592 FUNCTIONS OF ANIMALS. 



An account of the saliva has been given in a preceding chap- 

 ter of this work, to which the reader is referred for a minute de- 

 tail of its chemical properties. 



It is a liquid nearly colourless, somewhat viscid, and usually 

 containing a few white flocks, which gradually sink to the bottom 

 when the saliva is left at rest in a vessel. It is thrown into 

 the. mouth from the salivary glands, when it is secreted, and in 

 greatest abundance during the mastication of the food. The 

 whole amount of it in twenty-four hours from an adult indivi- 

 dual is about seven ounces and a-half avoirdupois.* 



Human saliva, when dried in the vacuum of an air-pump over 

 sulphuric acid, leaves 1*62 per cent, of solid residue. These 1-62 

 parts contain 0-42 1 of the white flocks which may be considered 

 as mucus. They contain 0-528 of a matter soluble in water, but 

 insoluble in alcohol of 0-863. This is the substance to which 

 the name of salivin has been given, and the properties of which 

 have been described in a former chapter of this volume. Its use 

 in digestion has not yet been ascertained ; but, as it possesses the 

 characters of a weak acid, it is highly probable that it facilitates 

 the conversion of the food into chyme in the stomach. 



The 1-62 of insoluble residue contains also 0-444 of a matter 

 soluble in water ; but insoluble in alcohol of 0*800. This mat- 

 ter consists chiefly of chlorides of potassium and sodium : but is 

 not quite free from salivin. The residue of the 1'62 amounting 

 to 0-288 is soluble both in water and in alcohol at 0-800. It 

 consists chiefly of lactate of soda, and of soda combined with mu- 

 cus, but is not quite free from salivin. 



Thus the solid contents of healthy saliva from 100 parts of 

 that liquid are, 



Mucus, . 0-421 



Salivin, v/i; 0-528 



Salts, 0-732 



1-681 



In this analysis, which was made by Mitcherlich, there is an ex- 

 cess of 0-061. 



The great use of the saliva is doubtless to lubricate the food, 

 and cause it to descend easily into the stomach ; but it is proba- 

 ble that the salivin which it contains contributes somewhat to the 

 conversion .of the food into chyme. Accordingly, it appears 



* Poggendorf's Annalen, xxvii. p. 320. 



