12 



ANATOMY AND PHYSIOLOGY 



enough mineral matter to make them hard, while the bones of 

 an aged person are brittle, because they no longer contain suffi- 

 cient animal matter to keep them elastic. 



The hardest part of any bone is at its surface; it is white in 

 color like ivory, and is called compact bone tissue. The deeper 

 part is porous, and is therefore called spongy tissue (also named 

 cancellous tissue, because its appearance suggests lattice work). 

 (See Fig. 7.) 



Compact tissue is most abundant on the shafts of the long 

 bones, which by their situation in the extremities are exposed to 

 external violence, and therefore need especial strength for resist- 

 ance. Since it is important that the bones be slender as well as 

 strong, these two results are gained by packing the bone tissue as 

 closely as possible. 



Periosteum 



Outer ground lamellae 



Haversian canals 



Haversian lamellae 



Interstitial lamellae 

 Inner ground lamellae 



Marrow 



FIG. 8. FROM A CROSS-SECTION OF A METACARP OF MAN. X 50. The Haversian 

 canals contain a little marrow (fat-cells). (Lewis and Stohr.) 



Cancellous tissue is more abundant in the parts of bones where 

 extent of surface is desirable. For example, the enlarged ex- 

 tremities of long bones are composed of cancellous tissue covered 

 with a thin compact layer; thus they can give attachment to many 

 tendons and ligaments, while the spongy character of the bone 

 prevents excessive weight. 



The marrow of bones is contained in the spaces of cancellous 

 tissue (where it is thin and red) and in little canals running through 

 the bone substance. Under the microscope may be seen small 

 channels in the compact tissue called Haversian canals, which con- 

 tain minute vessels and a little marrow. A large canal called the 



