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ossification, and thus the plane of articulation becomes obliterated, so that direct structural continuity between the osseous segments takes place. The primary features common to all synarthroses are (a) continuous and direct union of the opposing surfaces; (b) no joint cavity; (c) no movement.
Sutura. This form of synarthrosis is found only in connexion with the bones of the skull. In a large number of cases the bones which articulate by suture present irregular interlocking margins, between which there is the interposed fibrous membrane to which reference has already been made. When these interlocking margins present well-defined projections they are said to form a sutura vera (true suture); on the other hand, when the opposed surfaces present ill-defined projections, or even flat areas, they are described as sutura notha (false suture). In each of these subdivisions the particular characters of the articulating margins are utilised in framing additional descriptive terms. Thus true sutures may possess interlocking margins whose projections are tooth-like (sutura dentata), e.g. in the interparietal suture; saw-like (sutura serrata) (Fig. 289), e.g. in the interfrontal suture; ridge-like, or comparable to the parallel ridges on the welt of a boot (sutura limbosa). Similarly false sutures may articulate by margins which are scalelike (sutura squamosa), e.g. in the squamoso-parietal suture; or by rough opposed surfaces, sutura harmonia, e.g. in the suture between the palatine processes of the maxillary bones. There is one variety of synarthrosis which, in the adult, can scarcely be called a suture, although the differences are of minor importance, viz., schindylesis, which is an articulation between the edge of a platelike bone, such as the rostrum of the sphenoid, and the cleft in another, such as the vomer.
Synchondrosis.-Illustrations of this group can be found only FIG. 289.-SUTURA in the young growing individual, because as age advances and growth ceases, the process of ossification affects the hyaline cartilage which constitutes the uniting medium, and the plane of articulation disappears. Under this heading we may include the planes of junction between all epiphyses and the diaphyses to which they severally belong. The occipito-sphenoid (Fig. 288) and the petro-jugular articulations in the base of the skull provide other well-marked examples.
Intervening -hyaline cartilage
FIG. 288.—SECTION THROUGH
AMPHIARTHROSES-DIARTHROSES (MOVABLE JOINTS).
The leading features of this group are capability of movement and permanence. In very few instances do such joints ever become obliterated under normal conditions. Determining their permanence, and regulating the amount of possible movement, there is always more or less of interruption in the continuity of the structures which bind the osseous elements together. That is, there is always some evidence of a cavum articulare (joint cavity), although as a matter of course such interruption can never be so extensive as to entirely disassociate the articulating elements. Therefore in all movable joints a new class of structures is found, viz., ligamenta (the ligaments), by means of which continuity is maintained even when all the other uniting media have given place to an articular cavity. The further subdivision of this group is founded upon the amount of movement permissible, and the extent to which the articular cavity takes the place of the original continuous uniting medium. Thus we obtain the amphiarthroses, or partly movable, and the diarthroses, or freely movable.
An amphiarthrosis (Fig. 292) presents the following characteristics: (a) partial movement; (b) union by ligaments and by an interposed plate or disc of fibro-cartilage, in the interior of which there is (c) an incomplete or partial joint cavity, which may be lined by a rudimentary stratum synoviale (synovial membrane)
whose function it is to secrete a lubricating fluid, the synovia or joint-oil; (d) a plate of hyaline cartilage coating each of the opposing surfaces of the bones concerned. All the joints belonging to this group occur in the median plane of the body. It includes the symphysis pubis, the joints between the bodies of the vertebræ, and the joint between the manubrium sterni and the body of the sternum. A diarthrosis (Fig. 291) is the most elaborate as well as the most complete form of articulation. It is characterised by (a) capability of movement which is more or less free in its range; (b) a reduction of the uniting structures to a series of retaining ligaments; (c) an articular cavity which is limited only by the surrounding ligaments; (d) the constant presence of synovial membrane; (e) cartilago articularis (hyaline encrusting cartilage) which clothes the opposed surfaces of the articulating bones. The majority of the synovialis joints in the adult belongs to this group. This series of joints has been subdivided into a number of minor sections, in order to emphasise the occurrence of certain wellmarked structural features, or because of the articularis particular nature of the movement by which they are characterised. Although in all diarthroses there is a certain amount of gliding movement between the opposed surfaces of the bones which enter into their formation, yet, when this gliding movement becomes their prominent feature, as in most of the joints of the carpus and tarsus, they are termed arthrodia. But bones may be articulated together so as to permit of movement in one, two, or more fixed axes of movement, or in modifications of these axes. Thus in uniaxial joints the axis of movement may lie in the longitudinal axis of the joint, in which case the trochoid rotatory form of joint results, as in the proximal and distal radio-ulnar articulations; or it may correspond with the transverse axis of the articulation, as in the elbow-joint and knee-joint, when the ginglymus or hinge variety results. If movement takes place about two principal axes situated at right angles to each other, as in the radio-carpal joint, the terms ellipsoid (biaxial or condyloid) are applied. Movements occurring about three principal axes placed at right angles to each other, or in modifications of these positions, constitute multiaxial joints, in which the associated structural peculiarities provide the alternative terms of enarthrosis or ball-and-socket joints.
