74 CONNECTIVE TISSUES BONE. 



fying surface, which were previously in close apposition, separate consider- 

 ably from one another by the increase of material within the cells, the 

 nuclei themselves becoming larger and more transparent. Small connect- 

 ing spicuke of bone then form between the vertical ones, and the groups of 

 original cartilage-cells come to be inclosed within oblong loculi or areolne, 

 which constitute the primary medullary spaces. The young cells occupy- 

 ing these spaces may be divided, according to Waldeyer, into two groups, 

 one of which aids *iu the formation of the osseous tissue, whilst the other 

 develops into medullary tissue. The former may be appropriately termed 

 " osteoblasts," and consist of masses of granular protoplasm, or germinal 

 matter apparently destitute of any investing membrane, but each contain- 

 ing a nucleus, which arrange themselves side by side, like an epithelial layer, 

 on the inner surface of the primary areolre of bone-substance. Many of 

 these osteoblasts may be seen undergoing calcification, and half buried in 

 the newly-formed yellowish but homogeneous bony deposit, that appears as 

 an edging to the darker and more granular primary trabeculse, whilst in 

 other parts they may be seen to communicate with one another by long pro- 

 cesses. After the first set, which proceed directly from the cartilage-cells, 

 have been used up, fresh osteoblasts are developed from the cells of the 

 young medullary tissue. Dr. Sharpey, Waldeyer, and others, believe that 

 some of the osteoblasts become wholly converted into bone, in which case 

 the nuclei vanish, whilst others only undergo partial calcification, the cen- 

 tral portion of the protoplasmic mass remaining with the nucleus to form 

 the contents of the bone lacunae. Gegenbaur, however, thinks the lamellae 

 are formed from a material excreted by the osteoblasts, 1 and Kolliker seems 

 inclined to adopt this view. Whilst these processes are occurring, the bony 

 tissues become vascular, partly by bloodvessels from the investing membrane 

 of the bone penetrating or pushing their way in from without by absorption, 

 and partly by vessels lying on the surface becoming surrounded by de- 

 posits of new osseous substance, which form by degrees the series of inclos- 

 ing lamellae that constitute an Haversian system. Of the bony spiculte and 

 lamelke bounding the primary areoke, some have been shown by Dr. Sharpey 

 to be speedily absorbed again, and thus to form larger areoke, which are to 

 be seen on making a transverse section a little below the ossifying surface. 

 Even after the completion of the bone, moreover, interstitial changes are 

 continually taking place in its substance, as in that of the softer tissues, old 

 Haversian systems being partially or entirely removed by absorption, and 

 new ones being developed in their place. And it is to the persistence of por- 

 tions of those older Haversian systems, which have undergone partial ab- 

 sorption, that we are to attribute the presence of those intervening lamiiue 

 which fill up the spaces between the existing Haversian systems. The clavi- 

 cle and lower jaw are the first bones to ossify, their osseous centres or points 

 of ossification appearing at the close of the first month of foetal life. In- 

 crease in the length of a bone as may be shown in experiments made by 

 feeding an animal with madder, the newly formed bone being deeply tinged 

 takes place almost entirely between the epiphyses and the shaft; but 

 probably also results in part from the interstitial deposition of new material ; :1 

 increase in girth, by ossification immediately beneath the periosteum; ab- 

 sorption from within and deposition from without going on continually. 

 The former takes place by intracartilaginons or eudosteal, the latter by 

 intramembranous or ectosteal ossification. 



1 Jc'iuiisrlien Zc'it.chrift, Band iii, 1806. 2 Gewehclchre. 



3 For :ir^uincnts for and against the latter point sec Wolff., Chi., 18(i!t, p. 849; 

 1S70, p. Ml ; Volkinann in Centralblatt, 1870, p. 1:29; Licbcrkahn, Centralblatt, 

 1872, p. 420; Eanvicr, Comptes licndus, 1873, t. 77, p. 1105. 



