part of the joint, viz. that here, the oil is not only dropped, but made.*

In considering the joints, there is nothing, perhaps, 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 should attribute, in part, to the provision which is made for the preventing of wear and tear, first by the polish of the cartilaginous surfaces, secondly, by the healing 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.†

* 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 Mechanics.

+ 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 structure of the parts is originally perfected by the action of the vessels, the function or operation of the part is made the stimulus to those 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 off in squama, 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

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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.

CHAPTER IX.

OF THE MUSCLES.

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 Animal 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 suture; the inner from its brittleness is called vitreous, and therefore merely joined together in a straight line; this mode of union is not accidental-not the result of chance, but design. The author of the treatise 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 such 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."-Paxton.

to point out instances in which, and properties with respect to which, the disposition of these muscles is as strictly mechanical, as that of the wires and strings of a puppet.*

I. We may observe, what I believe is universal, an exact relation between the joint and the muscles which move it. Whatever motion the joint, by its mechanical construction, is capable of performing, that motion, the annexed muscles, by their position, are capable of producing. For example; if there be, as at the knee and elbow, a hingejoint, capable of motion only in the same plane, the leaders, as they are called, i. e. the muscular tendons, are placed in directions parallel to the bone, so as, by the contraction or relaxation of the muscles to which they belong, to produce that motion and no other. If these joints were capable of a freer motion, there are no muscles to produce it. Whereas at the shoulder and the hip, where the ball and socket joint allows by its construction of a rotatory or sweeping motion, tendons are placed in such a position, and pull in such a direction, as to produce the motion of which the joint admits. For instance, the sartorius or tailor's muscle, rising from the spine, running diagonally across the thigh, and taking hold of the inside of the main bone of the leg, a little below the knee, enables us, by its contraction, to throw one leg and thigh over the other; giving effect, at the same time, to the ball and socket joint at the hip, and the hinge-joint at the knee. [Pl. XII. fig. 1.]

There is, as we have seen, a specific mechanism in the bones, for the rotatory motions of the head and hands; there is, also, in the oblique direction of the muscles belonging to them, a specific provision for the putting of this mechanism of the bones into action. [Pl. XII. fig. 2.] And mark the consent of uses. The oblique muscles would have been inefficient without that particular articulation; that particular articulation would have been lost, without the oblique muscles. It may be proper, however, to observe with respect to the head, although I think it does not vary the case, that its oblique motions and inclinations are often motions in a diagonal, produced by the joint action of mus

* Muscles are the fleshy parts of the body which surround the bones, having a fibrous texture; a muscle being composed of a number of muscular faciculi, which are composed of fibres still smaller; these result from fibres of a less volume, until by successive division we arrive at very small fibres no longer divisible. These muscular fibres are longer or shorter according to the muscles to which they belong; and every fibre is fixed by its two extremities to tendon or aponeurosis, which are the "wires and strings" which conduct the muscular power when they contract.-Paxton.

cles lying in straight direction. But whether the pull be single or combined, the articulation is always such, as to be capable of obeying the action of the muscles. The oblique muscles attached to the head, are likewise so disposed, as to be capable of the steadying the globe, as well as of moving it. The head of a new-born infant is often obliged to be filleted up. After death, the head drops and rolls in every direction. So that it is by the equilibre of the muscles, by the aid of a considerable and equipollent muscular force in constant exertion, that the head maintains its erect posture. The muscles here supply what would otherwise be a great defect in the articulation; for the joint in the neck, although admirably adapted to the motion of the head, is insufficient for its support. It is not only by the means of a most curious structure of the bones that a man turns his head, but by virtue of an adjusted muscular power, that he even holds it up.

As another example of what we are illustrating, viz. conformity of use between the bones and the muscles, it has been observed of the different vertebræ, that their processes are exactly proportioned to the quantity of motion which the other bones allow of, and which the respective muscles are capable of producing.

II. A muscle acts only by contraction. Its force is exerted in no other way. When the exertion ceases, it relaxes itself, that is, it returns by relaxation to its former state; but without energy. This is the nature of the muscular fibre: and being so, it is evident that the reciprocal energetic motion of the limbs, by which we mean motion with force in opposite directions, can only be produced by the instrumentality of opposite or antagonist muscles; of flexors and extensors answering to each other. For instance, the biceps and brachiæus internus muscles, placed in the front part of the upper arm, by their contraction bend the elbow; and with such degree of force, as the case requires, or the strength admits of. [Pl. XIII. fig. 1.] The relaxation of these muscles, after the effort, would merely let the forearm drop down. For the back siroke, therefore, and that the arm may not only bend at the elbow, but also extend and straighten itself, with force, other muscles, the longus and brevis brachiæus externus, and the anconæus, placed on the hinder part of the arms, by their contractile twitch fetch back the fore-arm into a straight line with the cubit, with no less force than that with which it was bent out of it The same thing obtains in all the limbs, and in every movable part of the body. A finger is not bent and straighten.

ed, without the contraction of two muscles taking place. It is evident, therefore, that the animal functions require that particular disposition of the muscles which we describe by the name of antagonist muscles. And they are accordingly so disposed. Every muscle is provided with an adversary. They act, like two sawyers in a pit by an opposite pull; and nothing surely can more strongly indicate design and attention to an end, than their being thus stationed; than this collocation. The nature of the muscular fibre being what it is, the purposes of the animal could be answered by no other. And not only the capacity for motion, but the aspect and symmetry of the body, is preserved by the muscles being marshalled according to this order, e. g. the mouth is holden in the middle of the face, and its angles kept in a state of exact correspondency, by several muscles drawing against, and balancing each other. [See Pl. XIV. fig. 3.] In a hemiplegia, when the muscles on one side are weakened, the muscles on the other side draw the mouth awry.

III. Another property of the muscles, which could only be the result of care, is, their being almost universally so disposed, as not to obstruct or interfere with one another's action. I know but one instance in which this impediment is perceived. We cannot easily swallow whilst we gape. This, I understand, is owing to the muscles employed in the act of deglutition, being so implicated with the muscles of the lower jaw, that, whilst these last are contracted, the former cannot act with freedom. The obstruction is, in this instance, attended with little inconveniency; but it shows what the effect is where it does exist; and what loss of faculty there would be if it were more frequent. Now, when we reflect upon the number of muscles, not fewer than four hundred and forty-six in the human body, known and named,* how contiguous they lie to each other, in layers, as it were, over one another, crossing one another, sometimes embedded in one another; sometimes perforating one another; an arrangement, which leaves to each its liberty, and its full play, must necessarily require meditation and counsel.

IV. The following is oftentimes the case with the muscles. Their action is wanted, where their situation would be inconvenient. In which case, the body of the muscle is placed in some commodious position at a distance, and made to communicate with the point of action, by slender *Keill's Anat. p. 295, edit. 3. There are, however, five hundred and twenty-seven muscles described by more modern anatomists.

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Paxton.