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modern times, who has adopted the mechanical theory, that has carried it to this extremity of absurdity; but all of them are still carrying it too far, who reason concerning the principal motions of the body as mere mechanical motions, and the powers which the muscles exert as mere mechanical powers; in which the bones are the levers, the joints the fulcra, and the muscles the moving cords; for it so happens that all the effects for which the whole of this complicated machinery is absolutely necessary out of the body, are in many instances performed by a single part of it within the body, namely, by the moving cords or muscles alone, without either bones or joints, levers or fulcra. I do not mean to contend that there is no kind of resemblance or conformity of principle between the laws of animate and inanimate mechanics, for I well know that in a variety of points the two systems very closely concur; but I am obliged to affirm that they are still two distinct systems, and that in the one case the living power exercises an influence which finds no sort of similitude in the other.
It is, indeed, curious to observe the difference of result which has flowed from the calculations of the different promoters of this theory; and which alone, were there nothing else to oppose them, would be sufficient to prove the fallacy of their reasoning. Among those who have adopted this mode of explanation, and have pursued it with most acuteness, and may be regarded as the fathers of the school, I may be allowed to mention Borelli and Keil; but while the former, in order to account for the circulation of the blood in man, calculated the force with which the heart contracts to be equal to
not less than a hundred and eighty thousand pounds weight at every contraction, the latter could not, from his principles, deduce a higher estimate than eight ounces.
In like manner, Borelli, in applying the same theory to the power with which the human stomach triturates, or, as we now call it, digests its food, calculated it, in conjunction with the assistance it receives from the auxiliary muscles, which he conceived to divide the labour about equally with itself, as equal to two hundred and sixty-one thousand one hundred and eighty-six pounds; and Pitcairn has made it very little less, since he estimates the moiety contributed by the stomach alone at one hundred and seventeen thousand and eighty-eight pounds; which gives to these organs jointly a force more than equal to that of twenty mill-stones! "Had he," says Dr. Monro, " assigned five ounces as the weight of the stomach, he had been nearer the truth."*
The fallacy of this theory, however, and especially as it applies to the stomach, has been completely exposed in our own times, by the wellascertained fact, that though the muscular coat of the stomach in most animals bears some part in the process of digestion, this important operation is almost entirely performed by a powerful chemical solvent secreted by the stomach itself for this very purpose, and hence denominated the gastric juice; and which answers all the purposes of the most violent muscular pressure we can conceive, and with a curious simplicity of contrivance.
* Comp. Anat. pref. p. viii. note.
The laws of physical force will certainly better apply to the action of the heart and arteries than to that of the stomach, and in some measure assist us in accounting for the circulation of the blood; but the moment we reflect that one half of this very circulation, that I mean which depends upon the veins, and which has for the most part to contend against the attraction of gravitation, instead of being able to avail itself of its assistance, is produced without any muscular propulsion that we are able to discover, and that even the arteries do not, when uninfluenced by pressure, appear to change their diameter in a state of health*, we are necessarily driven to the conclusion, that there is in animal statics, as well as in animal mechanics, a something distinct and independent, and which the laws of physical force are altogether incompetent to explain. Dr. Young, in his excellent Croonian lecture, read before the Royal Society in 1809+, has endeavoured to revive the mechanical theory; but he is still compelled to admit a variety of phænomena in the animal machine, and especially in the circulatory system, which are altogether unaccountable upon any of the known principles of common hydraulics, and which can never fail to reduce us to the same result.
So far, therefore, as we at present know, the circulation of the blood is performed by a double projectile power; one moiety being dependent on the action of the living principle in the heart, and
See Lect. VIII. p. 178., as also the author's Study of Medicine, vol. ii. p. 16. Edit. second, 1825.
On the Functions of the Heart and Arteries, Phil. Trans. 1809, p. 1.
perhaps the arteries; and the other moiety on the common law of hydraulics, or the vacuum produced in the heart by that very contraction or systole which has just propelled the blood returned from the lungs into the arterial system. Whence the heart itself becomes alternately a forcing and a suction pump; being the former in respect to the arteries, and the latter in respect to the veins.*
Upon a moderate estimate, the common labourer may be said to employ a force capable of raising a weight of ten pounds to the height of ten feet in a second, and continued for ten hours a day. A moderate horizontal weight for a strong porter, walking at the rate of three miles an hour, is 200 pounds: the chairman walks four miles an hour, and carries 150 pounds. The daily work of a horse is equal to that of five or six men upon a plane; but from his horizontal figure, in drawing up a steep ascent, it does not exceed the power of three or four men. In working windmills, twenty-five square feet of the sails is equivalent to the work of a single labourer; whence a full-sized mill, provided it could be made to work eight hours a day, would be equivalent to the daily labour of thirty-four men. A steam-engine of the best construction, with a thirty inch cylinder, has the force of forty horses; and as it acts without intermission, will perform the work of 120 horses, or of 600 men; every square inch of the piston being equivalent to the power of a labourer.
There are many muscles given to us which the common customs and habits of life seldom
* See Stud. of Med. vol. ii. p. 19. Ed. second.
render it necessary to exert, and which in consequence grow stiff and immoveable. Tumblers and buffoons are well aware of this fact; and it is principally by a cultivation of these neglected muscles that they are able to assume those extravagant postures and grimaces, and exhibit those feats of agility, which so often amuse or surprise us.
The same muscles of different persons, however, though of the same length and thickness, and, so far as we are able to trace, composed of the same number of fibres, are by no means uniformly possessed of the same degree of power; and we here meet with an express deviation from the law of physical mechanics; as we do also in the curious fact, that whatever be the power they possess, they grow stronger in proportion to their being used, provided they are well used, and not exhausted by violence or over-exertion.
I have calculated the average weight carried by a stout porter in this metropolis at 200 pounds; but we are told there are porters in Turkey, who, by accustoming themselves to this kind of burden from an early period, are able to carry from 700 to 900 pounds, though they walk at a slower rate, and only carry the burden a short distance. "The weakest man can lift with his hands about 125 pounds, a strong man 400. Topham, a carpenter, mentioned by Desaguliers, could lift 800 pounds. He rolled up a strong pewter dish with his fingers. He lifted with his teeth and knees a table six feet long, with a half hundred weight at the end. He bent a poker, three inches in circumference, to a right angle, by striking it upon his left fore-arm; another he bent. and unbent about his neck, and snapped a hempen