HUMAN BIOLOGY 



Joint 

 cartilage 



FIG. 36. DIAGRAM OF A JOINT. 



to bind the bones together. Sudden jolts would jar the 

 bones and injure them ; shocks are prevented by a layer 



of elastic cartilage over the 

 end of each bone. The mov- 

 ing of one bone over another 

 in bending a joint would wear 

 the bone with friction un- 

 less the cartilages were very 

 smooth and lubricated with a 

 fluid called the synovial fluid. 

 The synovial fluid would be 

 constantly escaping into the 

 surrounding tissues except for 

 the collarlike ligament called 

 the capsule^ which surrounds the joint and is attached to 

 each bone entirely around the joint (Fig. 36). 



THOUGHT QUESTIONS. The Kinds of Joints. Write /?, //, G, E, 

 P, or / after these names according to the kind of joint (ball-and-socket, 

 hinge, gliding, elastic, pivot, immovable) : between bones of skull, 

 head nodding, head turning, vertebrae, lower jaw, ribs to 

 breastbone (Fig. 75), shoulder, elbow, wrist, fingers, 

 hip, knee, ankle, toes. 



Growth of Bones. The blood vessels pass into the bones from the 

 periosteum. If the periosteum is returned, the larger blood vessels are 

 taken away and the bone beneath it perishes. If the underlying bone is 

 removed and the periosteum left, the bone will be replaced. A curious 

 proof of the active circulation in the bone is furnished when madder is 

 mixed with the food of pigs. In a few hours the bones become a darker 

 pink than usual: and if the madder is fed to the pigs for a few days, 

 their bones become red. A child grows in height chiefly during three 

 or four months in spring and summer; but its body broadens and 

 becomes heavier during autumn. 



Health of the Bones. It is plain that a strong and free circulation 

 of pure blood contributes to the health and strength of the bones ; good 

 food and pure air make pure blood. Cases of "delayed union," or 

 slow mending of broken bones, occur more often with intemperate than 

 with sober people. This is because the vitality of the bone cells has 



