XVI 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 united 

 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 bones 

 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- 

 ment capable of prehension, and provided with organs for mechanical- 

 ly dividing the aliment ; and often, also, in this important part of 

 animals, the organs are placed which produce or facilitate the action 



