Bones meet each other at joints or articulations. Some of these articulations unite the bones firmly while others allow movement. Bones are united by the following:
a) dense connective tissues forming fibrous joints e.g. radius and ulna;
b) a cartilage forming cartilaginous joints e.g. between sternebrae, pelvic symphysis or the mandibles;
c) a fluid-filled cavity intervenes between the bones forming synovial joints.a + b are relatively immovable and are termed synarthroses while ( c ) is freely moveable and is termed diarthrosis.
1. Syndesmosis – the uniting material is connective tissue and if it is extensive as between radius and ulna it is called interosseous membrane.
2. Suture – special type of joint uniting bones of the skull. The uniting medium is either a fibrous seam or a narrow seam of cartilage from which the bone keep growing. The edges of the bones may have interlocking margins (sutura serrata) such as of the frontal bones; b) overlap as the tiles of a roof e.g. parieto-temporal (sutura squamosa) c) the edges are smooth or slightly roughened (sutura plana) and may completely ossify in old age e.g. nasal bones.
3. Gomphosis – the implantation and anchoring of the teeth in their alveoli by the periodontium (alveolar periosteum).
1. Synchodrosis – the bones are united by a hyaline cartilage e.g. between diaphysis and epiphysis of juvenile long bones, and between skull and hyoid bone (between tympanohyoid and temporal bone) except in carnivores where it is fibrous. Basisphenoid and presphenoid bones of juvenile animals are also joined by a synchondrosis.
2. Symphysis – the two bones are joined by a fibrocartilage or regular connective tissue.
Examples are the joints between the symmetrical halves of mandibles and of the pelvic girdle, also joint between successive vertebral bodies. When the uniting medium becomes ossified, these joints are converted into synostoses.
A typical synovial joint consists of the following parts:
a) articular extremities of at least two bones covered with a hyaline cartilage,
b) joint capsule which surrounds the joint cavity and consisting of
i) inner synovial layer, the synovial membrane (membrana synovialis)
ii) an outer tougher fibrous layer, the fibrous membrane (membrana fibrosa).
c) joint ligaments – these hold the bones together and restrict the movement of the joint. They are composed of dense connective tissue which consists of regularly arranged collagen fibers.
The articulating bones are separated by a fluid filled space, the joint cavity. The fluid is termed synovia or synovial fluid. The space is bound by a synovial membrane which is attached around the periphery of the articulating bones. A synovial membrane is strengthened externally by a fibrous capsule, while additional ligaments are placed to restrict the movement to the required directions and extent only.
is covered with articular hyaline cartilage. The cartilage is very thin in the dog, but thicker in the larger joints of horse and ox, being thickest in the centre of the convex surface and about the periphery in the concave ones. The cartilage is insensitive and avascular, drawing its nutrients by diffusion from the synovial fluid, vessels in the tissues periphery to the cartilage and from vessels close to the marrow spaces.
Synovial membrane
It completes the lining of the joint but may either
a) be left entirely unsupported and thus pouch out forming diverticular.
b) rest directly on a tough outer fibrous capsule,
c) may be separated from an outer fibrous capsule by the interposition of pads of fats which are called synovial pads or cushions.The inner surface of the membrane carries many projections of various sizes (synovial villi or folds, (villi or plica synoviales) which increases the surface area of the membrane. The membrane is vascular and sensitive. The synovial membrane is lined with a squamous epithelium (often incompletely) and secretes the aminoglycans (lubricant component) of the synovia. The nutritive component of the synovia is derived from blood plasma. The synovial membrane also have an absorptive function.In the joint cavity of the same articulations are fibrocartilaginous discs. When they separate the cavity into two as occurs in the temporo-mandibular joint, they are known as articular discs (disci articulares) while if they only partially separate the cavity and are semilunar in shape as in the femoro-tibial joint they are called menisci, menisci articulares. The discs and menisci act as shock absorbers and render the articular surfaces congruent. An articular labrus (labra articularia) or marginal cartilage is a fibrocartilaginous rim placed around the circumference of certain concave articular surfaces e.g. acetabulum.
Is attached around the margins of the articular surfaces. Local thickenings of this fibrous layer form ligaments, some of which may be intracapsular (e.g. cruciate ligaments of the stifle joint) while others are extracapsular (e.g. the collateral ligaments). The ligaments hold together the bones constituting the joint and guide them in their movements, or in some instances to check the extent of the movement.
Movement in joints
There are two main types of movements in a joint:
1. Rotation – the moving bones turn about an axis perpendicular to its articular surface.
2. Pendular movements in sagittal planes (parallel to the median plane). These predominate in the joints of limbs and include:
a) flexion – reduces the angle between the two segment of limbs,
b) extension – opens the angle and brings the two segments more closely into alignment,
c) over – extension (dorsal flexion) – e.g. of the fetlock at landing (metacarpal / tarsal – 1st phalanx),
d) adduction – carries the moving part towards the median plane,
e) abduction – carries the moving part further from the median plane,
f) circumduction – combination of extension, flexion, adduction and abduction allowing the extremity of the limb to describe a circle, thus defining a cone…………………………………………………………………………………
