316 PHILOSOPHY OF ZOOLOGY. 



man and many other animals *. Other inorganic substan- 

 ces are likewise employed for a variety of purposes. Many 

 savages make use of steatite and clay along with their food. 

 The common earth-worm swallows the soil, from which, in 

 its passage through the intestines, it extracts its nourish- 

 ment. 



In some cases, substances are swallowed for other pur- 

 poses than nourishment. Stones are retained in the stomach 

 of birds, to assist in triturating the grain. The wolf is 

 said to satisfy his hunger by filling his stomach with mud. 

 We have found in the stomach of the eared grebe (Podi- 

 ceps auritus), in the month of January, a large ball of its 

 own breast feathers, probably pulled off and swallowed for 

 the same purpose. 



When we consider the number of elementary substances 

 which enter into the composition of the bodies of animals, 

 and the varieties of combination which -they form in the in- 

 dividual parts, we may perceive how vain it is to assign to 

 any one substance the exclusive property of nourishing, 

 and to consider the value of the food to depend on its pre- 

 sence. In the ordinary secreted fluids, the number of ele- 

 mentary bodies of which they consist, is greater than one 

 would be led to expect, without some deliberation. The 

 spittle, for example, is, in man, necessary to mastication, 

 and, therefore, during the continuance of health, must be 

 constantly produced. Yet we find the following elements 

 entering into its composition : Oxygen, hydrogen, azote, 

 carbon, phosphorus, muriatic acid, lime, and soda. We 



* The eagerness with which many quadrupeds and birds press towards 

 salt-springs and lakes, situated in inland districts, for the purpose of tasting 

 their contents, indicates a constitutional fondness for salt. The saline mine- 

 ral spring of Dunblane, discovered a few years ago, first attracted notice by 

 its being the constant resort of pigeons, which nocked to it from great dis- 

 tances at all times of the day. 



