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other (as by joint), by force being applied, as here described, the opposing surfaces will be brought into closer compact and opposition; and by being brought into opposition by a force acting, or channeling in a course parallel with the long axis, it follows that a force acting in the line of the long axis of a rod, will, whilst passing through, or across its sectional partitions, run in the same course or direction as in the remaining portions which are not intersected, which course being that of the long axis of the rod, the force passing through the intersection or joint, will be in the line of the long axis also, or, in other words, in a straight line or nearly so, which line of force is here called the line of muscular force.
But supposing, as in figure 2, the muscular force, at the extremity M' P is interrupted at the point G with gravitating force, or as in figure 3, at the point G J, with the same force, then the muscular force being checked or interrupted at the said points, according to the amount of the gravitating force which meets the muscular at a more or less acute or obtuse angle, but seldom diverging very greatly from the right angle, and also according to the degree of counterpoising muscular force, a new or different line of force will result diagonal to the antagonistic forces. The force resultant upon the combined action of gravitating and muscular forces, whose direction, from the position, or rather Tine of direction, maintained for muscular force alone, must, in the present case, he more or less oblique or angular, in its deflections from one portion of the rod or bone to the other, and in such direction it must pass through the joint or intersection herein set forth.
If, then, the opposite states of consciousness of force, as those of power and strength, or of weight, result from the direction in which force is applied to the surfaces of joints, it follows, for all experiments conducted with the view of proving such a position that they should have within them, to a greater or less extent, the elements essential to the sustaining and demonstrating such a distinct feature, and, if I may so say, crucial a result. Experiment, therefore, only remains to prove and sustain the hypothesis, that, to synovial membranes sentient nerves are distributed, whose function is to take cognizance of force, as applied to joints, whether it be gravitating or muscular.
The standard from whence the experiments will be measured, will, as a rule, be based upon the supposition that muscular force, unchecked by gravitating, is accompanied by a feeling or consciousness of power or strength, which, when counterpoised by gravity, is exchanged to that of weight, and in the following experiments the circumstances and conditions under which weight can be best felt will occupy the greatest share of attention, whilst the contrasting feelings of strength or power shall not be overlooked.
Again, in attempting to repeat the experiments herein detailed it may be necessary to say a word or two. Force, like smell, taste, touch, etc., is very variable in different individuals, and unless due regard be paid to this part of the subject, no care in the conducting of the experiments will avoid a perfect failure. The best rule to attend to in this matter is to ascertain from the party operated upon, or the operator who may try them upon himself, whether he has a good idea of the weight of bodies by taking them in the hand, and trying to weigh them; if he has a good idea, then the experiments, cceteris paribus, will succeed, but if he is deficient upon this point, the probabilities are, that they will fail.
Moreover, in prosecuting these experiments great care will be required in attending to the details given with each experiment, and much patience in repeating them two or three times, until some facility is acquired in readily seeing through the conditions requisite for their successful prosecution.
In order that an experiment may determine the point which it is designed to elicit, it is necessary that the object to be experimented upon be somewhat understood in those parts in which the end to be elicited is closely connected; and in the present instance an examination of a few leading points connected with the mechanism and direction of forces pertaining to the joints, may be found convenient and suitable; each joint, in its course, receiving sufficient examination to enable the experimenter to judge of the value of any result obtained from such an ordeal as that of careful experiment.
Suppose we commence with the foot.1 So far as function is concerned, it may be said, that the foot possesses three fixed points or axes, for the weight of the body to rest upon, namely, the heel, or posterior axis; the ball of the great and second toes; and the cushion, or ball of the fourth and fifth toes, especially the fifth or little toe, the former of which might be called the internal^ and the latter the external axes of the foot.
It is from this mechanical arrangement that man is enabled to stand upon one foot for an indefinite period; and to recover himself when he has in some degree lost his balance. Birds, likewise, are enabled to stand for an indefinite period upon one leg, from a similar principle of mechanism pertaining to their feet, as in most aquatic species, etc.; the tarsal terminus constituting the posterior axis, and the internal and external toes the anterior axes, regulated
1 Before entering upon any remarks relative to the mechanism of the foot, I feel obliged to express my deep regret, that, from the nature of the subject matter of this paper, I am not capable of employing the improved detariptive nomenclature which its anatomical relations have naturally suggested to Mr F. 0. Ward, when describing the osseous structure of the foot in his work, already referred to, on Human Osteology ; but this paper was written previouslv to my having perused Mr W.'s work, and the nomenclature, as here supplied*, being adapted to function, in relation to the sense maintained, and not to descriptive anatomy, upon mature consideration it appeared best to me to persist in what I had first written, than to alter it for courtesy's sake, and at the expense of making the subject less intelligible; while at the same time, I do not in anywise desire to claim any superiority, far less priority, in giving, in many respects, an almost identical description of the functions"and motions of the foot.
by the middle toe; but where such arrangement is not made, as in the psittacidse, or parrot family, standing is difficult, or almost impossible. So likewise the ouran-outan (alias orang-outang), is obliged, in progression, to preserve its balance by a stick, from the absence of the heel (the os calcis being, in that animal, very small), and also from the want of a double arrangement of axis to the^anterior part of the foot, the weight being chiefly received upon the external border of the foot, or rather, modified hand; ana as for standing upon this instrument for any length of time, the execution, with such a mechanism, is impossible; for, whilst two axes, or fixed
points (with which signification the word axis is here used) are sufficient to balance and rest the superincumbent weight placed upon them, with the ability of changing position by throwing the body upon one or the other entirely; as when the body rests upon the heel and great toe, by raising the heel the body falls entirely upon the ball of the great toe; yet to preserve one position for a very limited period a second axis is necessary, owing to the variety of points from which muscular force is applied during the alternate contraction and relaxing of muscular structure; and when standing on one leg is performed with ease, and for long, as in birds, and also in man, a third, or regulating axis is indispensably necessary.
