DIGESTION IN THE MOUTH 



373 



piece of meat will at first evoke secretion in a dog until the animal 

 realizes it is a fraud ; then it ceases to do so. 



The smell of food causes a flow of saliva, varying according to the 

 appetite. It is particularly copious when the animal is hungry. 

 Hearing a noise associated with food e.g., the rattle of the food-plato 

 also induces a salivary flow. 



The amount of saliva secreted while eating depends 



1 . Upon the dryness of the food. 



2. Upon its chemical properties. 



3. Upon the length and thoroughness of the act of chewing. 



4. Upon the water intake. 



5. Whether accessory stimulants be present, such as mustard or 

 pepper. 



6. Upon the excitability of the nervous mechanism of the salivary 

 glands themselves. 



The quantity of saliva generally varies from 500 to 1,500 c.c. per 

 day. Experiments showed that a girl secreted while chewing 



150 grammes of sugar . . . . . . . . . . . . 200 c.c. of saliva. 



1,200 , of milk 200 



200 



700 



10 



10 



of bread 126 



of mixed diet: meat, soup, potatoes . . . . 300 



of thin bread and butter . . . . . . . . 2 



of dry bread . . . . . . . . . . . . 15 



The Mechanism of Secretion. During the process of secretion there 

 occur certain well-marked changes in the gland cells. In a serous 

 gland, prior to secretion, the cells are filled with granules which stain 

 readily, the nuclei of the cells being almost obscured or appearing 

 at one side as more or less irregular masses. During secretion, these 

 granules become discharged from the cell, the nuclei become more 

 prominent, while the cells markedly shrink in size. 



In the mucous gland the process is comparable. The resting cell, 

 with its large refractile granules and nuclei flattened towards the base 

 of the cell, is replaced after secretion by the smaller " exhausted " 

 cell, devoid of mucigen granules, with well-marked spherical nucleus 

 placed in the middle of the cell. During the period of rest, the cells 

 again elaborate new granular material, and pass into the " resting 

 state." 



The salivary secretion takes place under nervous influence, the 

 whole process being presided over by a centre in the medulla oblongata 

 (c) in the region of the glosso-pharyngeal nucleus. To this centre come 

 impulses directly by various sensory nerves, and also indirectly via 

 the higher centres (c ) of the brain (Fig. 192). Thus it is that contact 

 of food with the buccal mucous membrane or stimulation of the central 

 end of the lingual or glosso-pharyngeal nerve gives rise to a flow of 

 saliva. Exactly how the sensory discrimination and adaptation of 

 the flow of saliva are brought about is not known. The results show 

 that such discrimination is a function of the lower subconscious centres. 

 Sight and hearing act reflexly through the higher centres; so do past 



