3o 



HUMAN BIOLOGY 



— C Marrow. 



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

 broad and flat. The bones which furnish support arc thick 



and solid ; those designed to aid in 

 motion arc 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 ti\ v 

 of a long bone is shown V^f 

 in Fig. 29. It has a 

 long, Jwllorv 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 

 the bone. The long bones 

 are found in the limbs 

 (Fig. 28). The ribs and 

 other flat bones and the 

 irregular bones contain 

 no yellow marrow ; they 



Com pad 

 •A or dense 





Fig. 30. — 



Fig. 29. — Femur, sawed 

 lengthwise. The red 

 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 



ri .„. N Right Femur. 



cavities in the spongy parts of bones {rig. 29). 



Neiv red blood cells arc formed in this marrow. The bones 

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

 nective tissue and blood vessels. It is called periosteum. 



