« ПредыдущаяПродолжить »
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 consist 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 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 any thing to last, for any thing 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.
Moveable 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
died at Northampton, where he practised as a physician, in 1719.
The great Cheselden was a later authority, for he lived till 1752. His work, which had the same title as Keill's, passed through many editions, and tended materially to raise the reputation of its author.
Cheselden was certainly the first anatomist of his day, and his opinions are still regarded with reverence by the professors of medicine; and though his works were attacked with much asperity by Dr. Douglas and others, yet the great German physician, Laurence Heister, did ample justice to their merits. The celebrated John Hunter was one of his pupils.-Dr. Rees' Encyclop. Biog. Brit. The ingenuity of mechanics has not devised a joint or hinge, to which the anatomist cannot point out either a parallel, or superior example in the animal frame. To exhaust this subject, so replete in facts commanding admira tion, would occupy a volume: our observations in addition to those in the text, must therefore be desultory. Of the universal joint, that which permits motion in every direction, the hip and the shoulder are examples, the mechanic approaches nearest to it in the ball and socket joint. In man the moveable bone has a rounded head; the corres. ponding hollow is in the supporting bone. Let it be remarked that not only is this the most appropriate joint for attaching the limbs to the trunk, but that it is placed in the most advantageous situation. By being at the upper part of the limbs, the whole of those members partake of the same extensive motion. How inelegant, how awkward, how restricting the powers of the limbs, if the hips and shoulders permitted only a forward and backward motion, whilst the ball and socket joints were at the knees
and elbows; how much stronger are the more delicate
OF THE MUSCLES.
MUSCLES, with their tendons, are the instruments by which animal motion is performed. It will be our business 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 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 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. 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. 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 muscles lying in straight directions. 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 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
double joint; and as the elastic substance moves along with the condyle, to which its concave surface is well adapted, it is carried with it to the eminence, and while there provides for it a secure resting place. This is very distinct in man, but still more so in the elephant. It has been thought a very ingenious contrivance lately proposed for a carriage to carry its own rail-road with it is not this as worthy of admiration?
The beautiful mechanism of the human knee is detailed by Paley. It has a similar structure in most animals
requiring the same variety of movements. However, in some swimming birds, as the passage-duck, instead of a patella, or knee-cap, there is at the joint a process three inches long, proceeding from the lower bone of the leg. To the end of this process the muscles are attached, and it is easy to see what an increased power of motion, what an additional leverage is thus obtained to aid the progress of the bird when swimming.-See Sir Everard Home's and Cuvier's Lectures.
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. The relaxation of these muscles, after the effort, would merely let the fore-arm drop down. For the back stroke, 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 moveable part of the body. A finger is not bent and straightened, 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 two muscles drawing against, and balancing each other. In a hemiplegia, when the muscle on one side is weakened, the muscle on the other side draws 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 inconvenience; but it shews 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 imbedded 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 strings or wires. If the muscles which move the fingers, had been placed in the palm or back of the hand, they would have swelled that part to an awkward and clumsy thickness. The beauty, the proportions of the part, would have been destroyed. They are therefore disposed in the arm, and even up to the elbow; and act by long tendons, strapped down at the wrist, and passing under the ligaments to the fingers, and to the joints of the fingers, which they are severally to move. In like manner, the muscles which move the toes, and many of the joints of the foot, how gracefully are they disposed in the calf of the leg, instead of forming an unwieldy tumefaction in the foot itself! The observation may be repeated of the muscle which draws the nictitating membrane over the eye. Its office is in the front of the eye; but its body is lodged in the back part of the globe, where it lies safe, and where it encumbers nothing.
V. The great mechanical variety in the figure of the muscles may be thus stated. It appears to be a fixed law, that the contraction of a muscle shall be towards its centre. • Keill's Anatomy, p. 295. ed. 3.
Therefore the subject for mechanism on each occasion is, so to modify the figure, and adjust the position of the muscle, as to produce the motion required, agreeably with this law. This can only be done by giving to different muscles a diversity of configuration, suited to their several offices, and to their situation with respect to the work which they have to perform. On which account we find them under a multiplicity of forms and attitudes; sometimes with double, sometimes with treble tendons, sometimes with none: sometimes one tendon to several muscles, at other times one muscle to several tendons. The shape of the organ is susceptible of an incalculable variety, whilst the original property of the muscle, the law and line of its contraction, remains the same, and is simple. Herein the muscular system may be said to bear a perfect resemblance to our works of art. An artist does not alter the native quality of his materials, or their laws of action. He takes these as he finds them. His skill and ingenuity are employed in turning them, such as they are, to his account, by giving to the parts of his machine a form and relation, in which these unalterable properties may operate to the production of the effects intended.
VI. The ejaculations can never too often be repeated ;-how many things must go right for us to be an hour at ease! how many more for us to be vigorous and active! Yet vigour and activity are, in a vast plurality of instances, preserved in human bodies, notwithstanding that they depend upon so great a number of instruments of motion, and notwithstanding that the defect or disorder sometimes of a very small instrument, of a single pair, for instance, out of the four hundred and forty-six muscles which are employed, may be attended with grievous inconveniency. There is piety and good sense in the following observation, taken out of the Religious Philosopher: "With much compassion," says this writer, "as well as astonishment at the goodness of our loving Creator have I considered the sad state of a certain gentleman, who, as to the rest, was in pretty good health, but only wanted the use of these two little muscles that serve to lift up the eyelids, and so had almost lost the use of his sight, being forced, as long as this defect lasted, to shove up his eyelids every moment with his own hands!" In general, we may remark, in how small a degree those, who enjoy the perfect use of their organs, know the comprehensiveness of the blessing, the variety of their obligation. They perceive a result, but they think little of the multitude of concurrences and rectitudes which go to form it.
