A STUDY OF THE SKELETON 



175 



The structure of the bone itself is best seen in cross and 

 longitudinal sections. On the outside is a 

 layer of compact hard bone, that cannot be 

 cut or dug out with the point of a knife. 

 The whole interior of the rib is made up 

 of spongy bone (compare Fig. 74, 1, 2), the 

 spaces in the bone tissue being filled with a 

 soft substance called red marrow. Hence, 

 in cutting to the center of a rib, one would 

 meet successively periosteum, hard bone, 

 and spongy bone with its red marrow. 



Structure of a Soup Bone. A soup bone is 

 more or less cylindrical in form, with an en- 

 largement at either end (compare with Figs. 

 73 and 74). The longer central portion 

 is called the shaft, and the enlarged extremi- 

 ties are known as the heads of the bone. 

 The surface of each head is covered with 

 cartilage wherever it moves upon another 

 bone, as was the case with the rib. The 

 rest of the head and shaft is incased in 

 periosteum. A longitudinal section shows 

 the internal structure to be as follows: 

 All over the outside is found a layer of 

 hard bone which is thick in the shaft region 

 and relatively thin beneath the cartilage of ' 

 the heads (compare with Fig. 74). The 

 interior of the heads is largely composed 

 of spongy bone. The long central cavity of 

 the shaft is filled with fatty yellow marrow. 



On comparing the rib and soup bone, we 

 see that the marrow in the former is found 

 only in spongy bone ; in the latter it occurs 

 in the spongy bone of the heads, and in a 

 solid, fatty mass in the marrow cavity of 

 the shaft. 



^G. 74. Longitu- 

 dinal Section of 

 Tibia. 



spongy bone 

 in heads. 



3 = hard bone in 



shaft. 



4 = marrow cav- 



ity in shaft. 



5 = layer of car- 



tilage in 



6 = periosteum 

 c overing 

 outside. 



7= surface of 

 heads cov- 

 ered with 

 cartilage. 



