98 



HUMAN ANATOMY. 



Fig. 129. 



in the limited situations where the central spongy bone persists, as in the vertebrae, 

 the ribs, the sternum, and the ends of the long bones. 



The important fact may be here emphasized that the process sometimes spoken 

 of as the " ossification of cartilage" is really a substitution of osseous tissue for car- 

 tilage, and that even in the endochondral mode of formation cartilage is never 

 directly co7iverted i7ito bone. 



The ossijicatioji of the epiphyses (Fig. 130), which in the majority of cases 

 does not begin until some time after birth, the cartilage capping the diaphysis mean- 

 while retaining its embryonal character, repeats in the essential features the details 

 already described in connection with endochondral bone-formation of the shaft. 

 After the establishment of the primary marrow-cavity and the surrounding spongy 

 bone, ossification extends in two directions, — towards the periphery and towards the 

 adjacent end of the diaphysis. As this process continues, the layer of cartilage in- 

 terposed between the central spongy bone and the free surface on the one hand, 



and between the central bone of the epiphysis 

 and the diaphysis on the other, is gradually 

 reduced until in places it entirely disappears. 

 Over the areas which correspond to the later 

 joint-surfaces the cartilage persists and be- 

 comes the articular cartilage covering the 

 free ends of the bone. With the final ab- 

 sorption of the plates separating the epiphyses 

 from the shaft the osseous tissue of the seg- 

 ments becomes continuous, ''bony union" 

 being thus accomplished. 



Intramembranous Bone-Develop- 

 ment. — The foregoing consideration of the 

 formation of bone within cartilage renders it 

 evident that the true osteogenetic elements 

 are contributed by the periosteum when the 

 latter membrane sends its processes into the 

 ossific centre ; the distinction, therefore, be- 

 tween endochondral and membranous bone 

 is one of situation rather than of inherent 

 difference, since in both the active agents in 

 the production of the osseous tissue are the 

 osteoblasts, and in essential features the pro- 

 cesses are identical. Since in the produc- 

 tion of membrane-bone the changes within 

 pre-existing cartilage do not come into ac- 

 count, the development is less complicated 

 and concerns primarily only a formative pro- 

 cess. 



Although the development of all osseous 

 tissue outside of cartilage may be grouped 

 under the general heading of intramembranous, two phases of this mode of bone- 

 formation must be' recognized ; the one, the intramenibrayious , in the more literal 

 sense, applying to the development of such bones as those of the vault of the skull 

 and of the face, in which the osseous tissue is formed within the mesoblastic sheets, 

 and the other, the subperiosteal, contributing with few exceptions to the production 

 of every skeletal segment, in which the bone is deposited beneath rather than within 

 the connective-tissue matrix. In consideration of its almost universal participation, 

 the periosteal mode of development will be regarded as the representative of the 

 intramembranous formation. 



Subperiosteal Bone. — The young periosteum, it will be recalled, consists of 

 an outer and more compact fibrous and an inner looser osteogenetic layer. The 

 latter, in addition to numerous blood-vessels, contains young connecti\'e-tissue ele- 

 ments and delicate bundles of fibrous tissue. These cells, or osteoblasts, become 

 more regularly and closely arranged along the fibrillae, about which is deposited the 



Embryonal 

 cartilage 



Zone of calci- 

 ficalion 



Endochondral 

 spongy bone 



Periosteum 



Subperiosteal 

 bone 



Young mar- 

 row 



Endochondral 

 spongy bone 



Zone of calci-, 

 fication 



Embryonal 

 cartilage 



Longitudinal section of phalanx of fcetus of five 

 months. X 23. 



