Organs of Nutrition 117 



surely return to outdoor study of the daily life of these 

 creatures, with more balanced interest, and a "little 

 knowledge" which, instead of being a " dangerous thing," 

 will, in this instance, add many fold to our appreciation 

 of the external results of these functions, whose work- 

 ings are ever concealed from the light of day. 



The digestive apparatus of a bird, or indeed of any 

 creature with a back-bone, is in reality a tube or canal, 

 which begins at the mouth and extends through the 

 body. Certain portions are contracted or expanded, and 

 specialized to store up, moisten, grind, dissolve, digest, or 

 absorb the food substances which pass through. 



The Salivary Glands 



The mucous membrane, or lining, of the entire diges- 

 tive canal is very delicate and requires to be kept con- 

 stantly moist. The lining of the mouth and throat, being 

 so exposed to contact with the outer air, requires some 

 special provision to lubricate it. This is accomplished 

 by certain glands, some beneath the tongue, others situ- 

 ated in the upper portion of the mouth. These are not 

 found in fishes, nor in other creatures which live alto- 

 gether in the water; but in reptiles several groups are 

 distinguishable. In birds they vary greatly, some having 

 scarcely a trace, while others have large well-developed 

 glands. Salivary is the common name given to certain 

 of these, and we will let that name represent all. 



In ourselves, saliva is an important aid in digestion. 

 Besides moistening the food and softening all hard por- 

 tions, it exerts active chemical effects, as, for example, 



