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Joints and Bones Help

By — McGraw-Hill Professional
Updated on Aug 30, 2011

Introduction to Joints and Bones

A junction is a meeting or joining place. (Picture a railroad junction or crossing of two intersecting train tracks.) A joint , therefore, is a place of meeting or joining between bones. There are three main types or categories of joints within the human body (Figure 13.10).

Skins and Skeletons Joints: A Meeting of the Bones

Fig. 13.10 The three main types of joints.

Fibrous Joints (synarthroses)

The simplest group are the fibrous ( FEYE -brus) joints or synarthroses ( sin -ar- THROW -seez). As their name indicates, the adjacent bones in a fibrous joint are more-or-less strapped together by a set of collagen fibers. A good example is provided by the sutures ( SOO -churs) or jagged “seams” (sutur) running between individual skull and facial bones.

The sutures and other types of fibrous joints are immovable. This gives them the alternate name of synarthroses – literally “conditions of” (-oses) “joints” (arthr) with the bones strapped “together” (syn-).

Cartilaginous Joints (amphiarthroses)

The next group of joints are called amphiarthroses ( am -fee-ar- THROW -seez). These are “joint conditions” ( arthroses ) permitting movement “on both sides” ( amphi -) of the involved bones. Amphiarthroses are only partially movable joints, but (as their name states) their bones can move on both sides, and in all directions.

Consider, for instance, the intervertebral ( in -ter-ver- TEE -bral) joints that are sandwiched “between” ( inter -) the individual vertebrae. Because the intervertebral joints are slightly movable, you are able to bend your back and twist your trunk moderately as the jointed vertebrae move short distances on both of their sides (top and bottom) and in all directions. You may even be able to dance the “twist”!

An oval slab of cartilage connective tissue, called the intervertebral disc , lies between each two vertebrae and connects the vertebra lying above it to the one lying below.

Cutting a section out of an intervertebral disc, and looking at it through the microscope, would reveal a slab of cartilage connective tissue. (Hence the alternate name, cartilaginous joints.) The cartilage holding the vertebrae together is shot through with numerous collagen fibers. These fibers make the oval, disc-shaped slab of cartilage behave much like a stale marshmallow or tough cushion. Because there are so many intervertebral joints (containing intervertebral discs) between the vertebrae, the vertebral column has excellent shock absorption. When you jump up-and-down with excitement, then, you usually don’t break your back!

Synovial Joints (diathroses)

The third main category of joints are the diarthroses ( die -ar- THROW -seez) or synovial (sin- OH -vee-al) joints . The word, diarthrosis ( die -ar- THROW -sis), indicates a “double” ( di -) “joint” ( arthr ) “condition” (- osis ). An individual is said to be “double-jointed” when the interphalangeal ( in -ter-fah-lan- GEEL ) joints “between” ( inter -) his “finger or toe bones” ( phalange ) have an unusually high degree of mobility. Hence, the diarthroses are the freely movable joints, with bones so movable that many seem to be double-jointed!

Finally, the word, synovial, “pertains to” (- al ) “eggs” ( ovi ) “together” ( syn -). The amusing thinking behind this name reflects the appearance of the synovial fluid . This fluid is secreted by the synovial membrane lining the hollow joint cavity . The synovial fluid is clear, thick, and slimy, making it look like the raw white portion of many eggs poured into a frying pan together. Due to its slippery nature, the synovial fluid significantly reduces bone friction and wear while the body carries out most of its major movements.

Practice problems for these concepts can be found at:  Skins And Skeletons Test

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