Borelli, who explained the mechanical action of muscles with so much ingenuity, and had the merit of first placing the subject in a correct point of view, has m tdc many elaborate calculations to estimate the quantity of power which is lost by the different circumstances that have been pointed out, and some others which he mentions in addi tion to them. He concludes that, in many cases, not more than ata part of the power exerted is actually employed in performing the effect required ; and, although we may doubt whether all his calculations be equally well founded, yet, perhaps, upon the whole, the estimate is not exagge rated.
After pointing out so many circumstances in the animal mechanism, which contribute to the expenditure of mus cular power, we must not omit to notice, that there are a few instances of a contrary kind, where the structure of the parts tends to assist the muscles, and produces a saving of power. The heads of the bones into which the tendons are inserted, frequently swell out so as to form a conside rable projection, by which means the muscles act upon the bones at a less oblique angle ; and we observe this struc ture to exist more remarkably in those cases where the greatest exertion of muscular power is required, as about the trunk and the lower extremities. For the same pur pose, some bones have processes of considerable length, which appear to be intended solely for the insertion of muscles ; and the same would seem to be the use of the small detached bones which are occasionally found about the joints, such as the patella of the knee.
The cause of contractility, or the relation which it bears to the other powers or properties of matter, has given rise to many hypotheses and conjectures, some of which it will be proper for us to consider. The subject must be regard ed in two points of view : we may first inquire, what is the physical cause of the contraction of the fibre ? and, second, what is the cause or nature of that property of the fibre which enables it to contract ? Both these questions are obscure and of very difficult solution ; and we shall find, that, notwithstanding all the attempts that have been made to solve them, very little light has been thrown upon the subject. With respect to the efficient cause of muscular contraction, every explanation which depends upon mere mechanical principles must be necessarily abortive, nor shall we be more disposed to place confidence in the opi nions of those who refer it to some chemical operation, as fermentation, or effervescence, excited by the mixture of an acid and an alkali ; or sonic occult or mysterious ope ration of the animal spirits distending the fibres; or to any electrical or galvanic arrangement which causes muscular contraction by its explosion. It may not, however, be un
interesting to give a short account of one theory, not so much from any merit which it possesses, as from its being a specimen of the manner in which the most learned men of the seventeenth century misapplied their learning, by permitting themselves to confound a train of reasoning with a deduction of facts. The theory to which I refer is that of Keill, a man of considerable ingenuity ; and who was well versed in the mathematical sciences, which were then deemed the basis of all physical knowledge.
Keill began by adopting the notion of Borelli, that the muscular fibre consists of a series of rhomboidal vesicles which communicate with each other. He observes, that if these vesicles be distended, the length of the fibre must be diminished, and the muscle consequently shortened. 'I he blood and the animal spirits were made the agents for effecting this distention, which it was supposed was done by the fluids contained in the muscle heing rarefied. To produce this rarefaction the author has recourse to the supposition, that a quantity of condensed air is attached to the fluid particles of the blood, which is retained in its pre sent state by the pressure of the vesicle exactly balancing the distending force of the air ; but when a quantity of the animal spirits are brought into contact with the blood con taining the condensed air, the two fluids unite together, and a sudden rarefaction or increase of elasticity is the con sequence, which distends the vesicle and produces contrac tion. Having thus explained the nature of the effect that is produced, the author enters upon a long and abstruse train of mathematical reasoning, to show how the degree of effect may be indefinitely increased, without increasing the quantity of the elastic fluid which is set at liberty. He observes, that by diminishing the size of each vesicle, and proportionally increasing their number, the same shorten ing of the fibre will be produced at the same time that the capacity of the vesicles taken together will be diminished in proportion as their number is increased; and as there is no limit to this operation, it follows that any bulk of gas may produce any quantity of contraction, in the ratio of one to several millions. It is scarcely necessary to remark, that the whole of this hypothesis is built upon suppositions which are entirely gratuitous, and that the whole is con sequently without the slightest foundation.