132 
COMPARATIVE ANATOMY. 
with more difficulty, there the passage is circuitous and 
dilatory, that time and space may be allowed for the change 
and the absorption which are necessary. Where the food 
is soon dissolved, or already half assimilated, an unneces¬ 
sary, or perhaps hurtful, detention is avoided, by giving to 
it a shorter and a readier route. 
V. In comparing the bones of different animals, we are 
struck, in the bones of birds, with a propriety, which could 
only proceed from the wisdom of an intelligent and design¬ 
ing Creator. In the bones of an animal which is to fly, the 
two qualities required are strength and lightness. Where¬ 
in, therefore, do the bones of birds (I speak of the cylindri¬ 
cal bones) differ in these respects from the bones of quad¬ 
rupeds ? In three properties; first, their cavities are much 
larger in proportion to the weight of the bone than in those 
of quadrupeds; secondly, these cavities are empty; thirdly, 
the shell is of a firmer texture than the substance of other 
bones. It is easy to observe these particulars, even in 
picking the wing or leg of a chicken. Now, the weight 
being the same, the diameter, it is evident, will be greater 
in a hollow bone than in a solid one, and with the diame¬ 
ter, as every mathematician can prove, is increased, ccetems 
paribus, the strength of the cylinder, or its resistance to 
breaking. In a word, a bone of the same weight would 
not have been so strong in any other form; and to have 
made it heavier, would have incommoded the animal’s 
flight. Yet this form could not be acquired by use, or the 
bone become hollow and tubular by exercise. What appe¬ 
tency could excavate a bone? 
VI. The lungs also of birds, as compared with the lungs 
of quadrupeds, contain in them a provision, distinguishing^ 
calculated for this same purpose of levitation; namely, a 
communication (not found in other kinds of animals) be¬ 
tween the air-vessels of the lungs and the cavities of the 
body; so that by the intromission of air from one to the 
other (at the will, as it should seem, of the animal,) its body 
can be occasionally puffed out, and its tendency to descend 
in the air, or its specific gravity, made less. The bodies 
of birds are blown up from their lungs (which no other ani¬ 
mal bodies are,) and thus rendered buoyant. 
VII. All birds are oviparous. This likewise carries 
on the work of gestation with as little increase as possible 
of the weight of the body. A gravid uterus would have 
been a troublesome burden to a bird in its flight. The ad¬ 
vantage, in this respect, of an oviparous procreation is, that 
whilst the whole brood are hatched together, the eggs are 
