drum, to let the air pass freely into and out of the barrel of the ear, as the covering membrane vibrates. or as the temperature may be altered: the whole labyrinth hewn out of a rock, that is, wrought into the substance, as it were, of the hardest bone of the body.

The communication within, formed by the small bones of the ear, is, to look upon, more like what we are accustomed to call machinery, than any thing in animal bodies. It seems evidently designed to continue towards the sensorium, or brain, the tremulous motions which are excited in the membrane of the tympanum, or what is better known by the name of the "drum of the ear." The compages of bones consists of four, which are so disposed, and so hinge upon one another, as that if the membrane, the drum of the ear, vibrate, all the four are put in motion together; and, by the result of their action, work the base of that which is the last in the series, called the stapes, upon an aperture which it closes, and upon which it plays, and which aperture opens into the tortuous canals that lead to the brain.

The office of the drum of the ear is to spread out an extended surface, capable of receiving the impressions of sound, and of being put by them into a state of vibration. The office of the stapes is to repeat these vibrations. From which account of its action may be understood, how the sensation of sound will be excited, by any thing which communicates a vibratory motion to the stapes, though not, as in all ordinary cases, through the intervention of the drum or membrana timpani, as through the mouth or teeth when united to a sounding body. The use of the chain of bones is to propagate the impulse in a direction towards the brain, and to propagate it with the advantage of a lever; which advantage consists in increasing the force and strength

of the vibration, and at the same time diminishing the space through which it oscillates.

Behind the drum of the ear is a second cavity, or barrel, called the tympanum. The eustachian tube is a slender pipe, but sufficient for the passage of air, leading from this cavity into the back part of the mouth. The only remaining expedient, and that for which the eustachian tube serves, is to open to this cavity a communication with the external air. In one word: it exactly answers the purpose of the usual hole in a drum.

The membrana tympani is not found in the ears of fish; which furnishes an additional proof of what indeed is indicated by every thing about it, that it is appropriated to the action of air, or of an elastic medium. It bears an obvious resemblance to the pelt or head of a drum, from which it takes its name. It resembles also a drum-head in its principal property, that its use depends upon its tension.

Thus by means of the medium on which we live, we are enabled by the natural instruments of the eye and ear to discriminate actions on the medium at a distance, and to hold an intercourse with remote objects for our pleasure, convenience, and security. The delicacy of our perception, too, is such, that we can discriminate the action of different atoms, though it would require millions of the largest to form a grain of sand, and these differences of action we call colours, from red the largest to violet the smallest. And in like manner the ear enables us to discriminate the tones which are produced by various excitements, and proceed from those different vibrations, which are but the thousandth part of a second of time longer than each other.

Speaking more generally, it is the different action or motion of the same atoms, which the different structure of these instruments carries to the brain; while the atoms or medium being the same, and the

brain the same; so the proportions of the prismatic scale of colours, and of the diatonic scale of music, are also necessarily the same.

**

LETTER LXXVI.

The Bones of the Human Body.

MY DEAR CHILdren,

I challenge, says Paley, any man to produce, in the joints and pivots of the most complicated or the most flexible machine that was ever contrived, a construction more ingenious, or more evidently providential, than that which is seen in the vertebræ of the human neck. The head was to have the power of bending forward and backward, and at the same time, of turning itself round upon the body to a certain extent. For these two purposes, two distinct contrivances are employed: First, the head rests immediately upon the uppermost of the vertebræ, or back bone, and is united to it by a hingejoint; upon which joint the head plays freely forward and backward, as far either way as is necessary or as the ligaments allow. Secondly, to make the head capable of this, a further mechanism is introduced; not between the head and the uppermost bone of the neck, where the hinge is, but between that bone and the bone next underneath it, resembling a tenon and mortice. This second; or uppermost bone but one, has what anatomists call a process, viz. a projection, somewhat similar, in size and shape, to a tooth; which tooth, entering a corresponding hole or socket in the bone above it, forms a pivot or axle, upon which that upper bone, together with the head which it supports, turns freely in a circle; and as far in the circle as the attached

muscles permit the head to turn. Thus are both motions perfect, without interfering with each other. When we nod the head, we use the hingejoint, which lies between the head and the first bone of the neck. When we turn the head round, we use the tenon and mortice, which runs between the first bone of the neck and the second.

Another contrivance, not unlike the last in its object, but different and original in its means, is in the fore-arm, that is, in the arm between the elbow and the wrist. Here, for the perfect use of the limb, two motions are wanted; a motion at the elbow backward and forward, which is called a reciprocal motion; and a rotatory motion, by which the palm of the hand, as occasion requires, may be turned upward. The fore-arm consists of two bones, lying alongside each other, but touching only towards the ends. One, and only one of these bones, is joined to the cubit, or upper part of the arm, at the elbow; the other alone, to the hand at the wrist. The first, by means, at the elbow, of a hinge-joint, swings backward and forward, carrying along with it the other bone and the whole fore-arm. In the mean time, as often as there is occasion to turn the palm upward, that other bone to which the hand is attached, rolls upon the first, by the help of a groove or hollow near each end of one bone, to which is fitted a corresponding prominence in the other. If both bones had been joined to the upper arm at the elbow, or both to the hand at the wrist, the motions could not have been performed. The first was to be at liberty at one end, and the second at the other; by which means the two actions may be performed together.

The spine, or back-bone, is a chain of joints of very wonderful construction. Various, difficult, and almost inconsistent offices were to be executed by the same instrument. It was to be firm, yet flexible; firm, to support the erect position of

the body; flexible, to allow of the bending of the trunk in all degrees of curvature. It was further also to become a pipe or conduit for the conveyance from the brain, of the most important fluid of the animal frame, that, namely, upon which all voluntary motion depends, the spinal marrow. Now

the spine was not only to furnish the main trunk for the passage of this medullary substance from the brain, but to give out, in the course of its progress, small pipes therefrom, which being afterwards indefinitely subdivided, might, under the name of nerves, distribute themselves to every part of the body. The same spine was also to serve another use not less wanted than the preceding, viz. to afford a fulcrum, stay, or basis, for the insertion of the muscles which are spread over the trunk of the body; in which trunk there are not, as in the limbs, cylindrical bones, to which they can be fastened and, likewise, which is a similar uşe, to furnish a support for the ends of the ribs to rest upon.

:

It is composed of a great number of bones, (in the human subject of twenty-four) joined to one another, and compacted by broad bases. The breadth of the bases upon which the parts severally rest, and the closeness of the junction, give to the chain its firmness and stability; the number of parts, and consequent frequency of joints, its flexibility. In order to afford a passage for the descent of the medullary substance, each of these bones is bored through in the middle, in such a manner as that, when put together, the hole in one bone falls into a line, and corresponds with the holes in the two bones contiguous to it. By which means, the perforated pieces, when joined, form an entire, close, uninterrupted channel.

The strength of the compeges, and the security against luxation was a subject of studious precaution. For this purpose, the vertebræ are articula