MOVEMENTS.] 



PHYSIOLOGY. 



2S 



and very solid ; there is not much water in it. Bones 

 dry up very little. Cartilage is not nearly so hard as 

 bone ; there is very much more water in it. When it 

 is quite fresh it is very smooth, but because it has a 

 good deal of water in it, it shrinks very much when 

 it dries up, and when dried is not nearly so smooth as 

 when it is fresh. You can see the dried-up cartilage on 

 the ends of the bones in the skeleton — it is somewhat 

 smooth still, but you can form no idea of how smooth 

 it is in the living body by simply seeing it on the 

 dried skeleton. 



At the elbow, then, we have the ends 

 of two bones fitting into each other, so 

 that they will move in a certain direction ; 

 these ends are smoothed with cartilage, 

 kept moist with a fluid, and held in place 

 by ligaments. All this is a called a joint. 



15. There are a great many other joints in the body 

 besides the elbow-joint : there is the shoulder-joint, 

 the knee-joint, the hip-joint, and so on. These differ 

 from the elbow-join*: in the shape of the ends of the 

 bone, in the way the bones move on each other, and 

 in several other particulars, but we must not go into 

 these differences now. They are all like the elbow, 

 since in each case one bone fits into another, the 

 surfaces are coated with cartilage, are kept moist 

 with fluid (what the grooms call joint-oil, though it 

 is not an oil at all), and are held in place by ligaments. 



I dare say you will have noticed that though I have 

 been speaking only of the humerus and ulna at the 

 elbow, the other bone of the fore-arm — the radius — has 

 something to do with the elbow too. I left it out in 

 order to simplify matters, but it is nevertheless quite 



