16 INTRODUCTION. 
by particles of phosphate of lime. In the Mollusca, the Crustacea, and 
Insects, where they are external, and composed of a caicareous 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 appara- 
tus, 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 mx. It is only in paroxysms of the pas- 
sions and other powerful affections of the soul, which break down these 
barriers, that the empire of the mz is perceptible, and even then it is al- 
most always to disorder these vegetative functions. It is, also, in a state 
of sickness only that these functions are accompanied with sensations: di- 
gestion 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 portions of the ali- 
mentary canal that are 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 pre- 
paration. The parietes of the canal are pierced with pores which extract 
from this alimentary mass its nutritious portion; the useless residuum is 
rejected as excrement. 
The canal in which this first act of nutrition is performed, is a conti- 
nuation of the skin, and is composed of similar layers; even the fibres 
that encircle it are analogous to those which adhere to the internal sur- 
face of the skin, called the fleshy pannicle. Throughout the whole inte- 
rior of this canal there is a transudation which has some connexion with 
the cutaneous perspiration, ard which becomes more abundant when the 
latter is suppressed; the absorption of the skin is even very analogous to 
that of the intestines. It is only in the lowest order of animals that the ex- 
crements are rejected by the mouth, their intestines resembling a sac, 
having but one opening. 
