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to prevent the conduction of heat, and painted of one uniform colour. The first shall be composed of cork, the second of dry oak wood, the third of silver, the fourth of lead, and the fifth of platinum; and furthermore, let each ball be accurately fitted into a cup or die. In this condition neither sight, hearing, smelling, tasting, nor touching, could detect the difference between one ball and another ; but let the same balls be placed in the hand, and muscular force be applied to counter-balance gravitating, and immediately one mystery will be solved, namely, that although all had one colour and size, yet, that each of them was composed of different material. And why such an inward conviction ? Evidently because a feeling exists that each requires a different degree of muscular force to overcome the gravitating, as compared one with the other. Or, in other words, that each had a different weight compared one with the other, and, therefore, that each must be composed of different material.
The inference of their being composed of different materials, of course, depends upon previous experience; but that experience rests upon a feeling, which the conscious party describes as depending upon his acquired or past knowledge of weight in bodies, in other respects outwardly very similar.
If, then, there is a feeling within which will decide so curious and certain a result, and which none of the five senses could so do, what is it but that a different and independent sense determines what they have failed to accomplish ?
Having thus far pleaded for the existence of a sixth sense, sufficiently varied to claim for it the more weighty examination of results, elicited from an extensive series of experiments, I shall, for convenience sake, though the inference of there being a sixth sense be still sub judice, refer to it as existing and real, and shall, forthwith, enter into an inquiry as to the means used by nature to regulate and determine the different results experienced in the sensations, when muscular force is applied to counterpoise gravitating.
Though the present paper limits the inquiry, with regard to anatomical region, to the nerves distributed to synovial membranes of joints, yet no attempt is here made to demonstrate their presence by dissection. The more so, from the fact that numerous authorities, Sharpey and Quain in particular, who agree in inferring the presence of sentient nerves in bone, ligament, and synovial membrane from their sensibility to injuries and acute pain when diseased, especially synovial membrane; yet none of the authorities consulted have been able to trace the ultimate nerve fibres, or their modes of distribution, in these tissues. Nay, even in the very analogous structure of serous membrane to that of synovial, the same want of demonstration exists, though so susceptible to acute pain from the slightest amount of inflammation, or any other form of disease that may affect it. But the fact that none of these structures are subject to great pain when mechanically irritated, only shows that the stimulus applied to their structure is not adapted to excite their
sentient powers; but that sudden injury of a mechanical nature can excite their sentient powers when properly applied, is most evident from the fact, that when loose cartilage is compressed between a joint-as the knee-most excruciating pain is immediately experienced.
It therefore being assumed that synovial membranes have sentient nerves, the inquiry of the present paper is limited to their function, and not to their course and distribution.
Again, on the other hand, it is not deemed necessary to discuss at length the suitability in the anatomical seat of nerves, distributed to synovial membranes, for taking cognizance of such a stimulus as force, when applied to their periphery, since, saving the sphincters, the occipito-frontalis, and a few muscles pertaining to the face, palate, æsophagus, the eye, and the ear, with their appendages, the whole of the muscles of animal life effect and complete their leverage force through the appliance of joints; and, therefore, whatever of force is applied by muscles, must, according to its amount, pass in like degree through these fulcra of bony structures; and, if nerves capable of taking cognizance of muscular force be there distributed, they will, by such locality, be best secured from mechanical violence, and most suitably placed to fulfil the office of their own peculiar function.
Without, then, any further remarks upon locality in relation to nerves of force, let the attention be rather directed to the mechanical arrangements that determine a recognition of kinds or degrees of force, as already explained.
The kinds or degrees of force which have been referred to, are recognised, it is conceived, by the different mechanical modes in which force is conveyed to the sentient nerves of joints, by the structures with which they are connected, as bone, fibro-cartilage, and true cartilage, the mechanical structure of each having a very definite and certain effect upon the mode or way in which force is made to impress the peripheral nerves.
As bone is the chief material through which muscular and gravitating forces are made to channel ere they reach the joints, a few remarks upon its mechanical structure, not inicroscopical texture, may be here very appropriately introduced.
Bony structure, as is well known, is haril, inelastic, or nearly so, and, according to its mechanical arrangement, presents different facilities for resisting external injuries; for instance, the long bones have two distinct kinds of mechanical arrangements, the shaft being composed of dense and compact material, whilst the extremities, as the femur and humerus, are composed of cancellous structure, or delicate bands of osseous tissue, arranged in an irregular net-like order, not very dissimilar to the siliceous matter or spicula of the poripheræ, denuded of their soft substance, by boiling in nitric acid. But the long bones do not all present this cancellated structure in equal degrees at either extremity, as the inferior extremity of the tibia, compared with the superior; the radius and fibula, as compared with the humerus and femur. The texture of the short bones is very variable with regard to the amount of cancellated structure; but, as a rule, it may thus be stated, that in proportion to the sinallness of the bone, so does the hard compact external layer become more developed, and the cancellated interior become more limited, and less marked, and the larger the short bones are, the more of the reverse of this rule holds good.
