Part 15

1st. The motion of opposition is that which is made in two opposite directions, for example, from flexion to extension, from adduction to abduction, and vice versa. This motion is extensive or limited; extensive when it is made in all directions, first in the four named above, then in all the intermediate ones; limited, when it only takes place from flexion to extension, from adduction to abduction, &c. The thigh in its articulation with the pelvis enjoys an extensive motion of opposition. The tibia in its articulation with the femur has a limited motion of opposition.

2d. Circumduction is the motion in which the bone describes a kind of cone, the summit of which is in its superior articulation, and the base in the inferior; so that it is found successively in flexion, adduction, extension and abduction, or in abduction, extension, adduction and flexion, according to the motion by which it begins, and that moreover it goes through all the intermediate directions. Hence we see that circumduction is a motion composed of all those of opposition, and in which the bone, instead of moving from one direction to an opposite one, as in the preceding case, moves from one direction to another nearest to it, describing thus by its extremity a circle which is the base of a cone of which I have spoken, and which is so much the greater as the bone is so much the longer. We easily understand that among the bones, those only whose motion of opposition is loose, enjoy that of circumduction.

3d. Rotation is wholly different from the preceding motion. In this there was locomotion, a moving of the bone from one place to another; here it remains always in the same place; it only turns upon its axis. The humerus and the femur enjoy this motion which is simple.

4th. Sliding belongs to all the articulations. It is an obscure motion, by which two surfaces go in an opposite direction, by sliding as it were upon each other. In all the other motions, this is met with; but it often exists without them.

It is easy to understand, from these views upon the articular motions, the division into genera of the class of moveable articulations. In fact, there are articulations in which all the motions are united; in others, there is no rotation; in many rotation and circumduction are wanting, and opposition exists only in one direction; some have only rotation. Finally, there are those in which rotation, circumduction and opposition are nothing, and sliding alone remains.

Hence we see that nature moves here as elsewhere by gradation, that from the most moveable articulations to those that are the least so, there are different degrees of decrease, that she descends gradually to the immoveable articulations, that she is finally reduced to the motion of sliding alone, like that which exists at the carpus, the tarsus, &c. There is even an intermediate one between sliding and immobility; it is the articulation of the symphysis pubis; which can be considered with that of the humerus as forming the two extremes of the series of moveable articulations.

All the articulations of which I have spoken are with contiguous surfaces; this is the general character of those which are moveable. There is however an exception to this rule; it is the articulation of the body of the vertebræ, in which there is continuity and mobility. The symphysis pubis is also in part continuous in its surfaces, and yet has sometimes obscure motions. Hence arises a division of the moveable articulations, into those with continuous surfaces, and into those with contiguous ones.

_Immoveable Articulations._

The immoveable articulations are sometimes with surfaces inserted into each other, as the bones of the cranium, in which many projections and depressions reciprocally receive each other; sometimes with surfaces in juxta-position, as in the articulation of the temporal with the parietal, the two superior maxillary bones with each other; sometimes with implanted surfaces, as in the teeth.

All the different divisions that I have mentioned will be easily understood by the following table; it is not the same as that which I have given in my treatise on the membranes; I think it presents a classification a little more useful in this, that it offers for a characteristic two things essential to be known in all kinds of moveable articulations, viz. 1st, the relation of the articular surfaces which characterizes the orders; 2d, the number of motions of each which distinguishes the genera. There are no orders in the immoveable articulations because, except the relation of surfaces, the articulations have not differences sufficient to occasion them to be sub-divided.

_Table of the Articulations._

┌ CLASSES. ┌ ORDERS. ┌ GENERA. │ │ │1st. Extensive Opposition, │ │ │ Circumduction and │ │ │ Rotation. │ │1st. With │2d. Extensive Opposition │ │ Contiguous │ and Circumduction. │1st. Moveable. │ Surfaces. │3d. Limited Opposition. │ │ │4th. Rotation. │ │ └5th. Sliding. ARTICULATIONS. │ │2d. With Continuous │ └ Surfaces. │ │ ┌1st. With Surfaces │ │ in juxta-position. │ │2d. With Surfaces │ │ inserted └2d. Immoveable. │ into each other. │3d. With Surfaces └ implanted.

After having thus divided the articulations, let us offer upon each class some general observations. But let us first remark that the preceding table, considered in respect to the moveable articulations with contiguous surfaces, indicates perfectly the disposition of these articulations as to luxations, which are so much the more frequent as the motions are more extensive. The first genus is the most exposed to it, the last the least so; and the others are more or less so according to their distance from the first.

