SALIVARY GLANDS AND THEIR SECRETIONS. 265 



the contact of the substances which are passing through the alimentary 

 tube. When these are unusually acrid, the secretion of mucus is aug- 

 mented in quantity, and is increased in viscidity, so as to form an effec- 

 tive sheath to the membrane, which would otherwise suffer severely. 

 When this secretion is deficient, the membrane is irritated by the con- 

 tact of any but the blandest substances ; and the class of remedies 

 termed demulcents are useful in coating and protecting it. 



465. During the mastication of the food in the mouth, the Salivary 

 secretion is poured in, for the purpose of being mingled with it, and of 

 rendering the act of mastication more easy. This secretion is formed 

 by three pairs of glands, the Parotid, the Sub-lingual, and the Sub- 

 maxillary ; these are composed of minute follicles, 



whose diameter is about l-1000th of an inch, con- Fi S- 79 - 



nected together by branches of their duct, upon 

 which they are set like grapes upon their stalk, 

 surrounded by a plexus of blood-vessels, and bound 

 together by areolar tissue. Within the follicles 

 are the true secreting cells ( 238) ; by whose 

 growth and development, the material of the se- 

 cretion is separated from the blood. These sali- 

 vary cells are often to be recognised in the saliva ; 

 they must not however be confounded with the 

 epithelium-cells of the mucous membrane of the 

 mouth, which are much larger. The fluid ob- 

 tained from the mouth is not pure saliva ; for Lobule of Parotid 



,, ,, ., ,p *. . , , ' ., of new-born infant, filled with 



the mUCUS OI the mOUth itselt IS mingled With mercury; magnified 50 dia- 



the secretion from the salivary glands. If the m 

 proportion of the former be considerable, it gives to the fluid of the 

 mouth an acid reaction ; whilst, if the latter be predominant (which it 

 is directly before, and during, the act of eating), the fluid of the mouth 

 has an alkaline reaction. It may be sometimes observed, that the 

 saliva of the mouth will strike a blue colour with reddened litmus- 

 paper, whilst it turns blue litmus-paper red ; thus showing the presence 

 both of an acid and an alkali in a state of imperfect neutralization. 



466. The solid matter of the Salivary secretion is about 1 per cent, 

 of the whole ; and this consists in part of animal principles, and in 

 part of saline substances. The animal matter consists of osmazorne, 

 mucus, and a peculiar substance termed ptyaline or salivary matter ; 

 which is soluble in water and insoluble in alcohol, and which is yet dif- 

 ferent from both albumen and gelatine. This substance appears to 

 have a decided effect in producing the metamorphosis of certain alimen- 

 tary substances, on which it acts like a ferment. Starch may be 

 converted into sugar, and sugar into lactic acid, by its agency ; and, if 

 acidified, it has a certain solvent power for caseine, animal flesh, and 

 other proteine-compounds. Its chemical nature has not yet been pre- 

 cisely determined. The saline constituents of the Saliva are nearly 

 identical with those of the blood ; the chlorides of sodium and potassium 

 form considerably more than half; and the remainder consists chiefly 

 of the tribasic phosphate of soda, to which the alkaline reaction of the 

 fluid is due, with the phosphates of lime, magnesia, and iron. It is of 



