36 



HUMAN BIOLOGY 



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 



\JM Synovial 



membrane 



Fig. 36. — Diagram of a Joint. 



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 B, H, 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 removed^ the larger blood vessels are 

 taken aivay 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 