II. _Observations upon the Moveable Articulations._

The class of moveable articulations is the most important to be considered, because their mechanism is the most complicated of the two orders composing this class, as we have seen. The latter, or that of the articulations with continuous surfaces, will not be considered in our general observations, as it embraces only one species of motion, that of the vertebræ, this motion will be noticed in the examination of the spine. The order of the moveable articulations with contiguous surfaces, comprises, as we have said, five genera characterized by their respective motions.

_First Genus._

Extensive opposition, circumduction and rotation characterize this genus. The first by the extent and number of its motions. The articulations of the humerus with the scapula and the femur with the ilium are examples of it; they even exclusively compose it.

We see why it is at the superior part of the limbs that nature has placed this genus. A double advantage results from this situation. On the one hand, very far from the part of the limb immediately exposed to the action of external bodies, it more easily escapes luxations to which its want of solidity renders it liable. On the other hand, it can by this situation give to the limb the motions of a whole which compensate for those of the inferior articulations, the solidity of which prevents the power of motion in all directions. For example, the two articulations of which I have just spoken, are not only the articulations of the bones that form them, of the humerus and the femur, but also the articulations of the whole limb, which they direct in different directions; thus the anchylosis of these articulations renders the limb completely useless, whilst that of the inferior articulations only destroys partial motions.

The kind of motion of this genus of articulation requires a rounded form in the articular surfaces, whether they be concave that receive and convex that are received. This form is in fact the only one that can accommodate itself to extensive opposition, rotation and circumduction united; this is the form of the superior parts of the humerus with the scapula, and the femur with the os innominatum. The bone which moves has a convex surface, that which serves for support a concave one. There are in animals examples of an opposite arrangement: that is to say that a concavity is moved in all directions upon a convexity; but this is not found in man.

Though the two limbs have between them the greatest analogy in their motions, yet there are some differences relative especially to their respective uses, which in the one are for seizing and repelling bodies, in the other destined to locomotion. The principal of these differences is, that rotation and circumduction are found in them in an exactly inverse ratio. The mechanical reason and advantages of this arrangement are easily understood.

In the femur the length of the neck which is the lever of rotation, gives much extent to this motion, which supplies the pronation and supination that are wanting in the leg; so that every rotation of the foot is a motion of the whole of the limb. In the humerus on the contrary, the neck being very short and bringing the axis of the bone near the centre of the motion, limits rotation, which is less necessary on account of that of the fore-arm; the motion of the hand without or within is never communicated but by a part of the limb.

As to circumduction, the length of the neck of the thigh is an obstacle to it. In fact, let us remark that this motion is much the more easy, when it is performed by a rectilinear lever, because then the axis of the motion is the axis of the lever; that on the contrary, if the lever is angular, the motion becomes so much the more difficult because the axis of the motion is not that of the lever; and in general we can say, that the difficulty of the motion is in the direct ratio of the distance of the two axes.

This being settled, let us observe that the axis of the motion of circumduction of the thigh is evidently a straight line, obliquely directed from the head to the condyles, and distant consequently above from the axis of the bone, the whole length of the neck. Now, from what has just been said, it is evident, that the difficulty of circumduction will be in the direct ratio of the length of the neck, and consequently very great. In the humerus, on the contrary, the neck being very short, the axis of the bone and that of the motion are almost the same; hence the facility and extent of the circumduction. We might fix precisely the relation of these motions by this proportion; the circumduction of the humerus is to that of the femur, as the length of the neck of the humerus is to the length of the neck of the femur; which shows us how much more difficult the circumduction of the femur is than that of the humerus. To know this, it is sufficient in fact to know the excess of the length of the neck of the first over that of the second.

It is easy to perceive the advantages of this very great extent in the circumduction of the superior limbs destined to seize, and of the limits placed by nature to that of the inferior limbs destined to standing and locomotion. We understand also why luxations are more easy in the first than in the second. The displacement almost always takes place in fact, in one of the simple motions, the succession of which forms the compound motion of circumduction, for example, in elevation or depression, in adduction or abduction, &c. Now all these motions being carried much further in the humerus than in the femur, the surfaces are more easily separated.

_Second Genus._

This genus differs from the first by the absence of the motion of rotation. Opposition and circumduction are alone met with in it. We find examples of it in the temporal maxillary, sterno-clavicular, radio-carpal, meta-carpo-phalangeal articulations, &c.

