238 THE SKELETON 



lined with a thin layer of spongy tissue and filled with a 

 soft, fatty, reddish yeUow marrow. This marrow also fills 

 the spaces in the spongy tissue of the heads. It is this mar- 

 row that forms the red corpuscles of the blood. It is rich 

 in fat and gelatin (an albuminoid), and it is this last sub- 

 stance that forms the thickening matter of soup stock. The 

 structure of the long bone is excellently adapted to combin- 

 ing lightness ^^■ith strength. It is a law of mechanics that 

 a given amount of material in the form of a hollow cylinder 

 produces a stronger structure than the same amoimt of 

 material molded into a soUd rod. The bone, therefore, 

 by its hollow structure, secures the maximum strength 

 with the minimum of material. While it is true that they 

 would be stronger if they were kept of the same size but 

 made solid throughout, it would require great effort to 

 move such heavy structures. The structure of the heads, 

 also, gives strength and lightness at the same time, Awhile 

 their enlarged surfaces and roughened projections fit 

 them for the attachment of the various muscles and 

 tendons. These adaptations fit the long bones to form 

 the framework of movable parts, such as the legs and 

 arms, that must be strong and, at the same time, light 

 enough to admit of rapid movement. 



Structure of a flat bone.— (See Ex. XLIX., a.) A 

 sheep's rib serves well as a type of this form of bone. 

 Such a bone may show slight enlargements or heads at 

 one or both ends, but these heads are much smaller in 

 proportion to the rest of the bone than in the case of the 

 loiig bone, and are entirely lacking in case of the skull 

 plates. In the rib, there is only one such head, namely, 

 the one attaching the bone to the vertebrae of the spinal 

 column. The other end terminates in a mass of cartilage. 



