CHAR L] TISSUES AND MECHANISMS OF DIGESTION. 403 



toneum. This consists of a single layer of polygonal flattened 

 nucleated epithelioid cells (belonging in reality as we shall see to 

 the lymphatic system) resting on a thin connective tissue basis 

 which separates them from the longitudinal muscular coat. 



The general plan of structure of the alimentary canal then, in 

 its hypoblastic portion, is a compact muscular coat separated by a 

 loose more or less moveable submucous coat from a fairly compact 

 mucous coat. The mucous coat consists of a vascular connective 

 tissue basis, in which is embedded a thin special muscular sheet, 

 and of a single layer of special hypoblastic epithelial cells. The 

 muscular coat consists of a thick inner circular and a thin outer 

 longitudinal layer of plain muscular fibres; and the whole is covered 

 with an epithelioid peritoneal layer. 



209. Glands. The surface of the mucous membrane however 

 is not even and unbroken. It dips down at intervals, that is to say 

 it is involuted to form pockets or depressions sunk into the under- 

 lying connective tissue and differing in size and form in different 

 parts of the alimentary canal. Such an involution is called a gland. 

 The most simple kind of gland is a cylindrical depression with a 

 blind end, somewhat of the form of a test-tube, lined with a single 

 layer of epithelium cells, continuous at the mouth of the gland 

 with the rest of the epithelium of the mucous membrane. The 

 wall of the gland outside the epithelium is supplied by the 

 connective tissue of the mucous membrane, which generally forms 

 a distinct basement membrane, and is generally also richly supplied 

 with capillary blood vessels. Hence when two such glands lie side 

 by side, a certain quantity of connective tissue carrying blood 

 vessels runs up between them to reach the epithelial cells which 

 cover the surface of the mucous membrane between their mouths. 

 Such a simple tubular gland may have the same diameter through- 

 out, or may vary in diameter at different distances from the mouth, 

 and the epithelium lining it may be of the same character throughout 

 and similar to that on the surfaces between the mouths of the 

 glands; very frequently however at the lower part of the gland the 

 epithelium is modified and takes on certain special characters 

 which we shall speak of presently as those of a ' secreting ' 

 epithelium. When this occurs the upper part of the gland, where 

 the epithelium is not so modified, is often spoken of as ' the duct ' 

 of the gland. 



Very frequently the gland is not simple but branched, and the 

 branching may be slight or excessive. Such branched glands, 

 especially those in which the branching is considerable, are called 

 compound glands ; and in these there is always a very marked 

 distinction between the terminal portions of the several branchings 

 where the epithelial cells have secreting characters, and the proximal 

 portions or ducts where the cells have not these secreting characters. 

 In such a compound gland a tubular main duct (whose mouth opens 

 into the interior of the alimentary canal, and whose epithelial lining 



