ted, that is, the moveable joints between them are formed by means of projections of their substance; and these so lock in with, and overwrap one another, as to secure the body of the vertebræ, not only from accidentally slipping, but even from being pushed out of its place by any violence short of that which would break the bone. Let any one take, for example, into his hands, a piece of the clean-picked bone of a hare's back, (consisting we will suppose, of three vertebræ,) and he will find the middle bone of the three so implicated, by means of its. projections or processes, with the bone on each side of it, that no pressure which he can use, will force it out of its place between them. It will give way neither forward, nor backward, nor on either side. In whichever direction he pushes, he perceives, in the form, or junction, or overlapping of the bones, an impediment opposed to his attempt; a check and guard against dislocation.

To prevent the bones from being pulled asunder longitudinally, or in the direction of that line, a strong membrane runs from one end of the chain to the other, sufficient to resist any force which is ever likely to act in the direction of the back, or parallel to it, and consequently to secure the whole combination in their places. The general result is, that not only the motions of the human body necessary for the ordinary offices of life are performed with safety, but it is an accident hardly ever heard of, that even the gesticulations of a harlequin distort the spine.

The reciprocal enlargement and contraction of the chest to allow for the play of the lungs, depends upon a simple yet beautiful mechanical contrivance, referable to the structure of the bones which inclose it. The ribs articulated to the back-bone, or rather to its side projections, obliquely, that is, in their natural position they bend or slope from the place of articulation downwards. But the basis upon

which they rest at this end being fixed, the consequence of the obliquity, or the inclination downwards, is, that, when they come to move, whatever pulls the ribs upwards, necessarily, at the same time, draws them out; and that, whilst the ribs are brought to a right angle with the spine behind, the sternum, or part of the chest to which they are attached in front, is thrust forward. The simple action, therefore, of the elevating muscles does the business. The breast-bone, is an easy inspiration, is thrust out one-tenth of an inch, and this, added to what is gained to the space within the chest by the flattening or descent of the diaphragm, leaves room for forty-two cubic inches of air to enter at every drawing-in of the breath. When there is a necessity for a deeper and more laborious inspiration, the enlargement of the capacity of the chest may be so increased, by effort, as that the lungs may be distended with seventy or a hundred such cubic inches.

The patella, or knee-pan, is a curious little separate bone; in its form and office, unlike any other bone of the body. It is circular; the size of a crown piece; pretty thick; a little convex on both sides, and covered with a smooth cartilage. It lies upon the front of the knee: and the powerful tendons, by which the leg is brought forward, pass through it (or rather it makes a part of their continuation,) from their origin in the thigh to their insertion in the tibia. It protects both the tendon and the joint from any injury which either might suffer, by the rubbing of one against the other, or by the pressure of unequal surfaces,

The shoulder-blade is, in some material respects, a very singular bone; appearing to be made so expressly for its own purpose, and so independently of every other reason. In such quadrupeds as have no collar-bones, which are by far the greater number, the shoulder-blade has no bony communication

with the trunk, either by a joint or process, or in any other way. It does not grow to, or out of, any other bone of the trunk.

To almost all the bones belong joints; and in these still more clearly than in the form or shape of the bones themselves, are seen both contrivance and contriving wisdom. Every joint is a curiosity, and is also strictly mechanical. There is the hinge-joint, and the mortice and tenon joint; each as manifestly such, and as accurately defined, as any which can be produced out of a cabinet-maker's shop: and one or the other prevails, as either is adapted to the motion which is wanted: e. g. a mortice and tenon, or ball and socket joint, is not required at the knee, the leg standing in need only of a motion backward and forward in the same plane, for which a hinge-joint is sufficient; a mortice and tenon or ball and socket joint, is wanted at the hip, that not only the progressive step may be provided for, but the interval between the limbs may be enlarged or contracted at pleasure.

The hinge-joint is not formed by a bolt passing through the two parts of the hinge, and thus keeping them in their places; but by a different expedient. A strong, tough, parchment-like membrane, rising from the receiving bones, and inserted all round the received bones a little below their heads, encloses the joint on every side.

For the ball and socket joint, beside the membrane already described, there is in some important joints, as an additional security, a short, strong, yet flexible ligament, inserted by one end into the head of the ball, by the other into the bottom of the cup; which ligament keeps the two parts of the joint so firmly in their place, that none of the motions which the limb naturally performs, none of the jerks and twists to which it is ordinarily liable, nothing less indeed than the utmost and the most unnatural violence, can pull them asunder. It is hardly

imaginable, how great a force is necessary, even to stretch, still more to break, this ligament; yet so flexible is it, as to oppose no impediment to the suppleness of the joint.

Another no less important joint, and that also of the ginglymus sort, is the ankle; yet, though important (in order perhaps, to preserve the symmetry and lightness of the limb,) small, and on that account, more liable to injury. Now this joint is strengthened, i. e. is defended from dislocation, by two remarkable processes or prolongations of the bones of the leg which processes form the protuberances that we call the inner and outer ankle. is part of each bone going down lower that the other part, and thereby overlapping the joint: so that, if the joint be in danger of slipping outward, it is curbed by the inner projection, i. e. that of the tibia; if inward, by the outward projection, i. e. that of the fibula. Between both, it is locked in its position.

It

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.

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.

;

Moveable joints 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 enters the intervals of the other. We have sometimes one bone lapping over another, and plaued 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.

LETTER LXXVII.

The Muscles and Animal Motions.

MY ESTEEMED Children,

Muscles or flesh, with their tendons or strings, are the instruments by which animal motion is performed. Whatever motion the joint, by its mechanical construction, is capable of performing, that motion, the annexed muscles, by their position, aré capable of producing. For example; if there be, as at the knee and elbow, a hinge-joint, 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