The want of rotation evidently supposes, from what has been said above, the absence of an osseous head, the axis of which would make, as in the preceding genus, an angle with the axis of the body of the bone. Thus in all the bones of the articulations that I have just mentioned, the articular surface is at the extremity even of the bone, and not upon the side; the axis is the same in both cases. They form a rectilinear lever, instead of an angular one.

The articular surfaces are in general, as in the preceding case, uniform, without eminences and reciprocal depressions; which would embarrass and even prevent circumduction. In the bone which serves for support, there is a cavity more or less deep; in the bone which is moved, there is an analogous convexity. The corresponding surfaces of the temporal and the inferior maxillary bone, of the bones of the metacarpus and the first phalanges, are examples of this arrangement.

This articular mode is the most favourably disposed for circumduction, which is, as we have seen, constantly in the inverse ratio of rotation, and which consequently has the greatest possible facility when the lever is rectilinear, a circumstance that destroys rotation. Yet in many articulations of this genus, circumduction is evidently less extensive than in the humerus and the femur; but this arises from the arrangement of the moving powers which being in much greater number in the articulations of these two bones, compensate for the bad arrangement of the articular surfaces for circumduction.

In the genus of articulations of which we are treating, there is always one direction in which the motion of opposition is more easy than in the others; for example, it is elevation and depression in the jaw, flexion and extension in the first phalanges, in the wrist, &c. In general there are two lateral ligaments and the capsule in the direction in which the motions are most limited, the capsule only in that in which they are the most extensive.

_Third Genus._

As we advance in the examination of these articular genera, the extent of their motion diminishes. This has less opposition in many directions than the preceding, and less circumduction which always supposes an extensive opposition. Here this opposition is always limited to one direction only, to that of flexion and extension, for example.

We find this articular genus especially in the middle of the limbs, as at the elbow, the knee, the middle of the fingers in the articulations of the phalanges. Though the bone which composes them, inferiorly can move by itself but in one direction, yet it borrows from the loose motions of the superior articulation of the limb, so as to be able to be turned in every way.

The articular surfaces are found here as in the preceding genus, at the extremity of the bone, having the same axis as the bone; but they differ, 1st, in this that there are many eminences and cavities fitted to each other, an arrangement, which, by permitting the motion in one direction, prevents it in the others. Very commonly there are two kinds of round prominences, called condyles, which roll from before behind, or from without within, &c. upon two analogous cavities, that are separated by an eminence, which is received in the space between the condyles, as we see in the femoro-tibial, phalangeal articulations, &c. 2d. The breadth of the surfaces also distinguishes this genus from the preceding; this breadth insures its solidity, and prevents luxations, which besides are more to be feared when they happen here where more ligaments must be broken than elsewhere.

There is always in this genus greater extent of motion on one side, than on the opposite. In general flexion has always more extended limits than extension; observe in fact the condyles of the femur, of the phalanges, &c. they are extended much further in the first than the second direction; why? because all our principal motions are those of flexion, and the motions of extension are as it were but to moderate the first, and have for their object only to bring back the limb to the position from which it can be bent again. Hence why the number, and the strength of the fibres are much greater in the flexors than in the extensors; why the great vascular and nervous trunks are always on the side of flexion, as we see in the thigh, the leg, the fore-arm, the phalanges, &c. There is always something which limits the motion of extension, as the olecranon in the humero-cubital articulation, the crucial ligaments in the femoro-tibial articulation.

Though in the genus which we are describing, there is no well marked motion of circumduction, yet when the leg or the fore-arm are in flexion, they can move laterally and even in the form of a cone, but not in a very evident manner. In extension this is impossible, because the lateral ligaments being much stretched, do not yield enough to allow the bone to incline from one side to the other.

_Fourth Genus._

Every kind of opposition and circumduction is wanting in this genus, which presents rotation alone, as we see in the articulations of the ulna with the radius, and the atlas with the odontoid process. Sometimes it is a concave surface rolling upon a convex one, as at the lower end of the radius, and at the odontoid process; sometimes it is a convex surface moving upon a concave one, as at the head of the radius; there is always a kind of ligament which completes the concave surface, and which thus forms a ring turning upon the bone, or in which the bone turns.

Luxations are here very difficult, because the rotation being made upon the axis of the bone, the ligaments are hardly more distended on one side than the other, and are hence broken with difficulty, whatever may be the extent of the motion. The inferior part of the radius forms a slight exception to this rule, because it is upon the ulna, and not exactly upon its own axis, that the bone turns in this place.

