Fig. 26.—Elastic Tissue from the Ligaments about Joints. (Highly magnified.)
54. Perfect Joints. There are various forms of perfect joints, according to the nature and amount of movement permitted. They an divided into hinge joints, ball-and-socket joints and pivot joints.
The hinge joints allow forward and backward movements like a hinge. These joints are the most numerous in the body, as the elbow, the ankle, and the knee joints.
In the ball-and-socket joints—a beautiful contrivance—the rounded head of one bone fits into a socket in the other, as the hip joint and shoulder joint. These joints permit free motion in almost every direction.
In the pivot joint a kind of peg in one bone fits into a notch in another. The best example of this is the joint between the first and second vertebræ (see sec. 38). The radius moves around on the ulna by means of a pivot joint. The radius, as well as the bones of the wrist and hand, turns around, thus enabling us to turn the palm of the hand upwards and downwards. In many joints the extent of motion amounts to only a slight gliding between the ends of the bones.
55. Uses of the Bones. The bones serve many important and useful purposes. The skeleton, a general framework, affords protection, support, and leverage to the bodily tissues. Thus, the bones of the skull and of the chest protect the brain, the lungs, and the heart; the bones of the legs support the weight of the body; and the long bones of the limbs are levers to which muscles are attached.
Owing to the various duties they have to perform, the bones are constructed in many different shapes. Some are broad and flat; others, long and cylindrical; and a large number very irregular in form. Each bone is not only different from all the others, but is also curiously adapted to its particular place and use.
