VOLUME XV, SLICE V

Joints to Justinian I.

ARTICLES IN THIS SLICE:

JOINTS (anatomy) JUBILEES, BOOK OF JOINTS (engineering) JUBILEE YEAR JOINTS (geology) JÚCAR JOINTURE JUD, LEO JOINVILLE JUDAEA JOINVILLE, FRANÇOIS LOUIS MARIE JUDAH JOINVILLE, JEAN JUDAS ISCARIOT JOIST JUDAS-TREE JÓKAI, MAURUS JUDD, SYLVESTER JOKJAKARTA JUDE, THE GENERAL EPISTLE OF JOLIET JUDGE JOLLY JUDGE-ADVOCATE-GENERAL JOLY DE LOTBINIÈRE, GUSTAVE JUDGES, THE BOOK OF JOMINI, ANTOINE HENRI JUDGMENT JOMMELLI, NICCOLA JUDGMENT DEBTOR JONAH (prophet) JUDGMENT SUMMONS JONAH, RABBI JUDICATURE ACTS JONAS, JUSTUS JUDITH, THE BOOK OF JONATHAN JUDSON, ADONIRAM JONCIÈRES, VICTORIN JUEL, JENS JONES, ALFRED GILPIN JUEL, NIELS JONES, SIR ALFRED LEWIS JUG JONES, EBENEZER JUGE, BOFFILLE DE JONES, ERNEST CHARLES JUGGERNAUT JONES, HENRY JUGGLER JONES, HENRY ARTHUR JUGURTHA JONES, INIGO JUJU JONES, JOHN JUJUBE JONES, JOHN PAUL JU-JUTSU or JIU-JITSU JONES, MICHAEL JUJUY JONES, OWEN (Welsh antiquary) JUKES, JOSEPH BEETE JONES, OWEN (British architect) JULIAN JONES, RICHARD JÜLICH JONES, THOMAS RUPERT JULIEN, STANISLAS JONES, WILLIAM JULIUS JONES, SIR WILLIAM JULLIEN, LOUIS ANTOINE JÖNKÖPING JULLUNDUR JONSON, BEN JULY JOPLIN JUMALA JOPPA JUMIÈGES JORDAENS, JACOB JUMILLA JORDAN, CAMILLE JUMNA JORDAN, DOROTHEA JUMPING JORDAN, THOMAS JUMPING-HARE JORDAN, WILHELM JUMPING-MOUSE JORDAN (river) JUMPING-SHREW JORDANES JUNAGARH JORDANUS JUNCACEAE JORIS, DAVID JUNCTION CITY JORTIN, JOHN JUNE JOSEPH (Old Testament) JUNEAU JOSEPH (New Testament) JUNG, JOHANN HEINRICH JOSEPH OF ARIMATHAEA JUNG BAHADUR, SIR JOSEPH I. JUNG-BUNZLAU JOSEPH II. JUNGFRAU JOSEPH, FATHER JUNGLE JOSEPHINE JUNIN JOSEPHUS, FLAVIUS JUNIPER JOSHEKAN JUNIUS JOSHUA, BOOK OF JUNIUS, FRANZ JOSHUA THE STYLITE JUNK JOSIAH JUNKER, WILHELM JÓSIKA, MIKLOS [NICHOLAS] JUNKET JOSIPPON JUNO JOSS JUNOT, ANDOCHE JOST, ISAAK MARKUS JUNOT, LAURE JOTUNHEIM JUNTA JOUBERT, BARTHÉLEMY CATHERINE JUPITER (Roman deity) JOUBERT, JOSEPH JUPITER (planet) JOUBERT, PETRUS JACOBUS JUR JOUFFROY, JEAN JURA (department of France) JOUFFROY, THÉODORE SIMON JURA (island) JOUGS JURA (mountains) JOULE, JAMES PRESCOTT JURASSIC JOURDAN, JEAN BAPTISTE JURAT JOURNAL JURIEN DE LA GRAVIÈRE, JEAN EDMOND JOURNEY JURIEU, PIERRE JOUVENET, JEAN JURIS JOUY, VICTOR JOSEPH ÉTIENNE DE JURISDICTION JOVELLANOS, GASPAR MELCHOR DE JURISPRUDENCE JOVELLAR Y SOLER, JOAQUIN JURISPRUDENCE, COMPARATIVE JOVIAN JURJANI JOVINIANUS JURY JOVIUS, PAULUS JUS PRIMAE NOCTIS JOWETT, BENJAMIN JUS RELICTAE JOYEUSE JUSSERAND, JEAN ADRIEN ANTOINE JULES JOYEUSE ENTRÉE JUSSIEU, DE JUAN FERNANDEZ ISLANDS JUSTICE JUANGS JUSTICE OF THE PEACE JUAN MANUEL, DON JUSTICIAR JUAREZ, BENITO PABLO JUSTICIARY, HIGH COURT OF JUBA (kings of Numidia) JUSTIFICATION JUBA (African river) JUSTIN I. JUBBULPORE JUSTIN II. JUBÉ JUSTIN (Roman historian) JUBILEE (or Jubile), YEAR OF JUSTINIAN I.

