Chapter I: Part 1

Transcriber's notes:

(1) Numbers following letters (without space) like C2 were originally
printed in subscript. Letter subscripts are preceded by an
underscore, like C_n.

(2) Characters following a carat (^) were printed in superscript.

(3) Side-notes were relocated to function as titles of their respective
paragraphs.

(4) Macrons and breves above letters and dots below letters were not
inserted.

(5) [root] stands for the root symbol; [alpha], [beta], etc. for greek
letters.

(6) The following typographical errors have been corrected:

ARTICLE JONES, INIGO: "... and in the capacity of designer of the
masques he came into collision with Ben Jonson, who frequently made
him the butt of his satire." 'collision' amended from 'collison'.

ARTICLE JOPLIN: "Joplin is the trade centre of a rich agricultural
and fruit-growing district, but its growth has been chiefly due to
its situation in one of the most productive zinc and lead regions
in the country, for which it is the commercial centre." 'most'
amended from 'must'.

ARTICLE JORDANES: "... and their differentiation into Visigoths and
Ostrogoths, are next described. Chs. v.-xiii. contain an account of
the intrusive Geto-Scythian element before alluded to." 'next'
amended from 'nest'.

ARTICLE JURASSIC: "... similarly at the top of the system there is
a passage from the Jurassic to the Cretaceous rocks (Alps)."
'system' amended from 'sytsem'.

ARTICLE JURY: "... 'copied from this or that kindred institution to
be found in this or that German or Scandinavian land,' or brought
over ready made by Hengist or by William." 'or' amended from 'of'.

ENCYCLOPAEDIA BRITANNICA

A DICTIONARY OF ARTS, SCIENCES, LITERATURE
AND GENERAL INFORMATION

ELEVENTH EDITION

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._
iv. 403), but to this Fick replies by pointing out that muscular
movements begin before the muscle fibres are formed, and may be seen
in the chick as early as the second day of incubation.

The freely movable joints (true diarthrosis) are classified as
follows:--

(1) _Gliding joints_ (_Arthrodia_), in which the articular surfaces
are flat, as in the carpal and tarsal bones.

(2) _Hinge joints_ (_Ginglymus_), such as the elbow and
interphalangeal joints.

(3) _Condyloid joints_ (_Condylarthrosis_), allowing flexion and
extension as well as lateral movement, but no rotation. The
metacarpo-phalangeal and wrist joints are examples of this.

(4) _Saddle-shaped joints_ (_Articulus sellaris_), allowing the same
movements as the last with greater strength. The carpo-metacarpal
joint of the thumb is an example.

(5) _Ball and socket joints_ (_Enarthrosis_), allowing free movement
in any direction, as in the shoulder and hip.

(6) _Pivot-joint_ (_Trochoides_), allowing only rotation round a
longitudinal axis, as in the radio-ulnar joints.

_Embryology._

Joints are developed in the mesenchyme, or that part of the mesoderm which is not concerned in the formation of the serous cavities. The synarthroses may be looked upon merely as a delay in development, because, as the embryonic tissue of the mesenchyme passes from a fibrous to a bony state, the fibrous tissue may remain along a certain line and so form a suture, or, when chondrification has preceded ossification, the cartilage may remain at a certain place and so form a synchondrosis. The diarthroses represent an arrest of development at an earlier stage, for a part of the original embryonic tissue remains as a plate of round cells, while the neighbouring two rods chondrify and ossify. This plate may become converted into fibro-cartilage, in which case an amphiarthrodial joint results, or it may become absorbed in the centre to form a joint cavity, or, if this absorption occurs in two places, two joint cavities with an intervening meniscus may result. Although, ontogenetically, there is little doubt that menisci arise in the way just mentioned, the teaching of comparative anatomy suggests that, phylogenetically, they originate as an ingrowth from the capsule pushing the synovial membrane in front of them. The subject will be returned to when the comparative anatomy of the individual joints is reviewed. In the human foetus the joint cavities are all formed by the tenth week of intra-uterine life.

ANATOMY

_Joints of the Axial Skeleton._

The bodies of the vertebrae except those of the sacrum and coccyx are separated, and at the same time connected, by the _intervertebral disks_. These are formed of alternating concentric rings of fibrous tissue and fibro-cartilage, with an elastic mass in the centre known as the _nucleus pulposus_. The bodies are also bound together by _anterior_ and _posterior common ligaments_. The odontoid process of the axis fits into a pivot joint formed by the anterior arch of the atlas in front and the _transverse ligament_ behind; it is attached to the basioccipital bone by two strong _lateral check ligaments_, and, in the mid line, by a feebler _middle check ligament_ which is regarded morphologically as containing the remains of the notochord. This _atlanto-axial joint_ is the one which allows the head to be shaken from side to side. Nodding the head occurs at the _occipito-atlantal joint_, which consists of the two occipital condyles received into the cup-shaped articular facets on the atlas and surrounded by capsular ligaments. The neural arches of the vertebrae articulate one with another by the _articular facets_, each of which has a capsular ligament. In addition to these the laminae are connected by the very elastic _ligamenta subflava_. The spinous processes are joined by _interspinous ligaments_, and their tips by a _supraspinous ligament_, which in the neck is continued from the spine of the seventh cervical vertebra to the external occipital crest and protuberance as the _ligamentum nuchae_, a thin, fibrous, median septum between the muscles of the back of the neck.