Besides these observations, which belong to the muscular organs as such, we may notice some advantages of structure which are more conspicuous in muscles of a certain class or description than in others. Thus :
I. The variety, quickness, and precision, of which muscular motion is capable, are seen, I think, in no part so remarkably as in the tongue. It is worth any man's while to watch the agility of his tongue; the wonderful promptitude with which it executes changes of position, and the perfect exactness. Each syllable of articulated sound requires for its utterance a specific action of the tongue, and of the parts adjacent to it. The disposition and configuration of the mouth, appertaining to every letter and word, is not only peculiar, but, if nicely and accurately attended to, perceptible to the sight; insomuch, that curious persons have availed themselves of this circumstance to teach the deaf to speak, and to understand what is said by others. In the same person, and after his habit of speaking is formed, one, and only one, position of the parts, will produce a given articulate sound correctly. How instantaneously are these positions assumed and dismissed; how numerous are the permutations, how various, yet how infallible! Arbitrary and antic variety is not the thing we admire; but variety obeying a rule, conducing to an effect, and commensurate with exigencies infinitely diversified. I believe also that the anatomy of the tongue corresponds with these observations upon its activity. The muscles of the tongue are so numerous, and so implicated with one another, that they cannot be traced by the nicest dissection; nevertheless (which is a great perfection of the organ), neither the number, nor the complexity, nor what might seem to be the entanglement of its fibres, in any wise impede its motion, or render the determination, or success, of its efforts uncertain.
"The Religious Philosopher; or the right use of contemplating the works of the Creator," was written in the Dutch language, by Dr Nieuwentyt, and translated into
English by John Chamberlayne, eɛq, F. R. S. It has passed through several editions.
I here entreat the reader's permission to step a little out of my way, to consider the parts of the mouth, in some of their other properties. It has been said, and that by an eminent physiologist, that whenever nature attempts to work two or more purposes by one instrument, she does both, or all, imperfectly. Is this true of the tongue, regarded as an instrument of speech, and of taste; or regarded as an instrument of speech, of taste, and of deglutition? So much otherwise, that many persons, that is to say, nine hundred and ninety-nine persons out of a thousand, by the instrumentality of this one organ, talk, and taste, and swallow, very well. In fact, the constant warmth and moisture of the tongue, the thinness of the skin, the papillæ upon its surface, qualify this organ for its office of tasting, as much as its inextricable multiplicity of fibres do for the rapid movements which are necessary to speech. Animals which feed upon grass have their tongues covered with a perforated skin, so as to admit the dissolved food to the papillæ underneath, which, in the mean time remain defended from the rough action of the unbruised spiculæ.
There are brought together within the cavity of the mouth more distinct uses, and parts executing more distinct offices, than I think can be found lying so near to one another, or within the same compass, in any other portion of the body: viz. teeth of different shape, first for cutting, secondly for grinding; muscles most artificially disposed for carrying on the compound motion of the lower jaw, half lateral and half vertical, by which the mill is worked: fountains of saliva, springing up in different parts of the cavity for the moistening of the food, whilst the mastication is going on: glands, to feed the fountains; a muscular construction of a very peculiar kind in the back part of the cavity, for the guiding of the prepared aliment into its passage towards the stomach, and in many cases for carrying it along that passage; for, although we may imagine this to be done simply by the weight of the food itself, it in truth is not so, even in the upright posture of the human neck; and most evidently is not the case with quadrupeds, with a horse for instance, in which, when pasturing, the food is thrust upward by muscular strength, instead of descending of its own accord.
In the mean time, and within the same cavity, is going on another business, altogether different from what is here described,—that of respiration and speech. In addition, therefore, to all that has been mentioned, we have a passage opened from this cavity to the lungs, for the admission of air, exclusively of every other substance; we have muscles, some in the larynx, and without number in the tongue, for the purpose of modulating that air in its passage, with a variety, a compass, and precision, of which no other musical instrument is capable. And lastly, which in my opinion crowns the whole as a piece of machinery, we have a specific contrivance for dividing the pneumatic part from the mechanical, and for preventing one set of actions interfering with the other. Where various functions are united, the difficulty is to guard against the inconveniences of a too great complexity. In no apparatus put together by art, and for the purposes of art, do I know such multifarious uses so aptly combined, as in the natural organization of the human mouth; or where the structure, compared with the uses, is so simple. The mouth, with all these intentions to serve, is a single cavity; is one machine; with its parts neither crowded nor confused, and each unembarrassed by the rest: each at liberty in a degree sufficient for the end to be attained. If we cannot eat and sing at the same moment, we can eat one moment and sing the next: the respiration proceeding freely all the while.
There is one case, however, of this double office, and that of the earliest necessity, which the mouth alone could not perform; and that is, carrying on together the two actions of sucking and breathing. Another route therefore is opened for the air, namely, through the nose, which lets the breath pass backward and forward, whilst the lips, in the act of sucking, are necessarily shut close upon the body from which the nutriment is drawn. This is a circumstance which always appeared to me worthy of notice. The nose would have been necessary, although it had not been the organ of smelling. The making it the seat of a sense, was superadding a new use to a part already wanted; was taking a wise advantage of an antecedent and a constitutional necessity.
But to return to that which is the proper subject of the present section, the celerity and precision of muscular motion. These qualities may be particularly observed in the execution