JOINTS, in anatomy. The study of joints, or articulations, is known as Arthrology (Gr. [Greek: arthron]), and naturally begins with the definition of a joint. Anatomically the term is used for any connexion between two or more adjacent parts of the skeleton, whether they be bone or cartilage. Joints may be immovable, like those of the skull, or movable, like the knee.

Immovable joints, or _synarthroses_, are usually adaptations to growth rather than mobility, and are always between bones. When growth ceases the bones often unite, and the joint is then obliterated by a process known as _synostosis_, though whether the union of the bones is the cause or the effect of the stoppage of growth is obscure. Immovable joints never have a cavity between the two bones; there is simply a layer of the substance in which the bone has been laid down, and this remains unaltered. If the bone is being deposited in cartilage a layer of cartilage intervenes, and the joint is called _synchondrosis_ (fig. 1), but if in membrane a thin layer of fibrous tissue persists, and the joint is then known as a _suture_ (fig. 2). Good examples of synchondroses are the epiphysial lines which separate the epiphyses from the shafts of developing long bones, or the occipito-sphenoid synchondrosis in the base of the skull. Examples of sutures are plentiful in the vault of the skull, and are given special names, such as sutura dentata, s. serrata, s. squamosa, according to the plan of their outline. There are two kinds of fibrous synarthroses, which differ from sutures in that they do not synostose. One of these is a _schindylesis_, in which a thin plate of one bone is received into a slot in another, as in the joint between the sphenoid and vomer. The other is a peg and socket joint, or _gomphosis_, found where the fangs of the teeth fit into the alveoli or tooth sockets in the jaws.

Movable joints, or _diarthroses_, are divided into those in which there is much and little movement. When there is little movement the term half-joint or _amphiarthrosis_ is used. The simplest kind of amphiarthrosis is that in which two bones are connected by bundles of fibrous tissue which pass at right angles from the one to the other; such a joint only differs from a suture in the fact that the intervening fibrous tissue is more plentiful and is organized into definite bundles, to which the name of _interosseous ligaments_ is given, and also that it does not synostose when growth stops. A joint of this kind is called a _syndesmosis_, though probably the distinction is a very arbitrary one, and depends upon the amount of movement which is brought about by the muscles on the two bones. As an instance of this the inferior tibio-fibular joint of mammals may be cited. In man this is an excellent example of a syndesmosis, and there is only a slight play between the two bones. In the mouse there is no movement, and the two bones form a synchondrosis between them which speedily becomes a synostosis, while in many Marsupials there is free mobility between the tibia and fibula, and a definite synovial cavity is established. The other variety of amphiarthrosis or half-joint is the _symphysis_, which differs from the syndesmosis in having both bony surfaces lined with cartilage and between the two cartilages a layer of fibro-cartilage, the centre of which often softens and forms a small synovial cavity. Examples of this are the symphysis pubis, the mesosternal joint, and the joints between the bodies of the vertebrae (fig. 3).

The _true diarthroses_ are joints in which there is either fairly free or very free movement. The opposing surfaces of the bones are lined with articular cartilage, which is the unossified remnant of the cartilaginous model in which they are formed and is called the _cartilage of encrustment_ (fig. 4, c). Between the two cartilages is the _joint cavity_, while surrounding the joint is the _capsule_ (fig. 4, l), which is formed chiefly by the superficial layers of the original periosteum or perichondrium, but it may be strengthened externally by surrounding fibrous structures, such as the tendons of muscles, which become modified and acquire fresh attachments for the purpose. It may be said generally that the greater the intermittent strain on any part of the capsule the more it responds by increasing in thickness. Lining the interior of the capsule, and all other parts of the joint cavity except where the articular cartilage is present, is the _synovial membrane_ (fig. 4, dotted line); this is a layer of endothelial cells which secrete the synovial fluid to lubricate the interior of the joint by means of a small percentage of mucin, albumin and fatty matter which it contains.

A _compound diarthrodial joint_ is one in which the joint cavity is divided partly or wholly into two by a _meniscus_ or _interarticular fibro-cartilage_ (fig. 5, Fc).

The shape of the joint cavity varies greatly, and the different divisions of movable joints depend upon it. It is often assumed that the structure of a joint determines its movement, but there is something to be said for the view that the movements to which a joint is subject determine its shape. As an example of this it has been found that the mobility of the metacarpo-phalangeal joint of the thumb in a large number of working men is less than it is in a large number of women who use needles and thread, or in a large number of medical students who use pens and scalpels, and that the slightly movable thumb has quite a differently shaped articular surface from the freely movable one (see _J. Anat. and Phys._ xxix. 446). R. Fick, too, has demonstrated that the concavity or convexity of the joint surface depends on the position of the chief muscles which move the joint, and has enunciated the law that when the chief muscle or muscles are attached close to the articular end of the skeletal element that end becomes concave, while, when they are attached far off or are not attached at all, as in the case of the phalanges, the articular end is convex. His mechanical explanation is ingenious and to the present writer convincing (see _Handbuch der Gelenke_, by R. Fick, Jena, 1904). Bernays, however, pointed out that the articular ends were moulded before the muscular tissue was differentiated (_Morph. Jahrb._