If we examine some piece of machiner y, we shall find all the working surfaces perfectly smooth, and we know that the machinist is careful to keep these smooth surfaces well oiled, so that they may glide easily over each other. The movable bones are constructed on much the same principle. Their working surfaces are covered with a smooth coating of gristle, and each joint is enclosed in a membrane, which has the power of preparing and pouring into it a peculiar oily fluid known as synovia, or joint-oil.
In this way nature provides not only the machinery, butthe joint-oil to lubricate it, so that it may work easily and smoothly.
Lesson 09
Properties of Bodies
"Are you coming with me, boys?" asked Mr. Wilson, as he passed out of the playground gate, and in a moment Fred and Willie were trudging along by his side.