STRUCTURES 'WHICH ENTER INTO THE FORMATION OF JOINTS.
The structures which enter into the formation of joints vary with the nature of the articulation. In every instance there are two or more skeletal elements, whether bones or cartilages, and in addition there are the uniting media, which are either simple or elaborate according to the provision made for rendering the joint more or less rigid, or capable of movement. We have already seen that the uniting medium in synarthrodial joints is a remnant of the common matrix, whether fibrovascular membrane or hyaline cartilage, in which ossification has extended from separate centres. Among the amphiarthroses there is still extensive union between the opposing surfaces of the articulating bones, but the character of the uniting medium has advanced from the primitive embryonic tissue to fibrous and fibrocartilaginous material, as well as hyaline cartilage. These, with very few exceptions, are permanent non-ossifying substances, such as may be seen between the opposing osseous surfaces of two vertebral bodies. The joint cavity, more or less rudimentary, is confined to the centre of the fibro-cartilaginous plate, and may result from the softening or imperfect cleavage of the central tissue. It It may also present rudiments of a synovial membrane.
In the diarthrodial group the extensive cavity has produced great interruption in the continuity of the uniting structures which originally existed between the
bones forming such a joint. Ligaments have therefore additional importance in this group, for not only do they constitute the uniting media which bind the articulating bones together, but, to a large extent, they form the peripheral boundary of the joint cavity, although not equally developed in all positions. Thus, every diarthrodial joint possesses a fibrous or ligamentous envelope constituting the fibrous stratum of the articular capsule, which is attached to the adjacent ends of the articulating bones. For special purposes, particular parts of the fibrous stratum may undergo enlargement and thickening, and so constitute strong ligamentous bands, although still forming continuous constituents of the envelope.
The fibrous stratum is lined by a stratum synoviale (O.T. synovial membrane), the two strata constituting the capsula articularis. The synovial stratum is continued from the inner surface of the fibrous stratum to the surface of the intraarticular portion of each articulating bone. The part of the bone included within the joint consists of a "non-articular " portion covered by the synovial layer and articular " portion covered by encrusting hyaline cartilage. The latter provides the surface which comes into apposition with the corresponding area of another bone. In its general disposition the synovial layer may be likened to a cylindrical tube open at each end. This layer is richly supplied by a close network of
vessels and nerves.
Certain diarthroses present intracapsular structures which may be distinguished as interarticular ligaments and articular discs and menisci (O.T. interarticular fibro-cartilages).
Ligamenta Interarticularia.-Interarticular ligaments extend between, and are attached to, non-articular areas of the intracapsular portions of the articulating bones. They usually occupy the long axis of the joint, and occasionally they widen sufficiently to form partitions which divide the joint-cavity into two compartments, e.g. the articulation of the heads of the ribs with the vertebral column, and certain of the costo-sternal joints.
Articular discs and menisci (O.T. interarticular fibro-cartilages) (Fig. 291) are more or less complete partitions situated between and separating opposing articular surfaces, and when complete they divide the joint cavity into two distinct compartments. By its periphery, a disc is rather to be associated with the articular Cartilago capsule than with the articulating bones, although its attachments may extend to non-articular areas on the latter. Those found in the knee-joint are called menisci; those found in other joints are called articular discs.
