Biomechanics

“ However delicately a machine may be constructed, with the most exquisitely contrived detents to release a weight or spring by the exertion of the smallest possible amount of force, some external force will always, be required; so, in the animal machine, however minute may be the changes required in the cells or fibres of the brain, to set in motion the nerve currents which loosen or excite the pent up forces of certain muscles, some force must be required to effect those changes. ”

“ In the first case, ordinary mechanical or translational energy is spent as friction, an amount measurable in foot-pounds, and the factors we [Pg 98] know, a pressure into a distance. If the surface be of the same kind of molecules, the whole energy is spent as heat, and is presently radiated away. If the surfaces are of unlike molecules, the product is a compound one, part heat, part electrical. What we have turned into the machine we know to be a particular mode of motion. ”

“ And thus wherever we probe the problem we find ourselves able to explain many secondary problems, while the fundamental ones we still attribute to the vital properties of the active tissues. Why a muscle contracts or a gland secretes we have certainly not yet answered. The relation of the actions to the general problems of correlation of force is simple enough. That a muscle is a machine in the sense of our definition is beyond question. But the problem of why a muscle acts is not answered by showing that it derives its energy from broken food material. ”

“ Iron and brass are simpler things than flesh and blood, and dead wood than living nerve; and a machine constructed of the one works in more definite ways than an organism constructed of the other,—especially when the machine is worked by the inorganic forces of steam or water, while the organism is worked by the forces of living nerve-centres. Manifestly, then, the ways in which the machine will work are much more readily calculable than the ways in which the organism will work. Yet in how few cases does the inventor foresee rightly the actions of his new apparatus! ”

“ Two bodies, such for instance as a screw and nut, having such forms that at any instant there is only one possible motion for each relatively to the other, form the simplest combination available for machinal purposes—such bodies he calls a pair of elements. Two or more elements from as many different pairs can be combined into a link, and such links united into kinematic chains, and it is by fixing, that is, preventing the motion of, some one link of such a chain that a mechanism is obtained. ”

“ The biologist will admit that an organism is a mechanism, and (in so far as its bodily structure is concerned) a material mechanism. All the actions of this structure are therefore mechanical, in the two senses of the term: first, as being the actions of material adjustments; secondly,375 as being movements, and thereby included under the general laws of motion represented in Mechanics; the abstract laws of movement for an organic body are not different from the abstract laws of movement for an inorganic body. ”

“ It may deform a fixed body against its external supports and resistances. In making impact tests in the laboratory the test specimen is in reality in the nature of a cushion between two impacting bodies, namely, the striking weight and the base of the machine. It is important that the mass of this base be sufficiently great that its relative velocity to that of the common centre of gravity of itself and the striking weight may be disregarded. ”

“ The attractions of electricity or of magnetism do not apply philosophically to the illustration of the contraction of animal fibres, since the force of those attractions increases in some proportion inversely as the distance, but in muscular motion there appears no difference in velocity or strength during the beginning or end of the contraction, but what may be clearly ascribed to the varying mechanic advantage in the approximation of one bone to another. ”

“ In examining mechanisms we consider the motions of each body as a whole, ignoring altogether its molecular condition, or more strictly assuming that it is so arranged that its molecular stability is not disturbed during the motion. ”

“ It has been confounded, under the common name of Mechanics, with the other science, Mechanics Proper, or Machinery, which considers the effect of force transmitted by Mechanism from one part of a material combination to another. For example, the Mechanical Powers, as they are usually termed, (the Lever, the Wheel and Axle, the Inclined Plane, the Wedge, and the Screw,) have almost always been treated with reference to the relation between the Power and the Weight, and not primarily as a mode of changing the velocity and kind of the motion. ”

“ Energy distributed between two Bodies: Projection and Propulsion.—Hitherto the effort by which a machine is moved has been treated as a force exerted between a movable body and a fixed body, so that the whole energy exerted by it is employed upon the movable body, and none upon the fixed body. This conception is sensibly realized in practice when one of the two bodies between which the effort acts is either so heavy as compared with the other, or has so great a resistance opposed to its motion, that it may, without sensible error, be treated as fixed. ”

“ The Muscle. mechanical work is obtained by movement resulting from a change, it is supposed, in the elastic tension of the framework of the living cell. In the fibrils existing in the cell a sudden alteration of elasticity occurs, resulting in an increased tension on the points of attachment of the cell to the neighbouring elements of the tissue in which the cell is placed. These yield under the strain, and the cell shortens between those points of its attachment. This shortening is called contraction. ”

“ The modification of motion and the modification of force take place together, and are connected by certain laws; but in the study of the theory of machines, as well as in that of pure mechanics, much advantage has been gained in point of clearness and simplicity by first considering alone the principles of the modification of motion, which are founded upon what is now known as Kinematics, and afterwards considering the principles of the combined modification of motion and force, which are founded both on geometry and on the laws of dynamics. ”

“ Among the forces which determine the forms of cells, whether they be solitary or arranged in contact with one another, this force of surface-tension is certainly of great, and is probably of paramount importance. But while we shall try to separate out the phenomena which are directly due to it, we must not forget that, in each particular case, the actual conformation which we study may be, and usually is, the more or less complex resultant of surface tension acting together with gravity, mechanical pressure, osmosis, or other physical forces. ”

