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signed to each part. The arm is an instrument of motion, principally, if not solely. Accordingly the shallowness of the socket at the shoulder, and yieldingness of the cartilaginous substance with which its edge is set round, and which in fact composes a considerable part of its concavity, are excellently adapted for the allowance of a free motion and a wide range; both which the arm wants. Whereas, the lower limb, forming a part of the column of the body; having to support the body, as well as to be the means of its locomotion; firmness was to be consulted, as well as action. With a capacity for motion, in all directions indeed, as at the shoulder, but not in any direction to the same extent as in the arm, was to be united stability, or resistance to dislocation. Hence the deeper excavation of the socket; and the presence of a less proportion of cartilage upon the edge. The suppleness and pliability of the joints we every moment experience; and the firmness of animal articulation, the property we have hitherto been considering, may be judged of from this single observation, that, at any given moment of time, there are millions of animal joints in complete repair and use, for one that is dislocated; and this, notwithstanding the contortions and wrenches to which the limbs of animals are continually subject. II. The joints, or rather the ends of the bones which form them, display also, in their configuration, another use. The nerves, blood-vessels, and tendons, which are necessary to the life, or for the motion of the limbs, must, it is evident, in their way from the trunk of the body to the place of their destination, travel over the movable joints; and it is no less evident, that, in this part of their course, they will have, from sudden motions, and from abrupt changes of curvature, to encounter the danger of compression, attrition, or laceration. To guard fibres so tender against consequences so injurious, their path is in those parts protected with peculiar care; and that by a provision in the figure of the bones themselves. The nerves which supply the fore-arm, especially the inferior cubital nerves, are at the elbow conducted, by a kind of covered way, between the condyles, or rather under the inner extuberances of the bone, which composes the upper part of the arm.* At the knee, the extremity of the thigh-bone is divided by a sinus or cleft into two heads or protuberances: and these heads on the back part stand out beyond the cylinder of the bone. Through the hollow, which lies between the hind parts of these two heads, that is to say, under the ham, between the hamstrings, and within the concave recess of the bone formed by the extuberances on each side; in a word, along a defile, between rocks, pass the grea vessels and nerves which go to the leg.” Who led these vessels by a road so defended and so secured? In the joint at the shoulder, in the edge of the cup which receives the head of the bone, is a notch which is joined or covered at the top with a ligament. Through this hole, thus guarded, the blood-vessels steal to their destination in the arm, instead of mounting over the edge of the concavity.t III. In all joints, the ends of the bones, which work against each other, are tipped with gristle. In the ball and socket joint, the cup is lined, and the ball capped with it. The smooth surface, the elastic and unfriable nature of cartilage, render it of all substances the most proper for the place and purpose. I should, therefore, have pointed this out amongst the foremost of the provisions which have been made in the joints for the facilitating of their action, had it not been alleged, that cartilage in truth is only nascent or imperfect bone; and that the bone in these places is kept soft and imperfect, in consequence of a more complete and rigid ossification being prevented from taking place by the continual motion and rubbing of the surfaces; which being so, what we represent as a designed advantage, is an unavoidable effect. I am far from being convinced that this is a true account of the fact; or that, if it were so, it answers the argument. To me, the surmounting of the ends of the bones with gristle, looks more like a plating with a different metal, than like the same metal kept in a different state by the action to which it is exposed At all events, we have a great particular benefit, though arising from a general constitution: but this last not being quite what my argument requires, lest I should seem by applying the instance to overrate its value, I have thought it fair to state the question which attends it. IV. In some joints, very particularly in the knees, there are loose cartilages or gristles between the bones, and within the joint, so that the ends of the bones, instead of working upon one another, work upon the intermediate cartilages. [Pl. XI. fig. 3..] Cheselden has observed j that the * Ches. An. p. 35. # Ib. 30. # Ib. p. 13.