The combined effect of all these joints and ligaments is to allow the spinal column to be bent in any direction or to be rotated, though only a small amount of movement occurs between any two vertebrae.

The heads of the ribs articulate with the bodies of two contiguous thoracic vertebrae and the disk between. The ligaments which connect them are called _costo-central_, and are two in number. The anterior of these is the _stellate ligament_, which has three bands radiating from the head of the rib to the two vertebrae and the intervening disk. The other one is the _interarticular ligament_, which connects the ridge, dividing the two articular cavities on the head of the rib, to the disk; it is absent in the first and three lowest ribs.

The _costo-transverse ligaments_ bind the ribs to the transverse processes of the thoracic vertebrae. The _superior costo-transverse ligament_ binds the neck of the rib to the transverse process of the vertebra above; the _middle_ or _interosseous_ connects the back of the neck to the front of its own transverse process; while the _posterior_ runs from the tip of the transverse process to the outer part of the tubercle of the rib. The inner and lower part of each tubercle forms a diarthrodial joint with the upper and fore part of its own transverse process, except in the eleventh and twelfth ribs. At the junction of the ribs with their cartilages no diarthrodial joint is formed; the periosteum simply becomes perichondrium and binds the two structures together. Where the cartilages, however, join the sternum, or where they join one another, diarthrodial joints with synovial cavities are established. In the case of the second rib this is double, and in that of the first usually wanting. The _mesosternal joint_, between the pre- and mesosternum, has already been given as an example of a symphysis.

_Comparative Anatomy._--For the convexity or concavity of the
vertebral centra in different classes of vertebrates, see SKELETON:
_axial_. The intervertebral disks first appear in the Crocodilia, the
highest existing order of reptilia. In many Mammals the middle
fasciculus of the stellate ligament is continued right across the
ventral surface of the disk into the ligament of the opposite side,
and is probably serially homologous with the ventral arch of the
atlas. A similar ligament joins the heads of the ribs dorsal to the
disk. To these bands the names of anterior (ventral) and posterior
(dorsal) _conjugal ligaments_ have been given, and they may be
demonstrated in a seven months' human foetus (see B. Sutton,
_Ligaments_, London, 1902). The _ligamentum nuchae_ is a strong
elastic band in the Ungulata which supports the weight of the head. In
the Carnivora it only reaches as far forward as the spine of the axis.

The JAW JOINT, or _temporo-mandibular articulation_, occurs between the sigmoid cavity of the temporal bone and the condyle of the jaw. Between the two there is an interarticular fibro-cartilage or meniscus, and the joint is surrounded by a capsule of which the outer part is the thickest. On first opening the mouth, the joint acts as a hinge, but very soon the condyle begins to glide forward on to the eminentia articularis (see SKULL) and takes the meniscus with it. This gliding movement between the meniscus and temporal bone may be separately brought about by protruding the lower teeth in front of the upper, or, on one side only, by moving the jaw across to the opposite side.

_Comparative Anatomy._--The joint between the temporal and mandibular
bones is only found in Mammals; in the lower vertebrates the jaw opens
between the quadrate and articular bones. In the Carnivora it is a
perfect hinge; in many Rodents only the antero-posterior gliding
movement is present; while in the Ruminants the lateralizing movement
is the chief one. Sometimes, as in the Ornithorhynchus, the meniscus
is absent.

_Joints of the Upper Extremity._

The _sterno-clavicular articulation_, between the presternum and clavicle, is a gliding joint, and allows slight upward and downward and forward and backward movements. The two bony surfaces are separated by a meniscus, the vertical movements taking place outside and the antero-posterior inside this. There is a well-marked capsule, of which the anterior part is strongest. The two clavicles are joined across the top of the presternum by an _interclavicular ligament_.

The _acromio-clavicular articulation_ is also a gliding joint, but allows a swinging or pendulum movement of the scapula on the clavicle. The upper part of the capsule is strongest, and from it hangs down a partial meniscus into the cavity.

_Comparative Anatomy._--Bland Sutton regards the interclavicular
ligament as a vestige of the interclavicle of Reptiles and Monotremes.
The menisci are only found in the Primates, but it must be borne in
mind that many Mammals have no clavicle, or a very rudimentary one. By
some the meniscus of the sterno-clavicular joint is regarded as the
homologue of the lateral part of the interclavicle, but the fact that
it only occurs in the Primates where movements in different planes are
fairly free is suggestive of a physiological rather than a
morphological origin for it.

