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. It also gives to the tendons a very considerable mechanical advantage, by altering the line of their direction, and by advancing it farther out from the centre of motion; and this upon the principles of the resolution of force, upon which principles all machinery is founded. These are its uses. But what is most observable in it is, that it appears to be supplemental, as it were, to the frame; added, as it should almost seem, afterward; not quite necessary, but very convenient. It is separate from the other bones; that is, it is not connected with any other bones by the common mode of union. It is soft, or hardly formed, in infancy; and produced by an ossification, of the inception or progress of which no account can be given from the structure or exercise of the part.

VI. 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. [Pl. X. fig. 4.] 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. It does not apply to any other bone of the trunk; (I know not whether this be true of any second bone in the body, except perhaps the os hyoïdes.) [Pl. X. fig. 5.] In strictness, it forms no part of the skeleton. It is bedded in the flesh; attached only to the muscles. It is no other than a foundation bone for the arm, laid in separate, as it were, and distinct, from the general ossification. The lower limbs connect themselves at the hip with bones which form a part of the skeleton; but this connexion, in the upper limbs, being wanted, a basis, whereupon the arm might be articulated, was to be supplied by a detached ossification for the purpose.

I. The above are a few examples of bones made remarkable by their configuration: but 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, o 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. Now, observe, what would have been the inconveniency, i. e. both the superfluity and the defect of articulation, if the case had been inverted: if the ball and socket joint had been at the knee, and the hinge-joint at the hip. The thighs must have been kept constantly together, and the legs have been loose and straddling. There would have been no use, that we know of, in being able to turn the calves of the legs before; and there would have been great confinement by restraining the motion of the thighs to one plane. The disadvantage would not have been less, if the joints at the hip and the knee had been both of the same sort; both balls and sockets, or both hinges: yet why, independently of utility, and of a Creator who consulted that utility, should the same bone (the thigh-bone) be rounded at one end, and channelled at the other?

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. This membrane ties, confines, and holds the ends of the bones together; keeping the corresponding parts of the joint, i. e. the relative convexities and concavities, in close application o each other.*

For the ball and socket joint, beside the membrane already described, there is in one important joint, as an additional security, a short, strong, yet flexible ligament, inserted by one end into the head of the ball, by the other

* This membrane is the capsular, or bursal ligament, common to every movable joint. It certainly connects the bones together, but does not possess much strength: its chief use is to produce and preserve the synovia in the part where it is required. The security and strength of the hinge-joint depends on certain ligaments called lateral ligaments, and the endons of those muscles which pass over it. In the particular in stance of the knee, from its being the largest joint in the body, there is, as we shall presently find, an additional contrivance to prevent dislocation

Paxton.

into he 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 vio

*

lence, can pull them asunder [Pl. XI. fig. 1.] It is hardly imaginable, how great a force is necessary, even to stretch, still more to break, this ligament; yet so flexible s it, as to oppose no impediment to the suppleness of the joint. By its situation also, it is inaccessible to injury from sharp edges. As it cannot be ruptured, (such is its strength,) so it cannot be cut, except by an accident which would sever the limb. If I had been permitted to frame a proof of contrivance, such as might satisfy the most distrustful inquirer, I know not whether I could have chosen an example of mechanism more unequivocal, or more free from objection, than this ligament. Nothing can be more mechanical; nothing, however subservient to the safety, less capable of being generated by the action of the joint. I would particularly solicit the reader's attention to this provision, as it is found in the head of the thigh-bone; to its strength, its structure, and its use. It is an instance upon which I lay my hand. One single fact, weighed by a mind in earnest, leaves oftentimes the deepest impression. For the purpose of addressing different understandings and different apprehensions-for the purpose of sentiment, for the purpose of exciting admiration of the Creator's works, we diversify our views, we multiply examples; but for the purpose of strict argument, one clear instance is sufficient; and not only sufficient, but capable, perhaps, of generating a firmer assurance than what can arise from a divided attention.

The ginglymus, or hinge-joint, does not, it is manifest, admit of a ligament of the same kind with that of the ball and socket joint, but it is always fortified by the species of ligament of which it does admit. The strong, firm, investing membrane, above described, accompanies it in every part; and in particular joints, this membrane, which is properly a ligament, is considerably stronger on the sides than either before or behind, in order that the convexities may play truc in their concavities, and not be subject to slip sideways, which is the chief danger; for the muscular

* This ligament is also common to all quadrupeds, even in the more large and unwieldy, as the Hippopotamus and Rhinoceros-it is wanting in the elephant only, whose limbs, ill qualified for active movements, do not seem to require this security to the joint.-Paxton.

tendons generally restrain the part 3 from going farther than they ought to go in the plane of their motion. In the knee, which is a joint of this form, sad of great importance, there are superadded to the com .non provisions for the stability of the joint, two strong ligaments which cross each other; and cross each other in such a manner, as to secure the joint from being displaced in any assignable direction. [Pl. XI. fig. 2.] "I think," says Cheselden, "that the knee cannot be completely dislocated without breaking the cross ligaments.' We can hardly help comparing this with the binding up of a fracture, where the fillet is almost always strapped across, for the sake of giving firmness and strength to the bandage.