The design of such structures or mechanism, in addition to that of resisting force by its compactness in the shaft, and warding off sudden concussions in the joints, by the expanded and porous extremities diffusing the force applied to them, has assigned to it the further important function of regulating the direction of force; as in the long bones, when a force is communicated at one extremity it runs along in nearly a straight line, till it reaches the opposite extremity, where, by meeting with the cancellous structure at that extremity, according to its extent and delicacy, so must the force assume some new course, not in relation to the course or direction in which it was first received, but in the direction which the cancellated osseous structure will permit it; for of necessity, the force will channel along its structure, it being denser in its structure than the interstices, which are occupied with some softer substance or fluid, either liquid or gaseous; and, moreover, from the very direction or interlacing of the cancellous structure, much of the force will come to composition and resolution before it reaches the surface of the joint itself.
But, besides the different structures in bone regulating the direction of force, there are two other structures of great importance in relation to the present inquiry; the first of these is true cartilage, a tissue both hard and elastic, and whose structure is composed of cells arranged in a columnar order, very closely resembling in their regularity certain basaltic rocks, as those of Fingal's Cave and the Giant's Causeway. These columns being arranged in a line with the long axis of the bones, will of course transmit force received by them from the articular surface of bones to the synovial membranes, in the same direction in which it is applied; since, in addition to its being regular in structure, it is also hard and dense to a considerable degree. Hence, where joints are only tipped with true cartilage and not with articular fibro-cartilage, as in the ankle and wrist joints-save the ulnar border of the wrist joint—the nature of the osseous structure entering into the joints need only be considered in estimating in what direction force would reach the synovial membrane, after it has left the osseous tissue to pass through the cartilage, before reaching that membrane. Since, in whatever direction it was received by the cartilaginous structure, in the same direction-or very nearly so—it will be transmitted to the synovial membrane.
The remaining structure we have to attend to is the articular fibro-cartilage, which is found bordering and overlapping the true cartilage in almost all the large joints, as the shoulder, hip, and knee-joints, etc. This structure, it need scarcely be mentioned, is anything but regular in its density or depth of substance, from within outwards, and is much more yielding and soft than true cartilage, though very elastic. The effect of such a structure would be, from its very softness and irregular density from within outwards, to vary and cast into some constant and uniform direction, any force which might be received upon its surface; of course, the completeness with which such new direction of force would be effected, would depend very much upon the thickness of the cartilage against which it first impinged; but this consideration, in a practical light, is of very little importance, since, in nearly every instance, where this substance enters into the composition of joints, cancellated osseous structure is associated with it, which has as great or greater modifying effect upon the direction of force, according to its amount in any particular joint, as has the articular fibro-cartilage, and therefore their modifying effect in relation to the direction of force in joints may be counted as unity, both tending to accomplish one and the same end. And it must be observed, that in large joints where the fibro-cartilage is the thickest, namely, at the circumference of joints, there also, namely, towards the external surface of joints, the cancellated structure becomes more dense and less spongy, and will therefore transmit force in the direction in which it is received more correctly than towards the centre or interior of the joint; but here the fibro-cartilage is a mere lamina or entirely absent, whilst towards the circumference, to countercheck the denser cancellated structure, it affords a deeper border for force to pass through.
Granted that cartilage, fibro-cartilage, and the different textures of osseous tissue have an effect upon the course or direction of force transmitted from the shaft to the joint, and that the combination of cancellated osseous structure at the extremities of bones, and articular fibro-cartilage, in its relative densities and thickness, tend to compel force, whilst passing through their structures into one uniform and constant line or direction, it will be very naturally asked, what is that line or course which force, so circumstanced, pursues ? The probabilities are, that under such circumstances, it enters the joints in a straight line, or thereabouts, and the result of such direction, according to its intensity, is, that to the recipient, or conscious party, a feeling or consciousness of power or strength is experienced. Whilst, on the other hand, when the mechanism of the tissues entering into the formation of a joint, are so constituted as to admit of force entering into their structure assuming more directions or courses than one, that in such case, if the force, from any incidental cause, or opposing obstacle, shall be deflected from the straight line, or thereabouts, when travelling along the course of the long axis of bones, to an oblique course, or to a course almost at right angles to
NEW SERIES.-NO. II. FEBRUARY 1855.
the line of the muscular force (or that nearly in a line with the long axis of bones), that then the conscious party will experience within, according to the intensity of the force, a feeling or sense of weight.
It is, then, upon the stability of the direction of force, in these two opposite and distinct courses, that the entire integrity, and demonstrability of the accuracy of the present hypothesis is based.
Before proceeding with any experiments, I shall at this juncture of my paper, between theory and practice, take the opportunity of pressing home the above positions of the direction of forces in the oblique, or nearly midway right angle course, and the straight line, or thereabouts, by a few illustrations and diagrams.
Suppose M F and M' F represent either extremity of a rod whose continuous integrity shall be interrupted by an intersection, or joint, at the point J. Let cords extend from either extremity of the rod (above and below), endowed with contractile, or shortening powers. The cords contracting, or, in other words applying force, the residual force unexpended in the cords (alias muscles), will be expended in moving and regulating the position of the rod (or bone), and, in so doing, will run nearly parallel with the long axis of the rod, from the nature of the attachment of the cords; and, as a necessary consequence of the section or division of one part of the rod from the