330 



FOOD AND DIGESTION 



When the mucous type of gland is secreting, or on stimulation of the nerve, 

 mucinogen is converted into mucin, the cells swell up, appear more transparent 

 and stain deeply in logwood, figure 249. After stimulation, the cells become 

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

 contents, and the nuclei appear more distinct. 



Nerves of large size are found in the salivary glands. They are princi- 

 pally contained in the connective tissue of the alveoli, and certain glands, 

 especially in the dog, are provided with ganglia. Some nerves have special 



FIG. 248. 



FIG. 249. 



FIG. 248. Section of the Submaxillary Gland of a Dog, Resting Stage. Most of the 

 alveolar cells are large and clear, being filled with the material for secretion (in this case, 

 mucigen), which obscures their protoplasm; some of the cells, however, are small and 

 protoplasmic, forming the crescents seen in most of the alveoli. (Ranvier.) 



FIG. 249. Section of a Similar Gland after a Period of Activity. The mucigen has 

 been discharged from the mucin-secreting cells, which consequently appear shrunken and 

 less clear. Both the cells and the alveoli are much smaller, and the protoplasm of the cells 

 is more apparent. The crescents of Gianuzzi are enlarged, c, Crescent cells; g, mucus- 

 secreting cells; /, lumen of alveolus. (Ranvier.) 



endings in Pacinian corpuscles, some supply the blood vessels, and others 

 penetrate the basement membrane of the alveoli and end upon, but not in, 

 the salivary cells. 



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

 alveoli, being carried in by the fibrous trabeculae between the alveoli, in which 

 also the lymphatics begin by lacunar spaces. 



The Nervous Mechanism of the Secretion of Saliva. The secretion 

 of saliva is under the control of the nervous system. Under ordinary con- 

 ditions it is excited by the stimulation of the peripheral branches of two 

 nerves, the gustatory or lingual branch of the inferior maxillary division 

 of the fifth nerve, and of the glosso-pharyngeal, which are distributed to the 

 mucous membrane of the tongue and pharynx conjointly. The stimulation 

 occurs on the introduction of sapid substances into the mouth, and the 

 secretion is brought about in the following way: From the terminations of 

 the above-mentioned sensory nerves distributed in the mucous membrane 



