A STUDY OF THE SKELETON 177 



]ow marrow. Long bones are found in the limbs, where a 

 considerable range of motion is desired. 



A second group includes the so-called short bones, namely, 

 the carpals, .tarsals, and the kneecaps. Short bones are 

 more or less cubical in form, and in structure resemble the 

 head region of a long bone. Much of the outside surface is 

 covered with cartilage ; within is a thin layer of hard bone ; 

 while the central portion is composed of spongy tissue. 

 The short bones glide over each other, and so allow, in wrist 

 and ankle, a considerable range of motion in a great many 

 directions. 



The group of flat bones includes all those that are like the 

 rib in form and internal arrangement. They are the ribs, 

 breastbone, collar bones, shoulder blades, hip bones, and most 

 of the bones forming the top and sides of the cranium. 

 Their principal function is either that of furnishing pro- 

 tection for the brain and organs of the chest, or that of 

 supplying a means of attachment of the arms and legs to 

 the rest of the skeleton. In most cases flat bones have little 

 or no movemento 



Finally, the irregular bones include all those that do not 

 readily fall into the groups already enumerated. As exam- 

 ples may be mentioned the bones of the spinal column, in- 

 cluding sacrum and coccyx, the sphenoid, ethmoid, and the 

 bones of the face. Some of them serve as support (verte- 

 brae), others protect the eyes, ears, and nose, while in the 

 case of the lower jawbone and the atlas vertebra a consider- 

 able range of motion is possible. 



6. CHEMICAL COMPOSITION OP BONE* 



Effect of Burning Bones. Bones, we have found (p. 27), 

 consist of two kinds of material : (1) the living bone cells 

 that provide for the growth and repair of bone tissue, and 

 (2) the hard intercellular mineral matter (see Figs. 8 and 



1 See "Laboratory Exercises," No. 31. 



M 



