HUMAN BIOLOGY 



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

 broad and fiat. The bones which furnish support are thick 



and solid ; those designed to aid in 



motion are long and straight. Including 



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 

 gthwhe. The red no yellow marrow ; they 



blood cells are formed * 



in the red marrow of are spongy inside, and 



the spongy part. hard and c ct near 



*_ f Jr IG, 30* ~~ 



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

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

 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. 



