CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 389 



side of the nucleus towards the lumen has no special features, but 

 on the outside, towards the basement membrane or connective 

 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 near the centre of the cell but rather nearer the basement 

 membrane, and the cell-substance, which has the general appear- 

 ance, in an ordinary preparation, of somewhat densely granular 

 protoplasm, stains readily and uniformly all over. J-No cells cor- 

 responding to the demilunes of a mucous alveolus are present. 

 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 con- 

 dition as does a mucous cell. There are however differences 

 between the loaded and the discharged albuminous cell, but to 

 these we shall return presently. 



V" 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 

 contain no mucin. 