The SHOULDER JOINT is a good example of the ball and socket or enarthrodial variety. Its most striking characteristic is mobility at the expense of strength. The small size of the glenoid cavity in comparison with the head of the humerus, and the great laxity of the capsule, favour this, although the glenoid cavity is slightly deepened by a fibrous lip, called the _glenoid ligament_, round its margin. The presence of the coracoid and acromial processes of the scapula, with the _coraco-acromial ligament_ between them, serves as an overhanging protection to the joint, while the biceps tendon runs over the head of the humerus, inside the capsule, though surrounded by a sheath of synovial membrane. Were it not for these two extra safeguards the shoulder would be even more liable to dislocation than it is. The upper part of the capsule, which is attached to the base of the coracoid process, is thickened, and known as the _coracohumeral ligament_, while inside the front of the capsule are three folds of synovial membrane, called _gleno-humeral folds_.

_Comparative Anatomy._--In the lower Vertebrates the shoulder is
adapted to support rather than prehension and is not so freely movable
as in the Primates. The tendon of the biceps has evidently sunk
through the capsule into the joint, and even when it is intra-capsular
there is usually a double fold connecting its sheath of synovial
membrane with that lining the capsule. In Man this has been broken
through, but remains of it persist in the _superior gleno-humeral
fold_. The _middle gleno-humeral fold_ is the vestige of a strong
ligament which steadies and limits the range of movement of the joint
in many lower Mammals.

The ELBOW JOINT is an excellent example of the ginglymus or hinge, though its transverse axis of movement is not quite at right angles to the central axis of the limb, but is lower internally than externally. This tends to bring the forearm towards the body when the elbow is bent. The elbow is a great contrast to the shoulder, as the trochlea and capitellum of the humerus are closely adapted to the sigmoid cavity of the ulna and head of the radius (see SKELETON: _appendicular_); consequently movement in one plane only is allowed, and the joint is a strong one. The capsule is divided into anterior, posterior, and two lateral ligaments, though these are all really continuous. The joint cavity communicates freely with that of the superior radio-ulnar articulation.

The _radio-ulnar joints_ are three: the upper one is an example of a pivot joint, and in it the disk-shaped head of the radius rotates in a circle formed by the lesser sigmoid cavity of the ulna internally and the _orbicular ligament_ in the other three quarters.

The _middle radio-ulnar articulation_ is simply an interosseous membrane, the fibres of which run downward and inward from the radius to the ulna.

The _inferior radio-ulnar joint_ is formed by the disk-shaped lower end of the ulna fitting into the slightly concave sigmoid cavity of the radius. Below, the cavity of this joint is shut off from that of the wrist by a _triangular fibro-cartilage_. The movements allowed at these three articulations are called pronation and supination of the radius. The head of that bone twists, in the orbicular ligament, round its central vertical axis for about half a circle. Below, however, the whole lower end of the radius circles round the lower end of the ulna, the centre of rotation being close to the styloid process of the ulna. The radius, therefore, in its pronation, describes half a cone, the base of which is below, and the hand follows the radius.

_Comparative Anatomy._--In pronograde Mammals the forearm is usually
permanently pronated, and the head of the radius, instead of being
circular and at the side of the upper end of the ulna, is transversely
oval and in front of that bone, occupying the same place that the
coronoid process of the ulna does in Man. This type of elbow, which is
adapted simply to support and progression, is best seen in the
Ungulata; in them both lateral ligaments are attached to the head of
the radius, and there is no orbicular ligament, since the shape of the
head of the radius does not allow of any supination. The olecranon
process of the ulna forms merely a posterior guide or guard to the
joint, but transmits no weight. No better example of the maximum
changes which the uses of support and prehension bring about can be
found than in contrasting the elbow of the Sheep or other Ungulate
with that of Man. Towards one or other of these types the elbows of
all Mammals tend. It may be roughly stated that, when pronation and
supination to the extent of a quarter of a circle are possible, an
orbicular ligament appears.

The WRIST JOINT, or _radio-carpal articulation_, lies between the radius and triangular fibro-cartilage above, and the scaphoid, semilunar, and cuneiform bones below. It is a condyloid joint allowing flexion and extension round one axis, and slight lateral movement (abduction and adduction) round the other. There is a well-marked capsule, divided into anterior, posterior, and lateral ligaments. The joint cavity is shut off from the inferior radio-ulnar joint above, and the intercarpal joints below.

The _intercarpal joints_ are gliding articulations, the various bones being connected by palmar, dorsal, and a few interosseous ligaments, but only those connecting the first row of bones are complete, and so isolate one joint cavity from another. That part of the intercarpal joints which lies between the first and second rows of carpal bones is called the _transverse carpal joint_, and at this a good deal of the movement which seems to take place at the wrist really occurs.