same; both are solid bone covered by cartilage, and both have a rim of
a strong fibro-cartilaginous texture, not only for the purpose of rendering
the socket deeper, but for preventing fractures of the rim in robust exer
tises, to which, were it bony, it would be very liable-Parton. Ches. An o 255, ed. 7th
contrivance of a loose ring is practised by mechanics, where the friction of the joints of any of their machines is great; as between the parts of crooked-hinges of large gates, or under the head of the male screw of large vices. The cartilages of which we speak, have very much of the form of these rings. The comparison moreover shows the reason why we find them in the knees rather than in other joints. It is an expedient, we have seen, which a mechanic resorts to, only when some strong and heavy work is to be done. So here the thigh bone has to achieve its motion at the knee, with the whole weight of the body pressing upon it, and often, as in rising from our seat, with the whole weight of the body to lift. It should seem also, from Cheselden's account, that the slipping and sliding of the loose cartilages, though it be probably a small and obscure change, humored the motion of the end of the thigh-bone, under the particular configuration which was necessary to be given to it for the commodious action of the tendons; (and which configuration requires what he calls a variable socket, that is, a concavity, the lines of which assume a different curvature in different inclinations of the bones.) V. We have now done with the configuration: but there is also in the joints, and that common to them all, another exquisite provision, manifestly adapted to their use, and concerning which there can, I think, be no dispute, namely, the regular supply of a mucilage, more emollient and slippery than oil itself, which is constantly softening and lubricating the parts that rub upon each other, and thereby diminishing the effect of attrition in the highest possible degree.* For the continual secretion of this important liniment, and for the feeding of the cavities of the joint with it, glands are fixed near each joint; the excretory ducts of which glands dripping with their balsamic contents, hang loose like fringes within the cavity of the joints. A late improvement in what are called friction wheels, which consists of a mechanism so ordered, as to be regularly dropping oil into a box, which encloses the axis, the nave, and certain balls upon which the nave revolves, may be said, in some sort, to represent the contrivance in the animal joint; with this superiority, however, on the part of the joint, viz. that here, the oil is not caly dropped, but made.* In considering the joints, there is nothing, permaps, which ought to move our gratitude more than the reflection, how well they wear. A limb shall swing upon its hinge, or play in its socket, many hundred times in an hour, for sixty years together, without diminution of its agility: which is a long time for anything to last; for anything so much worked and exercised as the joints are. This durability I
* This mucilage is termed synovia ; vulgarly called joint oil, but it has no property of oil. It is very viscid, and at the same time smooth and slippery to the touch; and therefore better adapted than any oil to lubricate the interior of the joints and prevent ill effects from friction. Parton
should attribute, in so. for the preventing of wear and tear, first by the polish of the cartilagin y ing lubrication of the mucilage; and, in part, to that astonishing property of animal constitutions, assimilation; by which, in every
portion of the body, let it consist of what it will, substance is restored, and waste repaired.t
* A joint then consists of the union of two bones, of such a form as to permit the necessary motion; but they are not in contact; each articulating surface is covered with cartilage, to prevent the jar which would result from the contact of the bones. This cartilage is elastic, and the celebrated Dr. Hunter discovered that the elasticity was in consequence of a number of filaments closely compacted, and extending from the surface of the bone, so that each filament is perpendicular to the pressure made upon it. The surface of the articulating cartilage is perfectly smooth, and is lubricated by a fluid called synovia, sygnifying a mucilage, a viscous or thick liquor. This is vulgarly called joint oil, but it has no property of oil, although it is better calculated than any oil to lubricate the interior of the joint.
When inflammation comes upon a joint, this fluid is not supplied, and the joint is stiff, and the surfaces creak upon one another like a hinge without oil. A delicate membrane extends from bone to bone, confining this lubricating fluid, and forming the boundary of what is termed the cavity of the joint, although, in fact, there is no unoccupied space. External to this capsule of the joint, there are strong ligaments going from point to point of the bones, and so ordered as to bind them together without preventing their proper motions. From this description of a single joint, we can easily conceive what a spring or elasticity is given to the foot, where thirty-six bones are joined together.—Bell’s Treatise on ...Animal JMechanics.
t If the ingenious author’s mind had been professionally called to contemplate this subject, he would have found another explanation. There is no resemblance between the provisions against the wear and tear of machinery and those for the preservation of a living part. As the struc ture of the parts is originally perfected by the action of the vessels, the function or operation of the part is made the stimulus to these vessels. The cuticle on the hands wears away like a glove; but the pressure stimulates the living surface to force successive layers of skin under that which is wearing, or, as the anatomists call it, desquamating; by which they mean, that the cuticle does not change at once, but comes offin squamae, or scales. The teeth are subject to pressure in chewing or masticating: and they would, by this action, have been driven deeper in the jaw, and
Movable joints, I think, compose the curiosity of bones but their union, even where no motion is intended or wanted, carries marks of mechanism and of mechanical wisdom. The teeth, especially the front teeth, are one bone fixed in another, like a peg driven into a board. The sutures of the skull are like the edges of two saws clapped together, in such a manner as that the teeth of one enter the intervals of the other.” We have sometimes one bone lapping over another, and planed down at the edges; sometimes also the thin lamella of one bone received into a narrow furrow of another. In all which varieties, we seem to discover the same design, viz. firmness of juncture, without clumsiness in the seam.
MUscles, with their tendons, are the instruments by which animal motion is performed. It will be our business
rendered useless, had there not been a provision against this mechanical
effect. This provision is a disposition to grow, or rather to shoot out of their sockets; and this disposition to project, balances the pressure which they sustain; and when one tooth is lost, its opposite rises, and is in danger of being lost also, for want of that very opposition.—Bell’s Treatise on J1nimal Mechanics. * Most of the bones of the skull are composed of two plates or tablets, with an intermediate spongy, vascular substance; the outer tablet is fibrous, having the edges curiously indented and united by a dove-tailed si'nre; the inner from its brittleness is called vitreous, and therefore n erely joined together in a straight line ; this mode of union is not accidental-not the result of chance, but design. The author of the treatiss on “Animal Mechanics” gives the following admirable illustration of the structure:– “Suppose a carpenter employed upon his own material—he would join a box with regular indentations by dove-tailing, because he knows that the material on which he works, from its softness and toughness, admits of succi adjustment of its edges. The processes of bone shoot into the opposite cavities with an exact resemblance to the fox-tail wedge of the carpenter. “But if a workman in glass or marble were to join these materials, he would smooth the edges and unite them by cement; for if he could succeed in indenting the line of union, he knows that his material would chip off on the slightest vibration. “Now apply this principle to the skull; the outer table, which resembles wood, is indented and dove tailed; the inner glassy table has its edges simply laid in contact.”—Parton.