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dent; some are known as much by the absence of one class of facts or phenomena, as by their own actual presence : thus, the shining of the stars by day is as certain as that of the sun's; but the very existence of stars, were it not for the absence of the sun, would never have been detected; and by day they can only be seen by
such instruments as exclude, in part, the sun's rays. Again, in i chemical analysis the effects of chemical re-agents, where the agent
to be acted upon is in very minute quantities, are so slight that their action might be altogether questioned, were it not for the fact that the re-agent, with the agent, can be contrasted with pure water to which the re-agent only has been added, when the difference is evident; but without such contrast, so slight is the change produced upon any given agent, when in very minute quantities, by the re-agent, that its presence or absence could not be satisfactorily determined. Such, then, is exactly the case in the two last experiments recorded. How a very minute weight is tested best by the hand being inverted, arises from all weight being entirely absent when the hand is gently raised, and pressed against the inter-articular cartilage of the head of the ulna, by inversion; but by rather quickly depressing the hand, the cartilage is gently compressed, and the hand in falling downwards naturally turns a little outwards, and so the articular surfaces of the scaphoid and semi-lunar approximate the radius, and thus induce a feeling of weight, by impressing a larger synovial surface gently and feebly, but certainly; whilst, when this pressure is taken off, by elevating the hand again, during the act of elevation, the feeling of weight is suspended. It is a curious fact, also, and worthy of great attention, that if the hand alone, or with a slight weight, be long gently elevated and depressed, whilst inverted, to assist the eversion, when the hand is depressed, and so enable the free articular surface of the radius to be gently counterpoised by the opposing carpal bones, that the whole of the thumb becomes congested and loaded with blood, whereby the external part of the hand is made heavier than the internal, and in the act of depressing the hand greatly assists in its very slight eversion, as a natural consequence of gravity acting from that point. Therefore the feeling of slight weights, when the hand is inverted, is not owing to the hand being better able to feel weight in the inverted position than in the straight position; but from the fact that very slight weights are scarcely discernible to the sense of force, and the only way of proving the presence of weight is to contrast the feeling experienced by its entire absence, as compared with its most limited, or minute, presence. Such an explanation, with such a contrast, whilst it is quite in accordance with correct anatomy and physics, cannot, I think, be legitimately explained upon a more definite basis than that herein assumed.
I shall now give my last written experiment, but my first practical, which led to the investigation of which this paper may be termed the chief summary.
Let the forearm be placed at nearly right angles to the arm (or humeral division), with the hand extended, and the fingers and thumb semi-flexed, the first phalanges being but very slightly flexed. In this position let a book about the size and thickness of Whewell's Bridgewater Treatise,” be placed upon the tips of the fingers and thumb, taking care to let each member distinctly touch the book. This being done, after having gently raised and depressed the book, for the purpose of weighing it, carefully remove it, taking care at the same time to retain in their exact sites the thumb and fingers. Whilst thus retained place a pair of compasses, with the point of one shaft over the transverse ligament, binding the heads of the index and middle fingers, and extend to the tip of the index finger the point of the opposing shaft. With the compasses thus adjusted describe a circle, whose centre, x, shall be over the transverse ligament before referred to, and whose circumference shall pass over, in order, the tips of all the fingers, the base of the metacarpal bone of the little finger, and the pad of the thumb (for no weight naturally rests upon its tip).
But to better apprehend this description, instead of continuing to burden the memory, where abstraction is chiefly required, I will endeavour to explain it by diagrams, with the aid of geometrical precision, rather than anatomical detail, as far as the subject permits.
Let a book be balanced on the tips of the fingers and pad of the thumb, and when removed let the various parts be retained in situ, as in fig. 6. Thus adjusted, describe the circle a, a, a, a, b, c, which shall respectively pass over the tips of the fingers a, a, a, a, the base of the metacarpal bone of the little finger, b, and the pad of the thumb, c, and whose centre, x, shall be immediately over the transverse ligament between the heads of the first and second metacarpal bones.
Let any radii of the circle a, a, a, a, b, c, F1o. 7. be bisected at equi-distance from either terminal point, as the radius x, a, at the point p, then the bisected radius, from either terminal point, as at the points x, p, shall be the extent of any radii of a second circle, d, d, e, f, f, passing over (when the fingers and thumb are brought together, with the hand extended), in order the metacarpo-phalangeal articulations of the index and little fingers, d, d, the base of the metacarpal bone of the little finger, e, and, less perfectly, over the base and head of the metacarpal bone of the thumb, f, f, and whose centre shall be y, situated near the centre of the hand, between the metacarpal bones of the second and third fingers. (Vide fig. 7.)
Let a straight line, 8, 8, be drawn between the second and third fingers, over the palm of the hand and the wrist, to three or four inches, or less, midway between the radius and ulna, over the interosseous ligament. This line shall intersect the circle, d, d, e, f, f, whilst crossing the wrist at the point 2.
The point z shall be the centre of a third circle, g, h, i, the extent of whose radii shall be equal to any radii of the circle d, d, e, f, f, bisected at equi-distance from either terminal point; as the radius y, r, bisected at the point t.