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. [PI. XI. fig 4.] 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. It is part of each bone going down lower than 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 outer projection, i. e. that of the fibula. Between both, it is locked in its position. I know no account that can be given of this structure, except its utility. Why should the tibia terminate, at its lower extremity, with a double end, and the fibula the same, but to barricade the joint on both sides by a continuation of part of the thickest of the bone over it? †

* Ches. Anat. ed. 7th, p. 45.

The most obvious proof of contrivance is the junction of the foot to the bones of the leg at the ankle-joint. The two bones of the leg, called the tibia and the fibula, receive the great articulating bone of the foot (the astragalus) between them. And the extremities of these bones of the leg project so as to form the outer and inner ankle. Now, when we step forward, and whilst the foot is raised, it rolls easily upon the ends of these bones, so that the toe may be directed according to the inequalities of the ground we are to tread upon; but when the foot is planted, and the body is carried forward perpendicularly over the foot, the joint of the leg and foot becomes fixed, and we have a steady base to rest upon. We next observe, that, in walking, the heel first touches the ground. If the bones of the leg were perpendicular over the part which first touches the ground, we should come down with a sudden jolt, instead of which we descend in a semicircle, the centre of which is the point of the heel. And when the toes have come to the ground we are far from losing the

The joint at the shoulder compared with the joint at the Lp, though both ball and socket joints, discovers a difference in their form and proportions, well suited to the different offices which the limbs have to execute. The cup

or socket at the shoulder is much shallower and flatter than it is at the hip, and is also in part formed of cartilage set round the rim of the cup. The socket, into which the head of the thigh-bone is inserted, is deeper, and made of more solid materials.* This agrees with the duties asadvantages of the structure of the foot, since we stand upon an elastic arch, the hinder extremity of which is the heel, and the anterior the balls of the toes. A finely formed foot should be high in the instep. The walk of opera dancers is neither natural nor beautiful; but the surprising exercises which they perform give to the joints of the foot a freedom of motion almost like that of the hand. We have seen the dancers, in their morning exercises, stand for twenty minutes on the extremities of their toes, after which the effort is to bend the inner ankle down to the floor, in preparation for the Bolero step. By such unnatural postures and exercises the foot is made unfit for walking, as may be observed in any of the retired dancers and old figurantes. By standing so much upon the toes, the human foot is converted to something more resembling that of a quadruped, where the heel never reaches the ground, and where the is nothing more than the phalanges of the toes.

paw

This arch of the foot, from the heel to the toe, has the astragalus, resembling the keystone of an arch; but, instead of being fixed, as in masonry, it plays freely between two bones, and from these two bones, a strong elastic ligament is extended, on which the bone rests, sinking or rising as the weight of the body bears upon it, or is taken off, and this it is enabled to do by the action of the ligament which runs under it.

This is the same elastic ligament which runs extensively along the back of the horse's hind leg and foot, and gives the fine spring to it, but which is sometimes ruptured by the exertion of the animal in a leap, producing irrecoverable lameness.

Having understood that the arch of the foot is perfect from the heel to the toe, we have next to observe, that there is an arch from side to side for when a transverse section is made of the bones of the foot, the ex posed surface presents a perfect arch of wedges, regularly formed like the stones of an arch in masonry. If we look down upon the bones of the foot, we shall see that they form a complete circle horizontally, leaving a space in their centre. These bones thus form three different arches-forward; across; and horizontally: they are wedged together, and bound by ligaments, and this is what we alluded to when we said that the foun dations of the Eddystone were not laid on a better principle; but our ad niration is more excited in observing, that the bones of the foot are not only wedged together, like the courses of stone for resistance, but that solidity is combined with elasticity and lightness.

Notwithstanding the mobility of the foot in some positions, yet when the weight of the body bears directly over it, it becomes immovable, and the bones of the leg must be fractured before the foot yields.

Bell's Treatise on Animal Mechanics.

* The socket for the head of the thigh-bone is indeed deeper than that at the shoulder, but the "materials" which form the concavities are the