46 
MECHANICAL AND 1MMECHANICAL PARTS 
pose, according to the laws belonging to and governing 
the substance or the action made use of in that part; or 
that each part is not so constructed as to effectuate its 
purpose whilst it operates according to these laws; but it 
is because these laws themselves are not in all cases equal¬ 
ly understood; or, what amounts to nearly the same thing, 
are not equally exemplified in more simple processes, and 
more simple machines; that we lay down the distinction, 
here proposed, between the mechanical parts and other 
parts of animals and vegetables. 
For instance; the principle of muscular motion, viz. 
upon what cause the swelling of the belly of the muscle, 
and consequent contraction of its tendons, either by an 
act of the will, or by involuntary irritation, depends, is 
wholly unknown to us. The substance employed, whether 
it be fluid, gaseous, elastic, electrical, or none of these, or 
nothing resembling these, is also unknown to us: of course, 
the laws belonging to that substance, and which regulate 
its action, are unknown to us. We see nothing similar 
to this contraction in any machine which we can make, 
or any process which we can execute. So far (it is con¬ 
fessed) we are in ignorance, but no farther. This power 
and principle, from whatever cause it proceeds, being as¬ 
sumed, the collocation of the fibres to receive the princi¬ 
ple, the disposition of the muscles for the use and applica¬ 
tion of the power, is mechanical; and is as intelligible as 
the adjustment of the wires and strings by which a puppet 
is moved. We see, therefore, as far as respects the sub¬ 
ject before us, what is not mechanical in the animal frame, 
and what is. The nervous influence (for we are often 
obliged to give names to things which we know little 
about)—I say the nervous influence, by which the belly, 
or middle, of the muscle is swelled, is not mechanical. 
The utility of the effect we perceive; the means, or the 
preparation of means, by which it is produced, we do not. 
But obscurity as to the origin of muscular motion brings 
no doubtfulness into our observations upon the sequel of 
the process: Which observations relate, 1st, to the con¬ 
stitution of the muscle; in consequence of which consti¬ 
tution, the swelling of the belly or middle part is neces¬ 
sarily and mechanically followed by a retraction of the 
tendons: 2dly, to the number and variety of the muscles, and 
the corresponding number and variety of useful powers— 
which they supply to the animal; which is astonishingly 
great: 3dly, to the judicious, (if we may be permitted to 
use that term, in speaking of the author, or of the works 
