FOOD AND DIGESTION. 33 



Varieties. Certain differences in the structure of salivary glands may 

 be observed according as the glands secrete pure saliva, or saliva mixed 

 with mucus, or pure mucus, and therefore the glands have been classified 

 as: 



(1) True salivary glands (called most unfortunately by some, serous 

 glands), e.g., the parotid of man and other animals, and the submaxil- 

 lary of the rabbit and guinea-pig (fig. 235). In this kind the alveolar 

 lumen is small, and the cells lining the tubule are short granular colum- 

 nar cells, with nuclei presenting the intranuclear network. During rest 

 the cells become larger, highly granular, with obscured nuclei, and the 

 lumen becomes smaller. During activity, and after stimulation of the 

 sympathetic, the cells become smaller and their contents more opaque ; 

 the granules first of all disappearing from the outer part of the cells, and 



Fig. 235. From a section through a true salivary gland, a, The gland alveoli, lined with albumin- 

 ous "salivary cells;" 6, intralobular duct cut transversely. (Klein and Noble Smith.) 



then being found only at the extreme inner part and contiguous border 

 of the cell. The nuclei reappear, as does also the lumen. 



(2) In the true mucus-secreting glands, as the sublingual of man and 

 other animals, and in the submaxillary of the dog, the tubes are larger, 

 contain a larger lumen, and also have larger cells lining them. The cells 

 are of two kinds, (a) mucous or central cells, which are transparent 

 columnar cells with irregular or flattened nuclei near the basement mem- 

 brane. The cell substance is made up of a fine network, which in the 

 resting state contains a transparent substance called mucigen, during 

 which the cell does, not stain well with logwood (fig. 236). When the 

 gland is secreting, as well as on stimulation of the nerve, mucigen is con- 

 verted into mucin, and the cells swell up, appear more transparent, and 

 stain deeply in logwood (fig. 237). After stimulation, the cells become 

 smaller, more granular, and more easily stained, from having discharged 

 their contents. The nuclei appear more distinct, (b) Crescents of Gia- 

 nuzzi, sometimes called the Semilunes of Heidenhain (fig. 236), which 

 are crescentic masses of granular parietal cells found here and there be- 