Where the circle, g, h, i, intersects the straight line, ss, at the part farthest from the fingers shall be the point desired, w, being immediately over that part where gravity, from a book, etc., will pass through the wrist-joint.
After having given these successive proportions, it may be said, Why not feel for the joint at once, and so settle the matter? In reply, I may state that, as yet, we have not got quite through the experiment; when it is completed we shall be better able to measure the importance of the point, w, 80 obtained.
1 The centre, y, is best attained after having adjusted the compass to the points x and p, by retaining one shaft of the compasses on the point x, and revolving it till the opposite shaft has attained the anatomical seat indicated.
ewis point, unled to the set the tips of it before trod co
The point w being now attained mark it with ink, and then re-weight the book as at the first; after having carefully estimated its weight in the mind remove it, but let the fingers and thumb be retained in situ, and then balance the book upon one of its corners, at the point w; at this point the book will weigh as heavy, or rather heavier, than when balanced on the fingers and thumb. If, then, at this point there was not some special provision for taking cognizance of gravity when placed as a counterpoise to muscular force, it is impossible to conceive how a weight, with so much increased leverage at the extremities, should weigh as heavy, or heavier, at this point, unless at this point the resultant of two forces is more directly applied to the sentient nerves recognizing this property in matter-weight, than at the tips of the fingers and thumb, where, if it have to travel to the chief joint before truly recognised, must lose much of the original force by friction and composition, and, consequently, be less perfectly felt than where, from its proximity to the sentient membrane, or nerves, such expenditure of force will not take place.
It must be observed that, if the book be placed anterior or posterior to the point w, the weight felt will be considerably less than at this particular point, the hand and fingers being preserved in their original position. Of course this check to the experiment only tends to confirm the previous inference.
Why it is better to place the book upon the point w, and not directly over the joint, as determined by feeling at the wrist, arises
from the fact that, in balancing a Fig. 8.
book upon the fingers and thumb,
the hand and forearm are either kept G in a line at right angles with the
arm, or else adjust themselves to a line more or less acute to that part: in one man, the ability to weigh with comfort is obtained by the hand and forearm naturally assuming one position with regard to the arm; whilst, in another, a slightly different position is assumed; and according to the position of the forearm and hand to the arm, so must the weight be
placed exactly over the joint, or half a line to two lines anterior to this part, for the gravity to pass through the greater part of the joint, or sentient surface, as is illustrated in the etchings, A and B, where gravity, G, is represented as passing through the joints at variable angles, according to the degree of flexion of the forearm upon the arm.
It will be asked again, But do not different individuals present very different proportions in their hands, let them be adjusted in whatever way it may be thought proper to measure their dimensions ? To this I
must reply by saying, that where the hand is not much used in severe hard labour, very little difference will be found in the most delicate hand, or the strongest, in the proportions here given, when the hand is properly adjusted—the little finger being the member most usually out of proportion, and perhaps all in a family will have the same peculiarity, yet this is any thing but common. The left hand, as a rule, is the best to measure from ; this hand being generally least exposed to injury from severe or constant pressure, etc.
Before closing my remarks upon the hand, perhaps it will not be deemed unbecoming, though not directly related to the subject here discussed, if I give one or two measurements of the thumb in relation to the hand, in as few words as the subject will permit—a well proportioned hand being here assumed.
With a pair of compasses measure from the base of the metacarpal to the head of the first phalangeal bones of the thumb—the dorsal side is the best to measure from. Adjust the compasses, by one shaft, over the base of the metacarpal bone of the thumb on the palmer side, and with the free shaft describe a circle; the same shall pass over the metacarpo-phalangeal articulations of the index and little fingers. Next, extend the hand, approximate the fingers, but abduct the thumb: with the compasses adjusted as at first, place the point of one shaft on the outer, or radial side of the metacarpophalangeal articulation of the index finger, with the free shaft describe a circle, which shall pass over the thumb and little finger opposite their nails. Lastly, with the hand extended and fingers closed, adjust the compasses across the palm of the hand, parallel with the metacarpo-phalangeal articulations, when the first, or thumb measurement, shall be equal to the last. As it is not the design of this paper to enter upon the mechanism of the hand, I shall not press any inference from the above measurements, but leave the same to better and more able hands than mine. Yet I would suggest to any who might be so disposed, that in deducing inferences from the measurements of the thumb, it will be convenient to view the thumb from the metacarpal to the first phalangeal bones as a moveable fulcrum, and the last phalangeal as the lever.
In the foregoing sketch scarcely any thing has been said of the sense of force in relation to the feeling of resistance or strength, neither is it deemed suitable to make any lengthened remarks upon this feeling, when we consider that vigour or strength is the normal state of this sense in health, since muscular force, whilst unantagonised by gravitation (saving that pertaining to the limbs themselves), naturally channels along the osseous structures, through the joints, in that direction best adapted to excite the feeling of resistance or strength. Whilst, on the other hand, if a weight be placed upon any of the parts already indicated in making experiments for weight, and in place of using ‘muscular force sufficient to counterpoise the imposed weight, that force is considerably plus over the gravitation (from rigidly contracting the muscles), then the gravitating force