The relation which these axes have to the sense of force will appear evident w7hen we come to examine their osseous connections. But it may here be remarked, that the pads or balls of the several axes, consisting for the most part, of dense cellular tissue, in which is enveloped variable quantities of adipose tissue, serve the important function of checking concussions, from the weight of the body falling suddenly upon those parts wThich their structure is adapted to protect; yet, as these pads have no direct connection with the osseous arrangement of the foot, further examination is unnecessary.
As the astragalus is that bone through which all weight, or force, from the body is transmitted to the foot, and also, as it is the common receiver of all force applied to the foot, as by muscles, to raise the body from the surface upon which it rests, the tibio-astragaloid articulation being the communicating medium, it may be well, at this part of the descriptive outline, to mark out the order in which these several axes are impressed during progression, and with it the probable amount of weight received by the articulation referred to from the sucessive axes.
The posterior axis, situated on the posterior border of the os calcis, receives the weight of the body first, when the foot is projected forwards and placed upon the ground, as in the act of walking; whilst the anterior external axis of the foot reaches the ground nearly simultaneously with it, so that the force of the body and consequent weight is divided between them. These two axes form the first firm resting point or platform for the body to rest upon; whilst, by a combination of movements in the lower extremity, but especially by the combined contraction of the soleus and gastrocnemieus muscles, the weight of the body is thrown off the external axis to the anterior internal axis, or ball of the great toe, and the heel or posterior axis. These twro axes—the posterior and anterior internal—preserve their sustained function to the body but for an exceedingly limited period, the whole weight of the body being speedily thrown upon the anterior internal axis only—the immense force applied between the tibio-astragaloid articulation and the ball of the great toe being received upon a powerful arch or bridge, whose summit is immediately over the scaphoid bone. In these successive movements, by careful examination, it will be perceived that the heel or posterior axis has a most important function to perform as a balancing or regulating axis, whilst the direction of weight from the body is transferred from the external to the internal anterior axis; or that axis which is destined, in successive movements, to receive and sustain the whole weight of the body during the period occupied bv the opposite limb in wielding it off from behind forward to be planted upon a like posterior axis or heel, to undergo like successive motions, tillprogression ceases in rest.
From the foregoing remarks, aided by a consideration of their osseous connection, it will be perceived that if the weight of the body be received upon the posterior axis of the foot, though the os calcis is so firmly and closely articulated with the astragalus, yet that the whole force is not transmitted from the calcis to the astragalus, and thence to all parts with which the astragalus is articulated; since part of the force is directly transmitted from the calcis to the cuboid, and to all bones articulated with it, especially to the fourth and fifth metatarsal bones. Consequently, if sentient nerves distributed to the tibio-astragaloid articulation, give consciousness of weight, the same weiyht applied to this axis will be less perfectly determined than where it is more directly transmitted, especially as it is brought early to composition by the force or weight of the body being speedily received upon the external axis, towards which point it is travelling from the heel, at the time the weight of the body is being received upon this axis—the external.1
Again, the anterior external axis, including the heads of the fourth and fifth metatarsal bones, does not transmit force applied to it by a direct line of osseous continuity to the astragalus, but through the medium of the os calcis; by which means, much of the force applied to it is expended in the os calcis, etc., before reaching the astragalus. Hence, only a limited amount of force applied to the anterior external axis, reaches the tibio-astragaloid articulation.
Lastly, the anterior internal axis, situated between the heads of the first and second metatarsal bones, but chiefly occupying the head of the first metatarsal bone, transmits its force directly to the astragalus, through the medium of the internal and middle cuneiform bones, and the scaphoid. The articulations of the tarsal bones with each other, admit of such slight motion, that their ability to transmit force in the direct line in which it is received, is here taken as a groundwork for estimating the course and distribution of force, in the same way as a continuous solid structure, making suitable allowance for the amount of cancellous structure existing in the centres of these bones.
But before we leave this mechanical subject, it may be asked, What do I make of the third toe? Suppose we view it as a communicating and moderating medium, directing the force applied to it, according to the conditions in which it may be placed, a little more to the external axis at one time, and at another time a little more to the internal axis, of the foot; since the peculiar manner in which its osseous relations are wedged in between the cuboid and scaphoid bones allows, if the conditions are supplied, this variable way of directing or shifting force—in the same manner, but to a much more limited extent—as the middle claw of the bird is applied to equipoise and adjust the course of force.
Having thus far examined a few leading points in the mechanism of the foot, it may not be amiss to make a few remarks relative to the synovial membranes pertaining to this instrument.
1 Vide Fig. iv.