CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 413 



tissue basis, has frequently a longitudinal striation as if made up 

 of a number of rods or narrow prisms placed side by side. 



The larger ducts running between the lobules differ from such 

 a small intralobular duct chiefly in the greater thickness of the 

 connective tissue basis, which in these is developed into a distinct 

 coat containing in the case of the larger branches and the main 

 duct plain muscular fibres. In the main duct and its chief branches 

 the single layer of columnar cells is replaced by two or three 

 layers of cubical, or sometimes flattened cells not marked with 

 the striation spoken of above. When a small intralobular duct 

 is about to end in an alveolus or a group of alveoli it becomes 

 narrowed, the cells lose their striation, from being slender and 

 cylindrical in form become short cubical, and at the very end of 

 the duct change into flat spindle-shaped plates, the transition 

 from which to the characteristic cells of the alveolus is in the 

 case of most animals quite abrupt. Such a modified terminal 

 portion of a duct is sometimes spoken of as a " ductule." 



218. Albuminous glands. These differ from the mucous 

 glands in the constitution of the cells lining the alveoli, but the 

 structure of the ducts and the general arrangements of the gland 

 are the same in both; indeed, as we have already said, in the 

 same gland some alveoli may be albuminous and others mucous. 



In an albuminous alveolus the cells are rather smaller than 

 those in a loaded mucous gland, and their outlines are rather 

 more angular. In each cell the nucleus, which is spherical, is 

 placed at about the centre of the cell but rather nearer the base- 

 ment membrane, and the cell-substance, which has, in an ordinary 

 preparation, a somewhat densely granular protoplasmic appearance, 

 stains readily and uniformly all over. In fact an albuminous cell 

 does not at first sight appear to differ markedly from a discharged 

 mucous cell, and does not shew the same marked differences 

 between a loaded and a discharged condition as does a mucous cell. 

 There are however differences between the loaded and the dis- 

 charged albuminous cell, but to these we shall return presently. 



The parotid gland of man and indeed of all mammals is a 

 wholly albuminous gland, though in the dog a few cells are 

 mucous ; the submaxillary of man is on the whole a mucous gland 

 but some lobules in it are albuminous ; the submaxillary of the 

 rabbit is an albuminous gland. The sublingual may perhaps in 

 all mammals be regarded as a mucous gland, though it differs in 

 several respects from other mucous glands; the cells lining the 

 ducts are much shorter and less distinctly striated, the alveoli 

 are more obviously branched tubules, and the cells of some alveoli 



P contain no mucin. 

 The small buccal glands which lie in the substance of the 

 mucous membrane of the mouth, and whose secretion contributes 

 to " mixed " saliva, are formed, on a small scale, after the plan of 

 salivary gland, that is to say, they are composed of a duct (or 





