FORMATION OF OSSEOUS TISSUE. 147 



tlie under surface of tlie membrane into tlie tiny 

 channels (Haversian canals) which pass obliquely 

 into the compact tissue. The vessels of the bone 

 ramify throughout its substance, and if they have 

 been injected previous to the removal of the cal- 

 careous matter by the action of acid, they will be dis- 

 tinctly seen ramifying through the semi-transparent 

 animal substance. A preparation of this kind dried, 

 and afterwards preserved in spirits of turpentine, 

 serves beautifully to exhibit the disposition of the 

 vessels in bone. 



Formation of Osseo^is Tissue. 



The osseous tissue itself is formed in the same way 

 in the cancellated texture and the compact tissue, 

 and it is worth studying very carefully. The changes 

 may be beautifully seen in the formation of the cranial 

 bones of the frog, which continue to grow at their 

 edges, even in the full-grown animal. In this struc- 

 ture an opportunity is afforded of observing every 

 stage of the process of bone formation in a single 

 specimen. At the extreme edge is ordinary cartilage, 

 which gradually passes into tissue which is being 

 infiltrated with calcareous deposit ; and, lastly, we 

 come to the fully formed bone. 



208. Conversion of cartilage into bone. — At the 

 outer edge where the bone is growing, we may also 

 study the development of cartilage. A little further 

 inwards the formation of cartilaginous tissue is com- 

 plete. Passing in the same direction, we soon ob- 

 serve that a change is taking place in the matrix. 

 Granules and highly refracting globules have been 

 deposited in its substance. The deposition of this 

 material, which is easily proved to consist of cal- 

 careous salts, invariably commences in the matrix at 

 a point equidistant from contiguous masses of bio- 

 plasm — that is, in that part of the formed material 

 which is, of course, most distant from the bioplasm. 



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