There is no rotation in the leg as in the fore-arm, because, as we have seen, that of the thigh which is very extensive, supplies the place of it; which the humerus could hardly do for the fore-arm, as we know when this last is anchylosed.

_Fifth Genus._

Every kind of rotation, opposition and circumduction are wanting in this genus, which is the most numerous, and which embraces the articulations of the carpus, the metacarpus, the tarsus and metatarsus, of the vertebræ between themselves by their articular processes, of the atlas with the occiput, of the humeral extremity of the clavicle, the sternal of the ribs, and the superior of the fibula. There is only a kind of slipping more or less obscure, and in which the osseous surfaces hardly ever leave each other. These surfaces are almost all plain, very close together, united by a considerable number of ligaments, and so strong in their connexion, that luxations hardly ever happen to them. Another reason moreover which renders them difficult, is that all this genus of articulations belongs almost wholly to short bones; now we know, that the motion imparted to a bone has a power of action which is in direct ratio of its length, and in the inverse of its smallness; for example, the same power applied to the tibial extremity of the femur, would luxate much more easily the ischiatic extremity, than if it acted upon the middle of this bone.

As the separate motion of each of the articulations of the fifth genus is almost nothing, nature usually unites several at the same place, for the purpose of producing a sensible, general motion, as we see in the carpus, the tarsus, the vertebræ, &c.; this is also a reason of the difficulty there is in luxating this genus of articulations. In fact, how great the general motions may be, two bones, taken separately, move but little upon each other; now it is only the extent of the motion of the two separate bones from each other, that can produce the displacement.

III. _Observations upon the Immoveable Articulations._

We have only pointed out orders in this class, because its varieties are not sufficiently great to assign genera for them.

1st. The order of immoveable articulations with surfaces in juxta-position, is met with where the mechanism of the part alone is almost sufficient to insure the solidity of the bones which are found only placed at the side of each other, without holding by any insertion, and having only between them a slight cartilaginous layer; the superior maxillary bones, wedged in between the malar bones, the ossa ungues, the ethmoid, the ossa palati, the vomer, and the frontal bone, are supported more by the general mechanism of the face, than by any articular attachments that unite them to each other; thus the squamous portion of the temporal bone supports the parietal, more by the abutting arches, than by the manner of the union of their respective surfaces. Remove this general mechanism of the part, you will soon see all the articulations separate.

2d. The order of immoveable articulations with inserted surfaces, owes also in some measure its solidity to the general mechanism of the part; but this mechanism would be insufficient to insure this solidity; thus the bones, instead of having almost plain surfaces, exhibit very evident prominences and depressions which are inserted into each other, as we see in the articulations of the parietal bones with each other, with the sphenoid, the occipital, the frontal, &c.; these are called sutures. This order sometimes approximates the preceding, as in the union of the parietal and frontal bones, which, reciprocally aiding each other, are supported by this mechanism, more than by their insertions; sometimes it resembles the following order, as in the articulation of the occipital and parietal bones, in which the very deep insertions almost alone insure the solidity of the union. This order is never seen except upon the edges of the flat bones; the insertion of these edges compensates for their want of size, by multiplying the points of contact. The eminences and depressions forming the insertion are always of an irregular form and size. They are exactly fitted to each other, they are not alike in two bones of the same species, taken from two different subjects; so that we cannot unite to a detached left parietal bone, the right parietal bone of another individual. There has been much dispute upon the formation of the sutures; they are an effect of the laws of ossification, an effect which we can account for no more than we can for all the others, and all the general phenomena of growth; we shall see the progress they follow in this formation. This articular order is gradually effaced with age, and the bones unite together by the ossification of the thin intermediate cartilage. It is more rare that the preceding order disappears. I have seen, however, in extreme old age, different articulations of this order cease to be evident, those of the maxillary bones between themselves especially.

3d. The order of articulations with implanted surfaces borrows none of its solidity from the mechanism of the part; it owes it entirely to the relation of the surfaces, which are so united and embraced by each other, that displacement is impossible. There is but one example of this articular order, it is the teeth with the jaws.

Age does not here efface the articulation, and thus confound the two bones as in the preceding orders, because the medium of union is the palatine membrane, which belongs to the mucous system, and which by its organization has no tendency to ossification; whereas in the preceding cases, the intermediate cartilage has a natural disposition to become encrusted with the phosphate of lime.