The _carpo-metacarpal articulations_ are, with the exception of that of the thumb, gliding joints, and continuous with the great intercarpal joint cavity. The carpo-metacarpal joint of the thumb is the best example of a saddle-shaped joint in Man. It allows forward and backward and lateral movement, and is very strong.

The _metacarpo-phalangeal joints_ are condyloid joints like the wrist, and are remarkable for the great thickness of the palmar ligaments of their capsules. In the four inner fingers these _glenoid ligaments_, as they are called, are joined together by the _transverse metacarpal ligament_.

The _interphalangeal articulations_ are simple hinges surrounded by a capsule, of which the dorsal part is very thin.

_Comparative Anatomy._--The wrist joint of the lower Mammals allows
less lateral movement than does that of Man, while the lower end of
the ulna is better developed and is received into a cup-shaped socket
formed by the cuneiform and pisiform bones. At the same time, unless
there is pretty free pronation and supination, the triangular
fibro-cartilage is only represented by an interosseous ligament, which
may be continuous above with the interosseous membrane between the
radius and ulna, and suggests the possibility that the fibro-cartilage
is largely a derivative of this membrane. In most Mammals the wrist is
divided into two lateral parts, as it is in the human foetus, but free
pronation and supination seem to cause the disappearance of the
septum.

_Joints of the Lower Extremity._

The _sacro-innominate articulation_ consists of the _sacro-iliac joint_ and the _sacro-sciatic ligaments_. The former is one of the amphiarthroses or half-joints by which the sacrum is bound to the ilium. The mechanism of the human sacrum is that of a suspension bridge slung between the two pillars or ilia by the very strong _posterior sacro-iliac_ ligaments which represent the chains. The axis of the joint passes through the second sacral vertebra, but the sacrum is so nearly horizontal that the weight of the body, which is transmitted to the first sacral vertebra, tends to tilt that part down. This tendency is corrected by the great and small _sacro-sciatic ligaments_, which fasten the lower part of the sacrum to the tuberosity and spine of the ischium respectively, so that, although the sacrum is a suspension bridge when looked at from behind, it is a lever of the first kind when seen from the side or in sagittal section.

The _pubic symphysis_ is the union between the two pubic bones. It has all the characteristics of a symphysis, already described, and may have a small median cavity.

FIG. 6.--Dissection of the Hip Joint from the front.]

The HIP JOINT, like the shoulder, is a ball and socket, but does not allow such free movement; this is due to the fact that the socket or acetabulum is deeper than the glenoid cavity and that the capsule is not so lax. At the same time the loss of mobility is made up for by increased strength. The capsule has three thickened bands, of which the most important is the _ilio-femoral_ or _Y-shaped ligament of Bigelow_. The stalk of the Y is attached to the anterior inferior spine of the ilium, while the two limbs are fastened to the upper and lower parts of the spiral line of the femur. The ligament is so strong that it hardly ever ruptures in a dislocation of the hip. As a plumb-line, dropped from the centre of gravity of the body, passes behind the centre of the hip joint, this ligament, lying as it does in front of the joint, takes the strain in Man's erect position. The other two thickened parts of the capsule are known as _pubo-femoral_ and _ischio-femoral_, from their attachments. Inside the capsule, and deepening the margin of the acetabulum, is a fibrous rim known as the _cotyloid ligament_, which grips the spherical head of the femur and is continued across the cotyloid notch as the _transverse ligament_. The floor of the acetabulum has a horseshoe-shaped surface of articular cartilage, concave downward, and, occupying the "frog" of the horse's hoof, is a mass of fat called the _Haversian pad_. Attached to the inner margin of the horseshoe, and to the transverse ligament where that is deficient, is a reflexion of synovial membrane which forms a covering for the pad and is continued as a tube to the depression on the head of the femur called the _fossa capitis_. This reflexion carries blood-vessels and nerves to the femur, and also contains fibrous tissue from outside the joint. It is known as the _ligamentum teres_.

_Comparative Anatomy._--Bland Sutton regards the _ilio-femoral
ligament_ as an altered muscle, the scansorius, though against this is
the fact that, in those cases in which a scansorius is present in Man,
the ligament is as strong as usual, and indeed, if it were not there
in these cases, the erect position would be difficult to maintain. He
also looks upon the _ligamentum teres_ as the divorced tendon of the
pectineus muscle. The subject requires much more investigation, but
there is every reason to believe that it is a tendon which has sunk
into the joint, though whether that of the pectineus is doubtful,
since the intra-capsular tendon comes from the ischium in Reptiles. In
many Mammals, and among them the Orang, there is no ligamentum teres.
In others, such as the Armadillo, the structure has not sunk right
into the joint, but is connected with the pubo-femoral part of the
capsule.