“ Where, contrariwise, an exerted muscle which has undergone much molecular change in evolving much mechanical force, has soon again to evolve much mechanical force, and so on continually; it is clear that either the quantity of contractile substance present must be great, or the apparatus for nutrition and depuration must be very efficient, or both. Hence we may look for marked unlikenesses of minute structure between muscles which are markedly contrasted in activity. ”

“ In the production of motion three important systems are employed—the muscular system, the nervous system, and a system of mechanical devices which are found mainly in the skeleton. The muscular system supplies the energy for operating the mechanical devices, while the nervous system controls the movements.78 Although the skeleton serves other purposes, such as giving shape to the body and protecting certain organs, its main use is that of an aid in the production of motion. ”

“ This muscular electricity may be obtained in tolerable quantity by placing a number of slices of muscle together in the form of a pile. The positive pole of the system will be the natural surface of one of the terminal slices, and the negative pole the cut surface of the other. ”

“ Though, could the muscles withstand any tensile strain they were subject to, the power like the weight might increase with the cubes, yet since the texture of muscle is such that beyond a certain strain it tears, it results that there is soon reached a size at which flight becomes impossible: the structures must give way. In a preceding paragraph the limit to the size of flying creatures was ascribed to the greater physiological cost of the energy required ”

“ For from hence are easily deduced the forces of machines, which are compounded of wheels, pullies, levers, cords, and weights, ascending directly or obliquely, and other mechanical powers; as also the force of the tendons to move the bones of animals. ”

“ If I have devoted more space to particular people and episodes than is warranted by their contributions to the modern treatment of the subject, it is because I have found that the history of kinematics of mechanisms, like the history of any other branch of engineering, is more interesting and more plausible if it is recognized that its evolutionary development is the result of human activity. ”

“ Under Function in its widest sense, are included both the statical and the dynamical distributions of force which an organism opposes to the forces brought to bear on it. In a tree the woody core of trunk and branches, and in an animal the skeleton, internal or external, may be regarded as passively resisting the gravity and momentum which tend habitually or occasionally to derange the requisite relations between the organism and its environment ”

“ It is obvious that this mechanical improvement is especially applicable wherever cycles occur in the mathematical operations, and that, in preparing data for calculations by the engine, it is desirable to arrange the order and combination of the processes with a view to obtain them as much as possible symmetrically and in cycles, in order that the mechanical advantages of the backing system may be applied to the utmost. It is here interesting to observe the manner in which the value of an analytical resource is met and enhanced by an ingenious mechanical contrivance. ”

“ The study of kinematics has been considered for nearly 80 years as a necessary part of the mechanical engineer's training, as the dozens of textbooks that have been published over the years make amply clear. ”

“ Thus in animal mechanics we have to consider the various orders of levers, the pulley, the centre of gravity, specific gravity, the resistance of solids, semi-solids, fluids, etc. As the laws which regulate the locomotion of animals are essentially those which regulate the motion of bodies in general, it will be necessary to consider briefly at this stage the properties of matter when at rest and when moving. ”

“ It is impossible, however, to get all of the energy of the striking body stored in the specimen, though the greater the mass and the shorter the space through which it moves, or, in other words, the greater the proportion of weight and the smaller the proportion of velocity making up the energy of the striking body, the more energy the specimen will absorb. ”

“ In what manner this molecular motion of the particles of the body is transmuted into molar motion of the body, is not known except by a few scientific men who see that molecular motion of the particles is transmuted into the molar motion of the body through the metabolism of the muscle, and that this motility or self-activity is controlled by the will which controls the choice or affinity of the molecules of the muscles. ”

“ It must be remembered, however, that a more thorough and complete study of physiology can not be undertaken without a considerable acquaintance with such sciences as mechanics, hydraulics, optics, etc., without which the action of the muscles, the circulation of fluids, and vision can not be properly and fully comprehended.
Whenever a piece of mechanism, designed for some particular use, is brought under our notice, and we wish to understand its manner of working, we naturally inquire about its structure ”

“ The actions of the machine are subordinated; the actions of an organism are co-ordinated. The lever moves a wheel, and the wheel in moving liberates a spring, each transmitting a communicated impulse, but otherwise each acts independently—no slight modification in the structure or movement of the wheel will modify the structure or the movements of the lever, no alteration in the tension of the spring will affect the structure of the wheel. ”

“ The construction of the machine is such as to modify the natural motion which is impressed upon it, so that it may transmit to the object to be moved that peculiar species of motion which it is required to have. To give a very obvious example, let us suppose that a circular or rotatory motion is required to be produced, and that the only natural source of motion at our command is a perpendicular fall of water: a wheel is provided, placed upon the axle destined to receive the rotatory motion ”

“ Having thus outlined the conditions under which the muscular machine performs its work, we have now to consider briefly the external mechanisms with the aid of which muscular energy is utilized. Of course, the simplest application of this power, and the one universally [53] employed in the animal world is that in which a direct push or pull is given to the substance, the position of which it is desired to change. We have already pointed out that there is no essential difference between pushing and pulling. The fact receives another illustration in considering the muscular mechanism. ”