STRUCTURE OF BONES. 



49 



bearing surface in the joints, and also to provide space 

 on which to attach the muscles which move the bone ; the 

 various knobs on the extremities, and 

 the rough patches on the shaft, all mark 

 areas where muscles were fixed. 



Internal structure. If the humerus 

 were divided lengthwise we would find 

 that its shaft was hollow ; the space is 

 known as the medullary cavity, and in 

 life is filled with soft fatty marrow. Fig. 

 22 represents such a longitudinal section. 

 We see in it that the marrow cavity ends 

 near the articular extremities ; and that 

 in these the hone has a loose, spongy text- 

 ure, except a thin dense layer on the sur- 

 face. In the shaft the compact outer 

 layer is much the thicker, the spongy 

 portion only forming a thin stratum next 

 the medullary cavity.* To the unas- 

 sisted eye the spongy bone appears made 

 up of a trellis- work of thin bony plates 

 which intersect in all directions and sur- 

 round cavities about the size of the 



What do the knobs and rough patches on the 

 bone indicate? 



What should we find on dividing the hu- 

 merus lengthwise? What is its shaft cavity 

 called? What does it contain? Where does 

 the marrow cavity end? What is the texture of 



the articular extremities of the bone? How FIG. 22. The humerus 

 does the shaft differ in structure from the ex- cut open, a, marrow 

 tremities of the femur? What does the spongy Jg5 

 bone look like? lae. 



* These facts may readily be demonstrated by sawing in two lengthwise the 

 bones out of a leg of mutton. 



