HISTOLOGY OP BONM. 51 



articular extremities, are called long bones ; other examples 

 are tibia and fibula, radius and ulna, metacarpal and meta- 

 tarsal bones, and the phalanges of fingers and toes. Tab- 

 ular bones form thin plates, like those of the roof of the 

 skull, and the shoulder-blades. Short bones are rounded 

 or angular, and not much longer in one diameter than 

 another ; as the carpal andtarsal (Fig. 19) bones. Irregu- 

 lar bones include all which do not fit well into any of the 

 above classes ; they usually lie in the middle line of the 

 body and are divisible into similar right and left halves ; 

 the vertebrae are good examples. 



All bones are covered by periosteum except where they 

 enter into the formation of a joint, but in the human 

 body only the long bones possess a medullary cavity con- 

 taining yellow marrow. The rest are filled up by spongy 

 bone, covered by a thin layer of dense, and have red mar- 

 row in their spaces. 



The histology oi bone. The microscope shows that 

 compact bone is only so to the naked eye ; even a hand 

 lens shows minute holes in it; it but differs from spongy 

 bone in the fact that its cavities are much smaller, and the 

 hard bony plates between them thicker,, It a thin trans- 

 verse section of the shaft ot long bone (Fig. 23) be exam- 

 ined with a microscope magnifying about twenty diameters, 

 even its densest part will be seen to show numerous open- 

 ings which become gradually larger near the medullary 



What is a long bone? Give examples. A tabular bone? Exam- 

 ples. A short bone? Examples. An irregular bone? Examples. 



With what is most of the surface of bones covered? Where is the 

 periosteum absent? What bones contain yellow marrow? What do 

 the others contain? 



Does compact bone contain any cavities? How may these be 

 seen? How does it differ from spongy bone? What is seen when a 

 thin slice of bone is magnified twenty iimes? Where do the apertures 

 in it become larger? 



