42 



that the bone consists of two distinct kinds of substance. Its 

 outer portion is composed of a very hard and compact substance, 

 somewhat resembling ivory. This is called the compact or dense 

 tissue. It is thickest along the shaft, and is very thin around the 

 heads. The heads of the bone are filled with a less compact 

 mass, which is so porous and light that it is called the spongy or 

 cancellous (Lat. cancelli, a grating) tissue. In addition to these 

 two kinds of tissue, the central cavity of the bone contains a soft 

 pulpy substance, called the marrow ; the cavity itself is called the 

 medullary cavity. The marrow consists of fat cells and blood- 

 vessels supported by a very delicate connecting membrane. The 



Fig. 47. Transverse Section of the Compact Tissue of Bone. 

 Magnified 150 diameters. 



i, Haversian canals ; 2, lacunae ; 3, laminae. 



blood-vessels enter the cavity through small openings in the 

 compact tissue, which may be seen on the outer surface of the 

 bone. There is also a kind of marrow in the pores of the cancel- 

 lous tissue, but it differs from that of the medullary cavity, in that 

 it is reddish in colour, more fluid, and contains much less fat. 



The short and the irregular bones have no medullary cavity, 

 but consist of a thin outer layer of compact tissue, filled in with 

 cancellous tissue. The flat bones consist of a layer of cancellous 

 tissue between two layers of the compact tissue. 



That part of a bone which assists in forming a joint is covered 

 with a smooth layer of articular cartilage. The remainder of the 



