3160 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 im- 
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 flocked to it from great dis- 
tances at all times of the day. 
