20 THE FRAMEWORK OF THE BODY. 



cident, a "sprain" is the consequence. An injury of this 

 sort may be, and frequently is, quite as serious as the 

 breaking of a bone. 



9. The ligament, then, secures firmness to the joint; 

 it must also have flexibility and smoothness of motion. 

 This is accomplished by a beautiful mechanism the perfec- 

 tion of which is only feebly imitated by the most ingenious 

 contrivance of man. The ends of the bones are covered 

 by a thin layer of cartilage, which being smooth and elas- 

 tic, renders all the 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 con- 

 taining fluid. This fluid is constantly 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 unyielding, affords 

 strength, while the cartilage, elastic 

 and moist, gives ease and smooth- 

 ness 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 

 jg one bone is dove-tailed into another 

 by what are termed sutures. Others 

 have a hinge-like motion, such as 



Fro. 4.-CELLS op CARTILAGE. fa OS Q Q f ftie elbow, wrist, ankle, 



and knee; the most complete of these being the elbow- 

 joint (Fig. 5). Belonging to another class, the ball-and- 



9. Office of the ligament ? What must it have ? How accomplished ? Describe 



1 1 of "what do we observe as resards the composition of a joint ? The lament 

 and cart ilajre? What varies? Example of the skull? Other examples ? 

 ball-and-socket joint? 



