JOINTS (anatomy) JUBILEES, BOOK OF
JOINTS (engineering) JUBILEE YEAR
JOINTS (geology) JÚCAR
JOINTURE JUD, LEO
JOINVILLE JUDAEA
JOINVILLE, FRANÇOIS LOUIS MARIE JUDAH
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JOIST JUDAS-TREE
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JOKJAKARTA JUDE, THE GENERAL EPISTLE OF
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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.
[Illustration: FIG. 1.--Vertical section through a synchondrosis. b, b,
the two bones; Sc, the interposed cartilage; l, the fibrous membrane
which plays the part of a ligament.]
[Illustration: FIG. 2.--Vertical section through a cranial suture, b, b,
the two bones; s, opposite the suture; l, the fibrous membrane, or
periosteum, passing between the two bones, which plays the part of a
ligament, and which is continuous with the interposed fibrous membrane.]
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.
[Illustration: FIG. 3.--Vertical section through an amphiarthrodial
joint. b, b, the two bones; c, c, the plate of cartilage on the
articular surface of each bone; Fc, the intermediate fibro-cartilage;
l, l, the external ligaments.]
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.
[Illustration: FIG. 4.--Vertical section through a diarthrodial joint.
b, b, the two bones; c, c, the plate of cartilage on the articular
surface of each bone; l, l, the investing ligament, the dotted line
within which represents the synovial membrane. The letter s is placed
in the cavity of the joint.]
[Illustration: FIG. 5.--Vertical section through a diarthrodial joint,
in which the cavity is subdivided into two by an interposed
fibro-cartilage or meniscus, Fc. The other letters as in fig. 4.]
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.