26 INTRODUCTION. 



one over another, and may be considered as so many levers. These 

 parts are called bones in the vertebrated 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 epidermis, 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 therefrom 

 resulting, that animals are capable of executing the innumerable 

 movements that enter into walking and leaping, flight and natation. 



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 is almost always to disorder these vegetative func- 

 tions. It is, also, in a state of sickness only that these functions 

 are accompanied with sensations : digestion is usually performed un- 

 consciously. 



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 portions of the alimentary canal that is usually expanded into 

 one or more stomachs ; there it is penetrated with juices fitted to 

 dissolve it. Passing thence through the rest of the canal, it receives 

 other juices destined to complete its preparation. The parietes of 

 the canal are pierced with pcres which extract from this alimentary 

 mass its nutritious portion ; the useless residuum is rejected. 



The canal in which this first act of nutrition is performed is a 

 continuation of the skin, and is composed of similar layers ; even the 

 fibres that encircle it are analogous to those which adhere to the 

 internal surface of the skin, called the fleshy pannicle. Throughout 



