388 MUCOUS GLANDS. [BOOK n. 



small quantity of ordinary protoplasmic cell-substance, staining 

 readily with the usual dyes ; the rest of the cell-body consists of a 

 transparent material, which does not stain readily, and which occu- 

 pies the spaces or meshes of a very delicate meshwork continuous 

 apparently with the staining protoplasmic cell-substance around 

 the nucleus, and with a thin sheet of similar material forming the 

 wall of the cell. This transparent material is either mucin, which 

 we have seen to be a conspicuous constituent of submaxillary 

 saliva (in the dog) or a substance which can be easily converted 

 into ' actual mucin, that is to say an antecedent of mucin ; hence 

 the name ' mucous cell.' A resting or loaded mucous cell then 

 consists largely of mucin (or its antecedent) lodged in the meshes 

 of the protoplasmic cell-substance which over the greater part 

 of the cell exists, in a hardened gland at any rate, as a delicate 

 meshwork or reticulum, but is gathered into a compact mass in 

 a small area immediately around the nucleus. 



In many alveoli, a more or less triangular space left between 

 the diverging bases of two of the mucous cells and the basement; 

 membrane may be seen to be occupied by one or by two or more 

 peculiar small cells. These on examination are found to be 

 irregular in form but often half-moon shaped, and are hence called 

 demilune cells. Each consists of deeply staining cell-substance 

 with a spherical nucleus. From their size, and their staining 

 deeply, as well as from their position, these demilune cells contrast 

 strongly with the mucous cells. 



In the ' discharged,' or as it is often called the ' active ' phase, 

 the mucous cell has a different appearance, especially if the 

 activity of the gland has been great. The cell is now smaller, 

 and thus gives rise to a more distinct lumen in the alveolus, 

 a larger portion of the cell stains, especially on the outer side, 

 and sometimes the whole cell stains ; the nucleus, now spherical 

 even in hardened specimens, occupies a more central position. 

 The transparent, non-staining mucin has in large part or wholly 

 disappeared, its place has been taken by ordinary staining proto- 

 plasmic cell-substance, and the distinction between the demilune 

 cells and the proper cells of the alveolus is much less distinct. 

 We shall presently have to discuss the nature and meaning of this 

 change from the loaded to the discharged cell. 



217. A small duct of the submaxillary gland, even when cut 

 transversely in the section so as to present like many alveoli a 

 circular outline, has an appearance very different from that of 

 an alveolus. The duct is lined by a single layer of epithelium, 

 but these are slender, narrow, columnar cells leaving in the centre 

 a relatively wide lumen, and the outside of the duct is not so 

 sharply defined by a conspicuous basement membrane as is the 

 case in an alveolus. Each cell, which bears an oval nucleus placed 

 vertically in the cell at about the middle but rather nearer the 

 base, consists of a protoplasmic cell-substance which on the inner 



