180 MOUTH DIGESTION. 



glossopharyngeal and lingual nerves serve as the afferent nerves 

 in this act, carrying the impulses to a center in the medulla, from 

 which efferent impulses pass out through the secretory nerves to 

 the glands. This center may be stimulated by afferent impulses 

 reaching it through other nerves ; thus salivary secretion may be 

 increased, or " the mouth made to water," by the smell or even 

 the thought of food. This center may also be inhibited from 

 sending out secretory impulses, as through fright or other nervous 

 disturbance, thus diminishing the secretion of saliva and causing 

 the dry mouth and throat which so commonly accompany such 

 conditions. 



Changes in the Salivary Cells. Certain changes take place in 

 the salivary cells as a result of their activity. When a rabbit is 

 fasting, the cells of the parotid gland are granular throughout, 

 their outlines being faintly marked by light lines. If it is 

 fed, or pilocarpin injected, or the sympathetic stimulated, the 

 granules disappear from the outer portion of the cells, so that 

 there is an external clear border surrounding the granular in- 

 terior. If the stimulation continues, the granules diminish and 

 are collected near the lumen of the alveoli and at the margin 

 of the cells, the clear border becomes enlarged, and the cells 

 become smaller. The explanation of this is that the granules 

 contain a substance which is or which becomes the ptyalin 

 of the saliva. If it should be demonstrated hereafter that 

 there is a zymogen which precedes the ptyalin, it would receive 

 the name of ptyalinogen, but this has not as yet been proved to 

 occur. These granules in the cells are called zymogen granules. 

 While these granules are being used up to form the ptyalin new 

 material is being deposited by the blood at the base of the cells, 

 which in turn will later form the zymogen or ptyalin. 



Similar changes take place in the mucous glands, in the cells 

 of which are granules (125 to 250 to a cell) composed of mucin- 

 ogen, which becomes mucin, and is in this form discharged into 

 the lumen of the alveoli. The demilunes, before referred to, are 

 situated outside these cells which produce the mucinogen. 



Properties and Composition of the Saliva. The secretion of 

 the human parotid gland is a clear fluid, though sometimes turbid, 

 and contains some epithelial cells. It is alkaline in reaction, but 

 less so than the saliva of the submaxillary gland. Its specific 

 gravity is very variable, as is shoAvn by the following figures : 

 Mitscherlich gives it as 1.006 to 1.008 ; Oehl, 1.010 to 1.012 with 

 scanty, and 1.0035 to 1.0039 with plentiful secretion ; Hoppe- 

 Seyler, 1.0061 to 1.0088 ; the solids being from 5 to 16 parts per 

 1000. It contains ptyalin and potassium sulphocyanate, but no 

 mucin. 



The secretion of the human submaxillary gland is clear and 

 watery, always alkaline, and has a specific gravity between 1.0026 