The KNEE JOINT is a hinge formed by the condyles and trochlea of the femur, the patella, and the head of the tibia. The capsule is formed in front by the ligamentum patellae, and on each side special bands form the lateral ligaments. On the outer side there are two of these: the anterior or _long external lateral ligament_ is a round cord running from the external condyle to the head of the fibula, while the posterior is slighter and passes from the same place to the styloid process of the fibula. The _internal lateral ligament_ is a flat band which runs from the inner condyle of the femur to the internal surface of the tibia some two inches below the level of the knee joint. The posterior part of the capsule is strengthened by an oblique bundle of fibres running upward and outward from the semimembranosus tendon, and called the _posterior ligament of Winslow_.

The intra-articular structures are numerous and interesting. Passing from the head of the tibia, in front and behind the spine, are the _anterior_ and _posterior crucial ligaments_; the former is attached to the outer side of the intercondylar notch above, and the latter to the inner side. These two ligaments cross like an X. The _semilunar fibro-cartilages_--external and internal--are partial menisci, each of which has an anterior and a posterior cornu by which they are attached to the head of the tibia in front and behind the spine. They are also attached round the margin of the tibial head by a _coronary ligament_, but the external one is more movable than the internal, and this perhaps accounts for its coronary ligament being less often ruptured and the cartilage displaced than the inner one is. In addition to these the external cartilage has a fibrous band, called the _ligament of Wrisberg_, which runs up to the femur just behind the posterior crucial ligament. The external cartilage is broader, and forms more of a circle than the internal. The synovial cavity of the knee runs up, deep to the extensor muscles of the thigh, for about two inches above the top of the patella, forming the _bursa suprapatellaris_. At the lower part of the patella it covers a pad of fat, which lies between the ligamentum patellae and the front of the head of the tibia, and is carried up as a narrow tube to the lower margin of the trochlear surface of the femur. This prolongation is known as the _ligamentum mucosum_, and from the sides of its base spring two lateral folds called the _ligamenta alaria_. The tendon of the popliteus muscle is an intra-capsular structure, and is therefore covered with a synovial sheath. There are a large number of bursae near the knee joint, one of which, common to the inner head of the gastrocnemius and the semimembranosus, often communicates with the joint. The hinge movement of the knee is accompanied by a small amount of external rotation at the end of extension, and a compensatory internal rotation during flexion. This slight twist is enough to tighten up almost all the ligaments so that they may take a share in resisting over-extension, because, in the erect position, a vertical line from the centre of gravity of the body passes in front of the knee.

_Comparative Anatomy._--In some Mammals, e.g. Bradypus and
Ornithorhynchus, the knee is divided into three parts, two
condylo-tibial and one trochleo-patellar, by synovial folds which in
Man are represented by the ligamentum mucosum. In a typical Mammal the
external _semilunar cartilage_ is attached by its posterior horn to
the internal condyle of the femur only, and this explains the
_ligament of Wrisberg_ already mentioned. In the Monkeys and
anthropoid Apes this cartilage is circular. The _semilunar cartilages_
first appear in the Amphibia, and, according to B. Sutton, are derived
from muscles which are drawn into the joint. When only one kind of
movement (hinge) is allowed, as in the fruit bat, the cartilages are
not found. In most Mammals the superior tibio-fibular joint
communicates with the knee.

The _tibio-fibular articulations_ resemble the radio-ulnar in position
but are much less movable. The superior in Man is usually cut off i
from the knee and is a gliding joint; the middle is the interosseous
membrane, while the lower has been already used as an example of a
syndesmosis or fibrous half joint.

The ANKLE JOINT is a hinge, the astragalus being received into a lateral arch formed by the lower ends of the tibia and fibula. Backward dislocation is prevented by the articular surface of the astragalus being broader in front than behind. The anterior and posterior parts of the capsule are feeble, but the lateral ligaments are very strong, the external consisting of three separate fasciculi which bind the fibula to the astragalus and calcaneum. To avoid confusion it is best to speak of the movements of the ankle as dorsal and plantar flexion.

FIG. 7.--Dissection of the Knee-joint from the front: Patella thrown down.]

The _tarsal joints_ resemble the carpal in being gliding articulations. There are two between the astragalus and calcaneum, and at these inversion and eversion of the foot largely occur. The inner arch of the foot is maintained by a very important ligament called the _calcaneo-navicular_ or _spring ligament_; it connects the sustentaculum tali of the calcaneum with the navicular, and upon it the head of the astragalus rests. When it becomes stretched, flat-foot results. The tarsal bones are connected by dorsal, plantar and interosseous ligaments. The _long_ and _short calcaneocuboid_ are plantar ligaments of special importance, and maintain the outer arch of the foot.

The _tarso-metatarsal_, _metatarso-phalangeal_ and _interphalangeal joints_ closely resemble those of the hand, except that the tarso-metatarsal joint of the great toe is not saddle-shaped.

