20 INTRODUCTION. 
The action received by the external organs is continued by 
the nerves to the central masses of the nervous system, which, 
in the higher animals, consists of the brain and spinal mar- 
row. ‘The more elevated the nature of the animal, the more 
voluminous is the brain and the more is the sensitive power 
concentrated there; the lower the animal, the more the me- 
dullary masses are dispersed, and in the most imperfect genera, 
the entire nervous substance seems to melt into the general 
matter of the body. 
That part of the body which contains the brain and princi- 
pal organs of sense, is called the head. 
When the animal has received a sensation, and this has oc- 
casioned volition; it is by the nerves, also, that this volition is 
transmitted to the muscles. 
The muscles are bundles of fleshy fibres whose contractions 
produce all the movements of the animal body. The exten- 
sion of the limbs and every elongation, as well as every flexion 
and abbreviation of parts, are the effects of muscular contrac- 
tion. ‘The muscles of every animal are arranged, both as re- 
spects number and direction, according to the movements it 
has to make; and when these motions require force, the 
muscles are inserted into hard parts, articulated one over an- 
other, and may be considered as so many levers. ‘These parts 
are called bones in the vertebrated animals, where they are in- 
ternal, and are formed of a gelatinous mass, penetrated by par- 
ticles of phosphate of lime. In the Mollusca, the Crustacea, 
and Insects, where they are external, and composed of a cal- 
careous 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 con- 
tinuation 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 mus- 
cular apparatus, and the form and proportion of the members 
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