24 MOTION JOINTS. 



4. There are small holes in the bone, where blood tubes 

 passed in and out. 



5. Saw a femur in two, lengthwise, and make a drawing 

 showing : 



(a) The central marrow cavity. 



(b) The spongy extremities, noting especially the directions 

 of the bony plates and fibers. 



6. Observe the width of the lower end of the femur, where 

 it rests on the tibia. Suppose these two bones were as narrow 

 at their ends, where they meet to form the knee joint, as they 

 are at their centers, what kind of a joint would they make ? 

 Illustrate by piling up a number of spools on end ; the column 

 is more lightened than it is weakened by the hollowing out of 

 the sides of each spool. And the central hollow of the spool 

 does not greatly weaken it. A given weight of material has 

 more strength when in the form of a hollow cylinder. Note 

 carefully that this is not saying that a hollow pillar is stronger 

 than a solid one; but for the same weight of bone more 

 strength is given by having it hollow. The bones combine 

 well two very desirable qualities, lightness and strength. 



If in our column of spools we place a wide rubber band 

 around the junction of two spools, we have something very 

 similar to the Capsular Ligament, of which we shall learn 

 something soon. 



Joints. Use beef joints or sheep shanks for demonstra- 

 tion ; or, if the work is done by the individual members of the 

 class, supply them with the legs of rabbits. 



1. Cut into the knee joint. Observe the liquid Synovia 

 that lubricates the joint. Rub a drop of it between the 

 thumb and finger. 



2. Note the white, glistening bands, the Ligaments, that 

 hold the ends of the bones together. Carefully study their 

 arrangement. The ligaments are composed of a form of 

 connective tissue. 



