294 



PHYSIOLOGY AND MORPHOLOGY OF ANIMALS. 



secreting gland. It is evidently only a device to bring 

 as large a surface as possible of epithelial cells into a 

 small space; for the secretion is a product of epithe- 

 lial action. Fig. 179 shows the different stages of in- 

 folding. 



Excitant. — Any stimulation of the tongue and in- 

 terior of the mouth, even mechanical, will excite the 

 secretion, but especially any strong taste. Even the 

 sight of food or the idea of food will often cause it to 

 flow. The first effect of excitement is the rush of blood 

 to the gland, and then follows a flow of secretion. Evi- 

 dently the secretion is manufactured out of materials in 

 the blood. This preliminary rush of blood and conse- 

 quent swelling of the gland is the cause of the pain pro- 

 duced by food or even the sight of food in mumps, which 

 is a disease of the parotid gland. 



Composition and Use of Saliva. — It is easy to 

 collect the salivary secretion in considerable quantity, 

 and thus to determine its composition. In a horse the 

 duct of the parotid gland runs just beneath the skin 

 over the broad, flat surface of the jaw and may be easily 

 taken up, a metallic tube introduced and turned out- 

 ward. If, now, food be given, or even a sheaf of hay be 

 shown, immediately the liquid secretion begins to pour 

 from the tube, and continues to pour as long as the food 

 is masticated. In this way half a pint or a pint of the 

 clear liquid may be collected. 



Saliva is a watery liquid, consisting mainly of mucus 

 (water and broken-down epithelial cells), but containing 

 a peculiar ferment called ptyalin, which is its active prin- 

 ciple ; its composition and its chemical properties seem to 

 be identical with diastase of sprouting seeds. Like dias- 

 tase, it changes the insoluble forms of amyloids (starch) 

 into the soluble forms (sugar). It does so by hydra- 

 tion of the starch. 