_Comparative Anatomy._--The anterior fasciculus of the external
lateral ligament of the ankle is only found in Man, and is probably an
adaptation to the erect position. In animals with a long foot, such as
the Ungulates and the Kangaroo, the lateral ligaments of the ankle are
in the form of an X, to give greater protection against lateral
movement. In certain marsupials a fibro-cartilage is developed between
the external malleolus and the astragalus, and its origin from the
deeper fibres of the external lateral ligament of the ankle can be
traced. These animals have a rotatory movement of the fibula on its
long axis, in addition to the hinge movement of the ankle.

For further details of joints see R. Fick, _Handbuch der Gelenke_
(Jena, 1904); H. Morris, _Anatomy of the Joints_ (London, 1879);
Quain's, Gray's and Cunningham's _Text-books of Anatomy_; J. Bland
Sutton, _Ligaments, their Nature and Morphology_ (London, 1902); F. G.
Parsons, "Hunterian Lectures on the Joints of Mammals," _Journ. Anat.
& Phys._, xxxiv. 41 and 301. (F. G. P.)

DISEASES AND INJURIES OF JOINTS

The affection of the joints of the human body by specific diseases is dealt with under various headings (RHEUMATISM, &c.); in the present article the more direct forms of ailment are discussed. In most joint-diseases the trouble starts either in the synovial lining or in the bone--rarely in the articular cartilage or ligaments. As a rule, the disease begins after an injury. There are three principal types of injury: (1) sprain or strain, in which the ligamentous and tendinous structures are stretched or lacerated; (2) contusion, in which the opposing bones are driven forcibly together; (3) dislocation, in which the articular surfaces are separated from one another.

A _sprain_ or _strain_ of a joint means that as the result of violence
the ligaments holding the bones together have been suddenly stretched
or even torn. On the inner aspect the ligaments are lined by a
synovial membrane, so when the ligaments are stretched the synovial
membrane is necessarily damaged. Small blood-vessels are also torn,
and bleeding occurs into the joint, which may become full and
distended. If, however, bleeding does not take place, the swelling is
not immediate, but synovitis having been set up, serous effusion comes
on sooner or later. There is often a good deal of heat of the
surrounding skin and of pain accompanying the synovitis. In the case
of a healthy individual the effects of a sprain may quickly pass off,
but in a rheumatic or gouty person chronic synovitis may obstinately
remain. In a person with a tuberculous history, or of tuberculous
descent, a sprain is apt to be the beginning of serious disease of the
joint, and it should, therefore, be treated with continuous rest and
prolonged supervision. In a person of health and vigour, a sprained
joint should be at once bandaged. This may be the only treatment
needed. It gives support and comfort, and the even pressure around the
joint checks effusion into it. Wide pieces of adhesive strapping,
layer on layer, form a still more useful support, and with the joint
so treated the person may be able at once to use the limb. If
strapping is not employed, the bandage may be taken off from time to
time in order that the limb and the joint may be massaged. If the
sprain is followed by much synovitis a plaster of Paris or leather
splint may be applied, complete rest being secured for the limb. Later
on, blistering or even "firing" may be found advisable.

_Synovitis._--When a joint has been injured, inflammation occurs in
the damaged tissue; that is inevitable. But sometimes the attack of
inflammation is so slight and transitory as to be scarcely noticeable.
This is specially likely to occur if the joint-tissues were in a state
of perfect nutrition at the time of the hurt. But if the individual or
the joint were at that time in a state of imperfect nutrition, the
effects are likely to be more serious. As a rule, it is the synovial
membrane lining the fibrous capsule of the joint which first and
chiefly suffers; the condition is termed _synovitis_. Synovitis may,
however, be due to other causes than mechanical injury, as when the
interior of the joint is attacked by the micro-organisms of pyæmia
(blood-poisoning), typhoid fever, pneumonia, rheumatism, gonorrhoea or
syphilis. Under judicious treatment the synovitis generally clears up,
but it may linger on and cause the formation of adhesions which may
temporarily stiffen the joint; or it may, especially in tuberculous,
septic or pyæmic infections, involve the cartilages, ligaments and
bones in such serious changes as to destroy the joint, and possibly
call for resection or amputation.

The symptoms of synovitis include stiffness and tenderness in the
joint. The patient notices that movements cause pain. Effusion of
fluid takes place, and there is marked fullness in the neighbourhood.
If the inflammation is advancing, the skin over the joint may be
flushed, and if the hand is placed on the skin it feels hot.
Especially is this the case if the joint is near the surface, as at
the knee, wrist or ankle.

