NERVOUS MECHANISM OF THE SECRETION OF SALIVA 305 



nuclei and smaller lumen. During activity the cells become smaller and 

 their contents more opaque. 



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

 mucigen is converted into mucin t 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 principally 

 contained in the connective tissue of the alveoli, and 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 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 

 an impression is conveyed upward (afferent) to the special nerve center 

 situated in the medulla oblongata which controls the process, and by it is 

 reflected to certain nerves supplied to the salivary glands, which will be pres- 

 ently indicated. In other words, the center, when stimulated to action by 

 the sensory impressions carried to it, sends out impulses along efferent or 

 secretory nerves supplied to the salivary glands. These cause the saliva to 

 be secreted by and discharged from the gland cells. Other stimuli, however, 

 besides that of the food, and other sensory nerves than those mentioned, 

 may reflexly produce the same effects. For example, saliva may be caused 

 to flow by irritation of the mucous membrane of the mouth with mechanical, 

 chemical, electrical, or thermal stimuli, also by the irritation of the mucous 

 membrane of the stomach in some way, as in nausea which precedes vomiting, 

 when some of the peripheral fibers of the vagi are irritated. Stimulation of 

 the olfactory nerves by smell of food, of the optic nerves by the sight of it, and 

 of the auditory nerves by the sounds which are known by experience to ac- 

 company the preparation of a meal may also stimulate the nerve center to 

 action. In addition to these, as a secretion of saliva follows the movement 

 of the muscles of mastication, it may be assumed that this movement stimu- 

 lates the secreting nerve fibers of the gland, direct or reflexly. From the fact 

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