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niuscle which draws the nictitating membrane over the eye . Its office is in the front of the eye ; but its body is lodged in the back part of the globe , where it lies safe , and where it incunxbers nothing . "
It is a fixed law that the contraction of a muscle is towards its centre . Therefore the subject for mechanism on each occasion is , so to modify the figure and adjust the position of the muscle as to produce the motion required agreeably to this law . Hence different muscles have a different
configuration suited to their several offices , and to their situation with respect to the work which they have to perform , on which account they are found under a multiplicity of forms and attitudes . The shape of the organ
is susceptible of an incalculable variety , while the law and line of its contraction remain the same . Iu this , to refer again to the same writer , the muscular system may be said to bear a perfect resemblance to our works of art . An artist takes his materials as
he finds them , and employs his skill and ingenuity in turning them to his account , by giving to the parts of his machine a form and relation , in which these properties may operate to the production of the effects intended *
The muscles , it is said , act in the limbs with what is called a mechanical disadvantage , yet this is conducive to animal conveniency . Mechanism lias always in view one of these two purposes , either to move a great
weight slowly , or a light one rapidly . For the former of these purposes , a different arrangement of the muscles might be better than the present , but for the latter , the present structure is the true one . It is of much more
consequence to a man to be able to carry his hand to his head with due expedition , than it would be to have the power of raising from the ground a heavier load than he can at present lift . The last faculty may occasionally be desirable , but the other he wants and uses every day and hour .
On Muscular Motion . Muscular motions are of three kinds , viz . voluntary , involuntary and mixed . The voluntary motions of muscles are such as proceed from an immediate exertion of the active powers of the wilJ : thus the mind directs the arm to be raised , the knee to he bent , the tongue to move ,. &c . The involun-
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tary motions of the muscles are those which are performed by organs , see mingly of their own accord , and certainly without any attention of the mind , as the contraction and dilata - tion of the heart , arteries , veins ,
stomach , &c . The mixed motions are those ^ which are in fact under the controul of the will , but which usually act without our being conscious that they do so , as in the muscles of respiration , the abdominal muscles and the diaphragm .
Motion , as has been observed , \ % produced by the muscle contracting both its ends towards the centre , and since one end is fixed , the other must be drawn towards the centre of motion , and with it the bone to which
it is affixed , and thus by the co-operation of several muscles , the whole body is put into action . This is the case with all the muscles of voluntary motion ; their fibres contract on the application of the nervous influence , and the whole muscle shortens itself : on the same principle the other
muscles perform involuntary motion . The heart , for instance , contracts from the stimulating properties of the blood the arteries do the same . Motion in animals may be defined to be the contraction of the muscular fibre from the presence of some stimulating influence . But whence the muscular fibre derives this contractile power and what is its nature bafBes all inquiry . Its properties are , however , known , and it is distinguished from those feelings or motions which result from the nerves .
Irritability , or the contractile force of the muscular fibre , is that power which belongs to muscles of shortening themselves , when in any way irritated , and is the source of motion and animal life . The nervous power is that property by which , when a nerve is irritated by pressure or by puncture , the animal feels pain , and the muscles supplied by that nerve are brought into motion . This power is the cause of voluntary motion , and relates chiefly to the enjoyments and consciousness of life . Sensibility , therefore , depends upon the nerves , but motion upon the muscles ; both are equally admirable and
inscrutable , the one conduces to all the enjoyments and ail the sufferings of life , and to the intellectual faculties of
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708 Natural Theology * No . XI * Muscular Motion .
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Citation
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Monthly Repository (1806-1838) and Unitarian Chronicle (1832-1833), Nov. 2, 1815, page 708, in the Nineteenth-Century Serials Edition (2008; 2018) ncse.ac.uk/periodicals/mruc/issues/vm2-ncseproduct1766/page/44/
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