The treatment of an inflamed joint demands rest. This may be
conveniently obtained by the use of a light wooden splint, padding and
bandages. Slight compression of the joint by a bandage is useful in
promoting absorption of the fluid. If the inflamed joint is in the
lower extremity, the patient had best remain in bed, or on the sofa;
if in the upper extremity, he should wear his arm in a sling. The
muscles acting on the joint must be kept in complete control. If the
inflammation is extremely acute a few leeches, followed by a
fomentation, will give relief; or an icebag or an evaporating lotion
may, by causing constriction of the blood-vessels, lessen the
congestion of the part and the associated pain. As the inflammation is
passing off, massage of the limb and of the joint will prove useful.
If the inflammation is long continued, the limb must still be kept at
rest. By this time it may be found that some other material for the
retentive apparatus is more convenient and comfortable, as, for
instance, undressed leather which has been moulded on wet and allowed
to dry and harden; poro-plastic felt, which has been softened by heat
and applied limp, or house-flannel which has been dipped in a creamy
mixture of plaster-of-Paris and water, and secured by a bandage.

_Chronic Disease of a Joint_ may be the tailing off of an acute
affection, and under the influence of alternate douchings of hot and
cold water, of counter-irritation by blistering or "firing," and of
massage, it may eventually clear up, especially if the general health
of the individual is looked after. But if chronic disease lingers in
the joint of a child or young person, the probability of its being
under the influence of tuberculous infection must be considered. In
such a case prolonged and absolute rest is the one thing necessary. If
the disease be in the hip, knee, ankle or foot, the patient may be
fitted with an appropriate Thomas's splint and allowed to walk about,
for it is highly important to have these patients out in the fresh
air. If the disease be in the shoulder, elbow, wrist or hand, a
leather or poro-plastic splint should be moulded on, and the arm worn
in a sling. There must be no hurry; convalescence will needs be slow.
And if the child can be sent to a bracing sea-side place it will be
much in his favour.

As the disease clears up, the surface heat, the pains and the
tenderness having disappeared, and the joint having so diminished in
size as to be scarcely larger than its fellow--though the wasting of
the muscles of the limb may cause it still to appear considerably
enlarged--the splint may be gradually left off. This remission may be
for an hour or two every other day; then every other night; then every
other day, and so on, the freedom being gained little by little, and
the surgeon watching the case carefully. On the slightest indication
of return of trouble, the former restrictive measures must be again
resorted to. Massage and gentle exercises may be given day by day, but
there must be no thought of "breaking down the stiffness." Many a
joint has in such circumstances been wrecked by the manipulations of a
"bone-setter."

_Permanent Stiffness._--During the treatment of a case of chronic
disease of a joint, the question naturally arises as to whether the
joint will be left permanently stiff. People have the idea that if an
inflamed joint is kept long on a splint, it may eventually be found
permanently stiff. And this is quite correct. But it should be clearly
understood that it is not the _rest_ of the inflamed joint which
causes the stiffness. The matter should be put thus: in tuberculous
and other forms of chronic disease stiffness may ensue in spite of
long-continued rest. It is the destructive disease, not the enforced
rest which causes it; for inflammation of a joint rest is absolutely
necessary.

The _Causes of permanent Stiffness_ are the destructive changes
wrought by the inflammation. In one case it may be that the synovial
membrane is so far destroyed by the tuberculous or septic invasion
that its future usefulness is lost, and the joint ever afterwards
creaks at its work and easily becomes tired and painful. Thus the
joint is crippled but not destroyed. In another case the ligaments and
the cartilages are implicated as well as the synovial membrane, and
when the disease clears up, the bones are more or less locked, only a
small range of motion being left, which forcible flexion and other
methods of vigorous treatment are unable materially to improve. In
another set of cases the inflammatory germs quickly destroy the soft
tissues of the joint, and then invade the bones, and, the disease
having at last come to an end, the softened ends of the bones solidly
join together like the broken fragments in simple fracture. As a
result, osseous solidification of the joint (_synostosis_) ensues
without, of course, the possibility of any movement. And, inasmuch as
the surgeon cannot tell in any case whether the disease may not
advance in this direction, he is careful to place the limb in that
position in which it will be most useful if the bony union should
occur. Thus, the leg is kept straight, and the elbow bent.

In the course of a tuberculous or other chronic disease of a joint,
the germs of septic disease may find access to the inflamed area,
through a wound or ulceration into the joint, or by the germs being
carried thither by the blood-stream. A _joint-abscess_ results, which
has to be treated by incision and fomentations. If chronic suppuration
continues, it may become necessary to scrape out or to excise the
joint, or even to amputate the limb. And if tuberculous disease of the
joint is steadily progressing in spite of treatment, vigorous measures
may be needed to prevent the fluid from quietly ulcerating its way out
and thus inviting the entrance of septic germs. The fluid may need to
be drawn off by aspiration, and direct treatment of the diseased
synovial membrane may be undertaken by injections of chloride of zinc
or some other reagent. Or the joint may need scraping out with a sharp
spoon with the view of getting rid of the tuberculous material. Later,
excision may be deemed necessary, or in extreme cases, amputation. But
before these measures are considered, A. C. G. Bier's method of
treatment by passive congestion, and the treatment by serum
injection, will probably have been tried. If a joint is left
permanently stiff in an awkward and useless position, the limb may be
greatly improved by excision of the joint. Thus, if the knee is left
bent and the joint is excised a useful, straight limb may be obtained,
somewhat shortened, and, of course, permanently stiff. If after
disease of the hip-joint the thigh remains fixed in a faulty position,
it may be brought down straight by dividing the bone near the upper
end. A stiff shoulder or elbow may be converted into a useful, movable
joint by excision of the articular ends of the bones.

