THE SKELETON 



219 



of the long bones is hollow, being filled with a fatty 

 substance known as the yellow marrow. Around the 

 marrow cavity the bone is very dense and compact, but 

 most of the material forming the ends is porous and 

 spongy. These materials are usually referred to as the 

 compact substance and the cancellous, or spongy, substance of 

 the bones (Fig. 93). 



The arrangement of the compact and spongy substance 

 varies with the different bones. In the short bones (wrist 

 and ankle bones, vertebras, etc.) 

 and also in the flat bones (skull 

 bones, ribs, shoulder blades, etc.) 

 there is no cavity for the yellow 

 marrow, all of the interior space 

 being filled with the spongy sub- 

 stance. The, red marroiv, rela- 

 tions of which to the red cor- 

 puscles of the blood have already 

 been noted (page 27), occupies 

 the minute spaces in the spongy 

 substance. 



Minute Structure of Bone. 

 A microscopic examination of a 

 thin slice of bone taken from the 

 compact substance shows this to 

 be porous as well as the spongy 

 substance. Two kinds of small 

 channels are found running 

 through it in different directions, 

 known as the Haversian canals 

 and the canaliculi (Fig. 94). 



FIG. 94. Cross section of 

 bone showing minute structure. 

 Magnified, i. Surface layer of 

 bone. 2. Deeper portion. 3. Ha- 

 versian canals from which pass 

 the canaliculi. 4. A lacuna. Ob- 

 serve arrangement of lacunae at 

 surface and in deeper portion. 



These serve the general purpose of distributing nourish- 

 ment through the bone. The Haversian canals are larger 



