356 THE TISSUES. 



Development of the Compact Bone- Substance. 



This is developed from a layer of plasma, underneath and afforded 

 by the vessels of the periosteum. In the foetus of five months, this 

 layer is so firm as to be detached with the fully formed periosteum, 

 forming upon the latter a moderately thick, soft, whitish -yellow 

 lamella very much resembling immature collagenous (white fibrous) 

 tissue, and containing granular, oval, or round nucleated cells, 

 3uW to T2 1 o o f an i ncn i a diameter. This lamella is very exten- 

 sively connected with the superficial layer of the bone, and on being 

 detached, a few little fragments of bone and scattered masses of 

 reddish soft medulla from the most superficial cancelli, will be seen 

 on the inner surface. 



The cells just mentioned appear exactly like the foetal medulla- 

 cells, but not at all like those of cartilage. And it appears that the 

 collagenous matrix is next ossified by the simple uniform deposit 

 of the calcareous salts, though without the previous appearance of 

 calcareous granules as before described (p. 354); while from the 

 cells the lacunas and pores are developed. 



Bone formed in this way, however, does not constitute connected 

 and parallel layers, but interrupted reticular lamellas, and the spaces 

 left between the latter T ^ to ^ F of an inch in diameter, are the 

 rudiments of the Haversian canals of the compact bone-substance. 

 These spaces at first contain only the unossified portion of the 

 plasma just described. But vessels communicating with those of the 

 interior of the bone (of the cancelli), and with those of the perios- 

 teum, soon appear in them; as well as the usual light-reddish me- 

 dulla-cells, and certain peculiar cellular corpuscles, with from three 

 to twelve or more vesicular nuclei and nucleoli, which are probably 

 referable to the multiplication of the latter. The vessels just men- 

 tioned are the future Haversian vessels; and finally the Haversian 

 rods consisting of the concentric (or spiral) lamellae, with their 

 lacunas and pores, are developed around the vessels the outer la- 

 mellse first; and thus the development of the compact bone-sub- 

 stance is completed. The manner in which the interrupted laminaa 

 are formed, and which are now seen to constitute the interstitial (or 

 inter- Haversian) bone-substance (p. 330), is shown by Fig. 227. A 

 vertical section of the sub-periosteal layer of developing osseous 

 tissue is shown by Fig. 228. 



The compact tissue continues to be formed, as just described, 