Discus Stratum synoviale
FIG. 291.-DIAGRAM OF A DIARTHRODIAL JOINT
CAVITY INTO TWO COMPARTMENTS.
Both interarticular ligaments and articular discs and menisci have their free surfaces covered by the synovial stratum.
Adipose tissue, forming pads of varying size, is usually found in certain localities within the joint, between the synovial stratum and the surfaces which it covers. These pads are Soft and pliable, and act as packing material, filling up gaps or intervals in the joint. During movement they adapt themselves to
the changing conditions of the articulation.
In addition to merely binding together two or more articulating bones, ligaments perform very important functions in connexion with the different movements taking place at a joint. They do not appreciably lengthen under strains, and thus ligaments may act as inhibitory structures, and by becoming tense may restrain or check movement in certain directions.
Synovial strata, in the form of closed sacs termed mucous or synovial bursæ, are frequently found in other situations besides the interior of joints. Such bursæ are developed for the purpose of reducing the friction, (a) between the integument and certain prominent subcutaneous bony projections, as, for instance, the point of the elbow, or the anterior surface of the patella
(subcutaneous mucous bursæ); (b) between a tendon and some surface, bony or cartilaginous, over which it plays (subtendinous mucous bursæ); (c) between a tendon or a group of tendons and the walls of osteo-fascial tunnels, in which they play (vaginæ mucosa tendinum or mucous sheaths of tendons). Subtendinous mucous bursæ are often placed in the neighbourhood of joints, and in such cases it not infrequently happens that there is a direct continuity between the bursa and the synovial stratum which lines the cavity of the joint through an aperture in the articular capsule.
THE DIFFERENT KINDS OF MOVEMENT AT JOINTS.
Reference has already been made to the existence of fixed axes of movement as a basis for the classification of certain forms of diarthrodial joints. Hence it is evident that the movements which are possible at any particular joint depend to a large extent upon the shape of its articular surfaces as well as upon the nature of its various ligaments. Therefore the technical terms descriptive of movements either indicate the directions in which they occur, or else the character of the completed movement.
In the great majority of articulations between short bones, the amount of movement is so restricted, and the displacement of the opposing articular surfaces so slight, that the term gliding sufficiently expresses its character.
A gliding movement of an extensive kind, for example that of the patella upon the femur, in which the movement largely resembles that of the tyre of a wheel revolving in contact with the ground so that different parts are successively adapted to each other, is called co-aptation.
Articulations between long bones, on the other hand, are usually associated with a much freer range of movement, with a corresponding variety in its character. Rotation is a movement around an axis which is longitudinal. Sometimes it is the only form of movement which a joint possesses; at other times it is merely one of a series of movements capable of execution at the same joint. Flexion or bending is a movement in which the formation of an angle between two parts of the body is an essential feature. As it is possible to perform this movement in relation to two axes, viz., a transverse and an antero-posterior axis, it is necessary to introduce qualifying terms. Thus, when two anterior or ventral surfaces are approximated, as at the hip-, elbow-, or wrist-joints, the movement is called ventral, anterior, or palmar flexion; but if posterior or dorsal surfaces are approximated by the process of bending, then the flexion becomes posterior or dorsi-flexion, as at the knee- or wristjoints. Further, at the wrist-joint, the formation of an angle between the ulnar border of the hand and the corresponding aspect of the forearm, produces ulnar flexion, and similarly the bending of the hand towards the radial border of the forearm is radial flexion.
Extension or straightening consists in obliterating the angle which resulted from flexion. In the case of certain joints, therefore, such as the elbow, wrist, and knee, the segments of the limb occupy a straight line as regards each other when extended.
At the ankle-joint the natural attitude of the foot to the leg is flexion at a right angle. The diminution of this angle by approximating the dorsum of the foot towards the anterior aspect of the leg constitutes flexion; while any effort at placing the foot and leg in a straight line, i.e. obliteration of the angle, as in pointing the toes towards the ground and raising the heel, constitutes extension.
Abduction is a term which either expresses movement of an entire limb in a direction away from the median plane of the body, or of a digit, away from the plane of the middle finger in the hand, or the plane of the second toe in the case of the foot.
Adduction is the reverse of abduction, and signifies movement towards the median plane of the body, or towards the planes indicated for the digits of the hand and foot.
