DIGESTION. 233 



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 col- 

 umnar cells with nuclei near the basement membrane. The cell sub- 

 stance 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. 176). When the gland is secreting, mu- 

 cigen is converted into mucin, and the cells swell up, appear more trans- 

 parent and stain deeply in logwood (Fig. 177). During rest, the cells 

 become smaller and more granular from having discharged their con- 

 tents. The nuclei appear more distinct, (b) Demilunes of Heidenhain 

 (Fig. 176), which are crescentic masses of granular parietal cells found 

 here and there between the basement membrane and the central cells. 

 The cells composing the mass are small and have a very dense reticulum, 

 the nuclei are spherical, and increase 

 in size during secretion. In the mu- 

 cous gland there are some large tubes, 

 lined with large transparent central 

 cells, and having besides a few granu- 

 lar parietal cells ; other small tubes 

 are lined with small granular parietal 

 cells alone ; and a third variety are 

 lined equally with each kind of cell. 



(3) In the muco-salivary or mixed 

 qlands, as the human submaxillary FIG. 177. -A part of a section through 



J a mucous gland after prolonged electrical 



gland, part Of the gland presents the stimulation. The alveoli are lined with 



small granular cells. (Lavdovski. ) 



structure of the mucous gland whilst 



the remainder has that of the salivary glands proper. 



Nerves and Blood-vessels. Nerves of large size are found in the sali- 

 vary glands, they are principally contained in the connective tissue of 

 the alveoli, and in certain glands, especially in the dog, are provided 

 with ganglia. Some nerves have special endings in Pacinian corpuscles, 

 some supply the blood-vessels, and others, according to Pfliiger, pene- 

 trate the basement membrane of the alveoli and enter the salivary cells. 



The blood-vessels form a dense capillary network around the ducts 

 of the alveoli, being carried in by the fibrous trabeculse between the al- 

 veoli, in which also begin the lymphatics by lacunar spaces. 



Saliva. 



Saliva, as it commonly flows from the mouth, is mixed with the 

 secretion of the mucous glands, and often with air bubble, which, being 

 retained by its viscidity, make it frothy. When obtained from the 

 parotid ducts, and free from mucus, saliva is a transparent watery fluid, 

 the specific gravity of which varies from 1004 to 1008, and in which, 



