INTRODUCTION. XVII 
interruption of respiration, which would otherwise follow the 
rapidity of their motion through the resisting atmosphere ; and 
thus the Ostrich, though deprived of the power of flight, runs 
almost with the swiftness of the wind, and requires, as he 
possesses, the usual resources of air conferred on other birds. 
Were it possible for man to move with the rapidity of a Swal- 
low, the resistance of the air, without some such peculiar pro- 
vision as in birds, would quickly bring on suffocation. The 
superior vital heat of this class of beings is likewise probably 
due to this greater aération of the vital fluid. 
Birds, as well as quadrupeds, may be generally distinguished 
into two great classes from the food on which they are destined 
to subsist ; and may, consequently, be termed carnivorous and 
granivorous. Some also hold a middle nature, or partake of 
both. The granivorous and herbivorous birds are provided 
with larger and longer intestines than those of the carnivorous 
kinds. Their food, consisting chiefly of grain of various sorts, 
is conveyed whole into the craw or first stomach, where it is 
softened and acted upon by a peculiar glandular secretion 
thrown out upon its surface; it is then again conveyed into a 
second preparatory digestive organ; and finally transmitted 
into the true stomach, or gizzard, formed of two strong muscles 
connected externally with a tendinous substance, and lined in- 
ternally with a thick membrane of great power and strength ; 
and in this place the unmasticated food is at length completely 
triturated, and prepared for the operation of the gastric juice. 
The extraordinary powers of the gizzard in comminuting food, 
to prepare it for digestion, almost exceeds the bounds of cred- 
ibility. Turkeys and common fowls have been made to swal- 
low sharp angular fragments of glass, metallic tubes, and balls 
armed with needles, and even lancets, which were found 
broken and compressed, without producing any apparent pain 
or wounds in the stomach. The gravel pebbles swallowed by 
this class of birds with so much avidity, thus appear useful in 
bruising and comminuting the grain they feed on, and prepar- 
ing it for the solvent action of the digestive organs. 
Those birds which live chiefly on grain and vegetable sub- 
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