XVl INTRODUCTION. 
Such, in particular, are the frogs, and connected genera, and the 
whole class of insects. 
The moving power is another characteristic of animal organization. 
It is seated in the muscular fibre, which is formed of filaments of ex- 
cessive tenuity, capable of contraction, and of moving the parts upon 
which they are fixed. These fibres are distributed over the body, and 
produce all its exterior and interior motions. When they are unitea 
in a bundle, of which the mass co-operates in the same action, this 
bundle is termed a muscle. In animal bodies, there are as many 
different muscles as there are simple movements; and besides, there 
are generally, for the purpose of bringing back the parts to their 
original position, other bundles of fibres, destined to produce a con- 
trary effect, and which have been accordingly termed antagonist 
muscles. The element of the muscular fibre, chemically considered, 
appears to reside in a matter called fibrine. 
The other organs destined to the purposes of movement, are alto 
gether passive. Sometimes they are disposed outwardly, under the 
appearance of membranes, or integuments, more or less solid; some- 
times under the form of crusts or sheaths, in the interior of which the 
muscles are placed. The solidity of these parts, their structure, their 
articulation, and movements, correspond to the animal’s mode of life ; 
and these crusts, shells, scales, or sheaths, are of a calcareous or horny 
nature, and adapted to the efforts they are destined to sustain,—the 
more soft coverings of this kind, as may be conceived, being only 
calculated for motion in fluids. 
In the higher classes of animals, the solid articulated parts which 
form the frame-work of the body and modify its form, are almost 
always placed internally, and-serve the purpose of jointed levers, and 
as a fulcrum for their muscular coverings. These parts are the dones 
of animals, and when arranged as a whole, they are termed the bony 
skeleton. All these bones meet in a central stalk, or hollow and 
moveable column, called the spine, of which the pieces, more-or less 
solid and numerous, are termed vertebra. Among those which are, 
on this account, named Vertebrated Animals, the column is terminated 
at one end by the cranium, a bony cavity, inclosing the mass of cere. 
bral matter which gives sensation, and is the seat, generally, of four 
organs of sense. In the head is also placed the mouth, an instru- 
mont capable of prehension, and provided with organs for mechanical- 
Jy dividing the aliment; and often, also, in this important part of 
animals, the organs are placed which produce or facilitate the-action 
