Let AB represent the fore-arm, BD the shoulder bone, C the muscle, E the tendon, F the insertion of the tendon into the fore-arm, and B the elbow joint. The contrac tion of the muscle tends to bring F to D, and, as D is a fixed point, this is effected by bending the joint B, raising up the point F, and, consequently the weight A. By con sidering the manner in which the muscle acts in this case, and referring the action to the mechanical principles of the lever, we find that the mechanism of the animal body is calculated to produce a great loss of power. This de pends upon the principle in mechanics, that in the action of levers, the power is to the weight as the distance be tween the weight and the fulcrum is to the distance be tween the power and the fulcrum. In the present case, the power of the muscle will be to the effect produced by it as AB 'is to FB; and, supposing FB to be of the length of AB, .A only of the power of the muscle is ex erted in raising the weight, the rest is expended in acting against the disadvantage of the position. We shall find it to be a law of the animal economy, with respect to the ac tion of muscles, that power is always sacrificed to conve nience. Had the object been to raise the weight in the hand with the least possible power, the muscles would have been placed near the wrist, and the tendon inserted into the lower part of the shoulder bone ; but, in this case, the awkwardness of the limb would have much more than counteracted the supposed advantage of a saving of mus cular power. The remark applies with still greater force to the fingers ; they are moved by the contraction of mus cles placed on the fore-arm, and connected to them by long and delicate tendons, which pass over the wrist and the hand. But if this order had been inverted, and the flesh of the muscles had been placed on the fingers, the hand would have been almost useless from its clumsy form.
There is, moreover, a decided advantage in the present arrangement, that, although we lose power we acquire velocity, and this in the same proportion.
Let NVPF represent the bone of the fore-arm, in which NV is the weight, P the power, and F the fulcrum. Now, let us suppose that the elbow-joint is moved, so as to bring the arm into the position ABF; from the centre F draw the arcs ANN' and BP, and it will appear, that, while the power is passing through the small arc PB, the weight is describing the large arc A W. Now the arcs are to each other as the lines NVF and PF, and the arcs are passed over in the same time ; the velocity will therefore be as the lines NVF and PF, or, in this case, as ten to one.
There is another circumstance in the mechanical struc ture of the muscles which causes a large expenditure of power, but which is compensated by the advantage of its saving the quantity of contraction, or in enabling a muscle to perform more by the same degree of contraction than could otherwise have been done. \Vc refer to the oblique position of the muscular fibres.
Let A and C be two fixed points in a bone, and let B be a moveable point, which is to be brought down to D. If this were done by a straight fibre, passing in a perpendicular direction BC, it would be necessary that the fibre should contract equal to half its whole length, supposing BD equal to DC; but if we employ the oblique fibre AB, the effect is produced by its contracting through the space EB only, which is less than half its length by EF, AB being bisected in F.
It is evident that the smaller is the degree of the contrac tion of the fibre, and the less alteration the muscle conse quently experiences in its general form, the metre conve niently will the action be performed. There will be less displacement of parts, less pressure upon the vessels and nerves, the antagonist muscles will be less stretched, and, in short, there will be less change in the relative position of the organs, or of the different parts of the same organ.
A third source of loss of power depends upon the cir cumstance of two or more muscles concurring in the same object, and, therefore, having a certain degree of obliquity to each other, and, in so far as this is the ease, acting as antagonists. When two oblique muscles act upon the same point, the effect will be to draw it down through the diago nal ; we, therefore, have it in our power to alter the direc tion of the motion with great facility, by throwing a little more or less energy into one or other of the muscles, and thus moving the body into any of the intermediate posi tions, so that a number of motions may be performed by two muscles only.
A great loss of power, likewise, arises from the tendons being generally inserted into the bone at a very acute angle, whereas, in order that the power might have been exerted to the most advantage, the muscles ought to have acted upon the lever in a perpendicular direction ; and, upon the same principle, power is lost by having the mus cular fibres inserted obliquely into the tendons. But, in these cases, it is obvious, that the present arrangement is much more convenient, and conducive to the symmetry and beauty of the form. It is generally stated by writers on the mechanism of muscular motion, that half the power of the muscles is lost in consequence of the two ends pull ing against each other. Where one end of a muscle is attached to a fixed point, as much power is expended on this as on the moveable extremity ; and before the latter can produce any effect, it must counteract and overcome the resistance offered by the former.
There are probably other causes, besides those which have been mentioned above, by which there is a loss of power in the contraction of the muscles ; but, in all cases, this loss of power will be found to be compensated by some corresponding advantage. It may be conjectured, that, in proportion as the contraction of the fibre advances, a great force becomes necessary to produce each suc cessive degree of contraction, and the more the fibre is shortened the more painful the exertion becomes, and the more fatigue is induced by its continuance. Hence we may percieve why power is always sacrificed for the pur pose of diminishing the quantity of contraction necessary for effecting a certain change of position, as well as for increasing the velocity of the motion, or promoting the symmetry of the form.