144 TEXT-BOOK OF EMBRYOLOGY. 



play an essential part not only in the formation of the bone itself but also in 

 the establishment of its form and internal structure. 



GROWTH OF BONES. The way in which the cranial cavity enlarges has been 

 described on page 139. While the process of enlargement is going on, the 

 individual bones increase in size principally by the addition of new bone along 

 their edges. 



Intracartilaginous bones grow both in diameter and in length. It has 

 already been stated that the primary spongy bone formed in cartilage is dis- 

 solved and that new bone is deposited under the periosteum. This naturally 

 brings about an enlargement of the primary marrow cavity and at the same 

 time an increase in the diameter of the bone as a whole. From this it is obvious 

 that the compact bone of the shaft of a long bone is of subperiosteal origin, the 

 intracartilaginous bone having been completely absorbed. 



A. B 



FIG. 119. Diagram representing growth in diameter of a long bone. 



from Flour ens. 



The fact that the osseous tissue bordering the marrow cavity is absorbed and that new 

 bone is deposited under the periosteum can be quite clearly demonstrated. A young 

 growing animal is fed for a few weeks on madder, which colors all the bone formed during that 

 time a distinct red. If the animal is then killed and sections made of the long bones, the 

 outer part of the latter will appear a distinct red. Another growing animal is fed on madder 

 for a few weeks, then allowed to live a few weeks longer without madder. Then if it is 

 killed and sections made of the bones, the red bone is found to be covered with a layer of 

 uncolored bone which was deposited after the madder feeding had been stopped. If a 

 young growing animal is fed on madder for a time and then allowed to live long enough 

 without madder, the red bone will be found lining the marrow cavity. (See Fig. 119.) 



Growth in length of the long bones takes place in a different manner. The 

 primary center of ossification is situated near the middle of the piece of cartilage, 

 and ossification proceeds in both directions toward the ends of the cartilage to 

 produce the diaphysis or shaft of the bone. In each end of the cartilage there 

 appears a secondary center from which ossification proceeds in all directions to 

 produce the epiphysis. Between the shaft and epiphysis a disk of cartilage 

 remains, and here, so long as the bone is growing, new cartilage continues to be 

 formed. At the same time new bone is being formed in the new cartilage, 





