THE FRAMEWORK OF THE BODY 25 



movements of the joint very easy. In. addition to this, there 

 is an arrangement introduced for " lubricating " the joint, by 

 means of a delicate sac containing fluid. This fluid is con- 

 stantly supplied in small quantities, but only so fast as it is 

 used up in exercise. In appearance, it is not unlike the white 

 of an egg, and hence its name synovia, or egg-like. 



10. Thus, we observe that two very different substances 

 enter into the composition of a joint. The ligament, very un- 



FIG. 9. ELBOW-JOINT. A, Bone of the arm ; B, C, Bones of the fore-arm 



yielding, affords strength, while the cartilage, elastic and moist, 

 gives ease and. smoothness of motion. The amount of motion 

 provided for varies greatly in different joints. In some there 

 is none at all, as in the skull, where one bone is dove-tailed 

 into another by what are termed sutures. Others have a hinge- 

 like motion, such as those of the elbow, wrist, ankle, and knee ; 

 the most complete of these being the elbow-joint (Fig. 9). 

 Belonging to another class, the ball-and-socket joint, is that at 

 the shoulder, possessing a freedom of motion greater than any 

 other in the body. 



11. The Spinal Column. The spinal column is often spoken 



10. What do we observe as regards the composition of a joint ? The ligament and carti- 

 lage ? What varies? Example of the skull? Other examples ? The ball-and-socket joint ? 



11. What is the spinal column ? What does it connect and form ? Joints of the verte- 

 brae ? Amount of motion ? Result ? 



