THE JOINTS.

That bones may be of any use, they must be jointed together. Joints are of the greatest importance. It is necessary they should be so constructed that there shall be no harsh grating of the bones upon each other, and no injurious jars in walking, etc. To prevent these things, a hard, smooth, and yet yielding, cushion-like substance is required between them in joints. Such are the cartilages. Fig. 16 gives a specimen of these intervening cartilages. D, is the body of a bone, at the end of which is a socket; C, the cartilage lining the socket, thin at the sides and thick in the centre; B, the body of a bone, at the end of which is a round head; C, the investing cartilage, thin at the sides and thick in the centre.

Cartilage grows thinner, harder, and less elastic in old age. Hence old people are not quite as tall as in mid dle life, and a little stiffer in their joints.

The synovial membrane is a thin layer covering the cartilage, and being bent back upon the inner surface of the ligaments, it forms a closed sac. From its inner surface a sticky fluid oozes out, which helps the joints to play easily.

There are other smaller sacs connected with the joints, called bursa mucous. They secrete a fluid similar to that from the synovial membrane.

The ligaments. To retain the bones in their places at the joints, some strong, flexible straps are required to stretch across from one to the other, and to firmly unite them. Such are the ligaments.

They are the pearl-colored, lustrous, shining parts about the joints, in the form of straps and cords. There are a number of them so woven together as to form a complete covering of the joint, called a capsular ligament. In Fig. 17 : 1, 2, are ligaments extending from the hip-bone, 6, to the femur, 4. In Fig. 18: 1, is the socket of the hip-joint; 2, head of the femur, lodged in the socket; 3, the ligament within the socket. In Fig. 19: 1, is the tendon of the muscle which extends the leg; 2, the knee-cap (patella); 3, the anterior ligament of the patella; 6, the long external lateral ligament; 4, 4, the synovial membrane; 5, the internal lateral ligament; 7, the anterior and superior ligament that unites the tibia with the fibula.