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Untitled Article
Of the involuntary muscles the appropriate stimuli are various , though some of these muscles are obedient only to specific stimulus . Thus the aliment is the appropriate stimulus of the muscular fibres of the stomach ; the chyme of the duodenum , or second stomach ; the chyle of the small intestines , and the blood of the heart . *
* Now the mere contact , and the gentlest contact , of the muscular fibre with its appropriate stimulus , will cause the muscle to contract . If in an animal recently dead , the inner surface of the ventricle of the heart be pricked in the gentlest manner with the point of a needle , the ventricle will contract so as to bring the needle deep into its substance . If in the living animal , if in man , volition command , the arm will lift a weight of a hundred pounds , will overcome a degree of resistance to this extent , no force having been previously exerted to cause it to do so ; nothing having preceded but a mental act .
* And this is the true physiological distinction between the production of motion by a living substance in an organized body , and the production of motion in a machine put into action by some physical agent . The living agent generates the power it exerts ; the machine merely accumulates , or directs the power already in existence . Power of the first kind is vital ; power of the second kind is mechanical . All vital motion is the produce of this one agent , the
muscular fibre , and is obtained by this one action of it , contractility . All the motion that can be required in the economy , is capable of being produced by this one agent , and this one action , and , therefore , with a simplicity that marks all the works of nature ; this is the only agent that is employed . Mechanical principles , without doubt , are put in requisition , and made to cooperate whenever this can be done
with convenience and effect ; whenever the doing so will economize the production of muscular fibre ; but to the extent in which muscular fibre is really neces 3 ary , it is dealt out with no niggard hand ; and the study of the muscles becomes a most interesting and beautiful study , when pursued with a view to observe the arrangements and combinations made to accomplish the infinitely varied and complex motions required and performed in the animal economy .
' The muscular fibres of the heart are curiously arranged . They almost all take their origin from one point , where the structure becomes tendinous ; and this point is the pivot of the heart ' s movements . Tendon is highly elastic . The arrangement of the whole is such that the general contraction of the fibres must necessarily bring all the parts of the heart towards the central tendinous point , and the result is the compression of all the cavities , and the forcible ejection of their contents by their natural openings . The contraction is instantly followed not only by dilatation , but by the recoil of the elastic tendon .
* The two auricles contract together , and the two ventricles contract together , and these motions alternate with each other , and go on in regular succession . When the ventricles contract , the apex of the heart is drawn upwards , and raised or tilted forwards ; it is this motion which is felt between the fifth and sixth ribs , and which is called the beating of the heart ; it just perceptibly precedes the pulsation at the wrist . The different chambers of the heart open into and communicate with each other , and the effect of the contractions is to eject the blood they contain with great force . It was necessary to make a pro-
Untitled Article
Dr . Southwood Smith on the Animal Economy . 129
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Citation
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Monthly Repository (1806-1838) and Unitarian Chronicle (1832-1833), Feb. 2, 1833, page 129, in the Nineteenth-Century Serials Edition (2008; 2018) ncse.ac.uk/periodicals/mruc/issues/vm2-ncseproduct2608/page/61/
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