A _stiff joint_ may remain as the result of long continued
inflammation; the unused muscles are wasted and the joint in
consequence looks large. Careful measurement, however, may show that
it is not materially larger than its fellow. And though all tenderness
may have passed away, and though the neighbouring skin is no longer
hot, still the joint remains stiff and useless. No progress being made
under the influence of massage, or of gentle exercises, the surgeon
may advise that the lingering adhesion be broken down under an
anaesthetic, after which the function of the joint may quickly return.

There are the cases over which the "bone-setter" secures his greatest
triumphs. A qualified practitioner may have been for months
judiciously treating an inflamed joint by rest, and then feels a
hesitation with regard to suddenly flexing the stiffened limb. The
"bone-setter," however, has no such qualms, and when the case passes
out of the hands of the perhaps over-careful surgeon, the unqualified
practitioner (because he, from a scientific point of view, knows
nothing) fears nothing, and, breaking down inflammatory adhesions,
sets the joint free. And his manipulations prove triumphantly
successful. But, knowing nothing and fearing nothing, he is apt to do
grievous harm in carrying out his rough treatment in other cases.
Malignant disease at the end of a bone (sarcoma), tuberculosis of a
joint, and a joint stiffened by old inflammation are to him the same
thing. "A small bone is out of place," or, "The bone is out of its
socket; it has never been put in," and a breaking down of everything
that resists his force is the result of the case being taken to him.
For the "bone-setter" has only one line of treatment. Of the
improvement which he often effects as if by magic the public are told
much. Of the cases over which the doctor has been too long devoting
skill and care, and which are set free by the "bone-setter," everybody
hears--and sometimes to the discomfiture of the medical man. But of
the cases in which irreparable damage follows his vigorous
manipulation nothing is said--of his rough usage of a tuberculous hip,
or of a sarcomatous shoulder-joint, and of the inevitable disaster and
disappointment, those most concerned are least inclined to talk! A
practical surgeon with common-sense has nothing to learn from the
"bone-setter."

_Rheumatoid Arthritis_, or chronic _Osteo-arthritis_, is generally
found in persons beyond middle age; but it is not rare in young
people, though with them it need not be the progressive disease which
it too often is in their elders. It is an obscure affection of the
cartilage covering the joint surfaces of the bones, and it eventually
involves the bones and the ligaments. A favourite joint for it is the
knee or hip, and when one large joint is thus affected the other
joints may escape. But when the hands or feet are implicated pretty
nearly all the small joints are apt to suffer. Whether the joint is
large or small, the cartilages wear away and new bone is developed
about the ends of the bones, so that the joint is large and
mis-shapen, the fingers being knotted and the hands deformed. When the
spine is affected it becomes bowed and stiff. This is the disease
which has crippled the old people in the workhouses and almshouses,
and with them it is steadily progressive. Its early signs are
stiffness and creaking or cracking in the joints, with discomfort and
pain after exercise, and with a little effusion into the capsule of
the joint. As regards _treatment_, medicines are of no great value.
Wet, cold and damp being bad for the patient, he should be, if
possible, got into a dry, bright, sunny place, and he should dress
warmly. Perhaps there is no better place for him in the winter than
Assuan. Cairo is not so suitable as it used to be before the dam was
made, when its climate was drier. For the spring and summer certain
British and Continental watering-places serve well. But if this luxury
cannot be afforded, the patient must make himself as happy as he can
with such hot douchings and massage as he can obtain, keeping himself
warm, and his joints covered by flannel bandages and rubbed with
stimulating liniments. In people advanced or advancing in years, the
disease, as a rule, gets slowly worse, sometimes very slowly, but
sometimes rapidly, especially when its makes its appearance in the
hip, shoulder or knee as the result of an injury. In young people,
however, its course may be cut short by attention being given to the
principles stated above.

_Charcot's Disease_ resembles osteo-arthritis in that it causes
destruction of a joint and greatly deforms it. The deformity, however,
comes on rapidly and without pain or tenderness. It is usually
associated with the symptoms of locomotor ataxy, and depends upon
disease of the nerves which preside over the nutrition of the joints.
It is incurable.