INTRODUCTION. 17 



The muscles are bundles of fleshy fibres whose contractions produce 

 all the movements of the animal body. The extension of the limbs, and 

 every elongation, as well as every flexion and abbreviation of parts, are 

 the effects of muscular contraction. The muscles of every animal are 

 arranged, both as respects number and direction, according to the move- 

 ments it has to make ; and when these motions require force, the muscles 

 are inserted into hard parts, articulated one over another, and may be 

 considered as so many levers. These parts are called bones in the verte- 

 brated animals, where they are internal, and are formed of a gelatinous 

 mass, penetrated by particles of phosphate of lime. In the Mollusca, 

 the Crustacea, and Insects, where they are external, and composed of 

 a calcareous or horny substance that exudes between the skin and epi- 

 dermis, they are called shells, crusts, and scales. 



The fleshy fibres are attached to the hard parts by means of other 

 fibres of a gelatinous nature, which seem to be a continuation of the 

 former, constituting what are called tendons. 



The configuration of the articulating surfaces of the hard parts, limits 

 their motion, which are also restrained by cords or envelopes, attached 

 to the sides of the articulations, called ligaments. 



It is from the various arrangements of this bony and muscular 

 apparatus, and the form and proportion of the members resulting 

 therefrom, that animals are capable of executing the innumerable move- 

 ments that enter into walking, leaping, flight and swimming. 



The muscular fibres, appropriated to digestion and the circulation, 

 are independent of the will ; they receive nerves, however, but the chief 

 of them are subdivided and arranged in a manner which seems to have 

 for its object their independence of the will. It is only in paroxysms 

 of the passions and other powerful affections of the soul, which break 

 down these barriers, that its empire is perceptible, and even then it 

 almost always tends to disorder these vegetative functions. It is, also, 

 in a state of sickness only that these functions are accompanied with 

 sensations : digestion is usually performed unconsciously. 



The aliment, divided by the jaws and teeth, or sucked up when 

 liquids constitute the food, is swallowed by the muscular movements of 

 the hinder parts of the mouth and throat, and deposited in the first por- 

 tions of the alimentary canal which is usually expanded into one or more 

 stomachs ; there it is penetrated with juices fitted to dissolve it. Pass- 

 ing thence through the rest of the canal, it receives other juices destined 

 to complete its elimination. The parietes of the canal are pierced with 

 pores which extract from this alimentary mass its nutritious portion ; the 

 useless residuum is rejected. 



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