HUMAN BIOLOGY 



Compad 

 or dense 

 tissue 



flat, and irregular. Those whose chief use is to protect are 

 broad and flat. The bones which furnish support are thick 

 and solid ; those designed to aid in 

 motion are long and straight. I ncluding 

 six small bones in the ear, there are two 

 hundred and six bones E 

 in the adult skeleton. 



Gross Structure of 

 Bones. The structure 

 of a long bone is shown 

 in Fig. 29. It has a 

 long, hollow shaft of 

 hard, compact bone, and 

 enlarged ends composed 

 of spongy bone. The 

 hollow in the shaft is 

 filled with yellow mar- 

 row, which is composed 

 of blood vessels and fat, 

 and aids in nourishing 

 thebone. Thelongbones 

 are found in the limbs 

 (Fig. 28). The ribs and 

 other flat bones and the 

 FIG. 29. -FEMUR, sawed irregular bones contain 

 lengthwise. The red no yellow marrow ; they 



blood cells are formed . 



in the red marrow of are spongy inside, and 

 the spongy part. hard and compact near 



the surface. There is a red marrow in the FRONT VIEW OF 



... /. N RIGHT FEMUR. 



cavities in the spongy parts of bones (Fig. 29). 



New red blood cells are formed in this marrow. The bones 

 have a close-clinging, fibrous covering composed of con- 

 nective tissue and blood vessels. It is called periosteum. 



FIG. 30. 



