascular glands. 



1. Mucous Membrane of the Oral Cavity. 



The epithelium of the oral mucous membrane is of the com- 

 pound scaly variety, and is continuous with the cuticle and stratum 

 Malpighii of the skin at the margins of the lips. It is not, how- 

 ever, distinctly divisible into the two layers just mentioned; but 

 resembles the stratum Malpighii of the skin, with the undermost 

 laminae of the cuticle. It undergoes certain modifications upon the 

 papilla of the tongue, hereafter to be described. Unlike the cuticle, 

 the oral epithelium is easily permeated by fluids; and also from 

 within outwards, byplasma passing from the vessels of the mucous 

 membrane into the mouth. 



The corium of the oral mucous membrane, though continuous 





