24 SCIENCE PRIMERS. [§ m. 



II 



did this you would be imitating very closely the 

 manner in which the bones at the elbo>v are kept 

 together in your own arm. Only the slips are not made 

 of india-rubber, but are flat bands of that stringy, \ 

 or as we may now call it fibrous stuff, \vhich in the 

 preceding lessons you learnt to call connective 

 tissue. These flat bands have a special name, and 

 are called ligaments. 



At the elbow the two ends of the ulna and humei*us 

 are kept in place by ligaments or flat bands of con- { 

 nective tissue. I 



In the skeleton, the surfaces of the two bones at ( 

 the elbow where they rub against each other, though 

 somewhat smooth, are dry. If you ever looked at the 

 knuckle of a leg of mutton before it was cooked, you 

 will have noticed that you have there two bones 

 slipping over each other somewhat as they do at the 

 elbow, and will remember that where the bones meet 

 they are wonderfully smooth, and very moist, so as to 

 be quite slipper}-. It is just the same in your own 

 elbow ; the end of the ulna and the end of the 

 humerus are beautifully smooth and quite moist, so 

 that they slip over each other as easily as possible. 

 You know that your eye is always moist. It is kept 

 moist by tears, though you don't speak of tears until 

 your eyes overflow with moisture ; but in reality you 

 are always crying a little. Well, there are, so to 

 speak, tears always being shed inside the wrapping 

 of ligaments around the elbow, and they keep the two 

 surfaces of the bones continually moist. 



The ends of bones where they touch each other 

 are also smooth, because they are coated over with 

 what is called gristle or cartilage. Bone is very hard 