Circumduction is a movement peculiarly characteristic of multiaxial or balland-socket joints. It consists in combining such angular movements as flexion, extension, abduction, and adduction, so as to continue the one into the other, whereby the joint forms the apex of a cone of movement, and the free end of the limb travels through a circle which describes the base of this cone.
THE DEVELOPMENT OF JOINTS.
Just as the question of structure determines to a large extent the presence or absence of movement in joints, so in tracing their development it will be found that the manner of their appearance forecasts their ultimate destination as immovable or movable articulations.
All joints arise in mesodermic tissue which has undergone more or less differentiation. When this differentiation has produced a continuous membranous layer, in which ossific centres representing separate skeletal segments make their appearance, we get the primitive form of suture. The plane of the articulation merely indicates the limit of the ossific process extending from different directions. If, again, the differentiation of the mesoderm has resulted in the formation of a continuous cartilaginous layer, in which ossification commences at separate centres, the plane of the articulation is marked out by the unossified cartilage-in other words, the articulation is a synchondrosis. Ultimately this disappears through the extension of the process of ossification.
To some extent sutures also disappear, although their complete obliteration is not usual even in aged people. Developmentally, therefore, synarthroses or immovable joints do not present any special structural element, and, speaking generally, they have only a temporary existence.
The development of all movable joints is in marked contrast to that of synarthroses. Not only are they permanent arrangements so far as concerns normal conditions, but they never arise merely as planes which indicate the temporary phase of an ossific process. From the outset they present distinct skeletal units, from which the special structures of the joint are derived.
The primitive movable joint is first recognised as a mass of undifferentiated mesodermic cells situated between two masses, which have differentiated into primitive cartilage.
The cell-mass which constitutes the joint-unit presents the appearance of a thick cellular disc, the proximal and distal surfaces of which are in accurate apposition with the primitive cartilages, while its circumference is defined from the surrounding mesoderm by a somewhat closer aggregation of the cells of which the disc is composed. From this cellular disc or joint-unit all the structures characteristic of amphiarthrodial and diarthrodial joints are ultimately developed.
Thus, by the transformation of the circumferential cells into fibrous tissue the investing ligaments are produced. Within the substance of the disc itself a transverse cleft, more or less well-defined and complete, makes its appearance. In this manner the disc is divided into proximal and distal segments, separated from each other by an interval which is the primitive articular cavity. This cleft, however, never extends so far as to interrupt the continuity of the circumferential part of the disc which develops into the fibrous tissue of the investing ligaments. From the proximal and distal segments of the articular disc the various structures, distinctive of movable joints, are developed.
Thus, in amphiarthrodial joints the cellular articular disc or primitive joint-unit gives origin to the following structures :-From its circumference, investing ligaments; from its interior, the fibro cartilaginous plate in which an imperfect articular cavity with corresponding imperfect synovial stratum may be found.
In the case of a diarthrodial joint the changes take place on a more extended scale. The articular cavity becomes a prominent feature, in relation to which the surrounding fibrous structures form an investing capsule, lined with a synovial stratum.
When a single cleft arises, but does not extend completely across the longitudinal axis of the articular disc, the undivided portion develops into fibrous interarticular ligaments. On the other hand, when two transverse clefts are formed, that portion of the cellular disc which remains between them becomes transformed into a fibro-cartilaginous disc (or in the case of the knee-joint, menisci), which in its turn may either be complete or incomplete, and thus we may obtain two distinct synovial joint cavities belonging to one articulation.1
In considering the development of the synovial layer, and the surfaces on which it is found in the interior of a joint, it is necessary to keep clearly in mind that a synovial layer is a special structure, whose function it is to produce a lubricating fluid or synovia, and that, therefore, its position is determined by the essential necessity of proximity to a direct blood-supply. In other words, this condition is provided by all parts of
1 From a series of observations upon the development of diarthrodial joints, the writer considers that there is evidence to show that the "cellular articular disc" is directly responsible for the production of the epiphyses which adjoin the completed articular cavity, and that, among such amphiarthroses as exist between the bodies of vertebræ, not only the intervertebral fibro-cartilage, but the proximal and distal epiphyses which ultimately unite with the vertebral bodies have a common origin in the joint-unit.