1S80.] «-*^l [Ryder. 



eventually forms the matrix of the calcified skeleton of the adult. This 

 new matrix, after the hollowing-out process has been accomplished by the 

 agency of the ingrowth of the blood vessels and amoebiform osteoclastic 

 cells into the cartilage, is deposited not only within the bone but also on 

 its outside ; at first the amoebiform cells, which now begin to be included 

 within it, as the bone substance grows in thickness, are known as osteo- 

 blasts, and are joined together by fine protoplasmic processes and to the 

 lymph and blood spaces which have been eroded by the latter within the 

 bone substance. In this way an elaborate metabolic cycle is established 

 with the blood vascular system in which the fine protoplasmic threads, 

 joining the bone cells into an almost infinitesimally fine reticulum, are the 

 ultimate ramifications, while the system of blood and lymphatic vessels 

 are the gross bonds through which the whole is brought into relation and 

 continuity with the general metabolism of the body. 



The ultimate ramifications of the vessels through the adult bony tissue 

 are known as the Haversian canals and the canals of Volkmann. The 

 bonj'^ matrix around the former is concentrically laminated, around the 

 latter it is not. In the very young of higher animals, such as a cliild 

 under a year old, the bony tissue does not exhibit the lamination around 

 the Haversian canals such as it shows in the bones of the adult. This in- 

 teresting fact is confirmed by the structure of the bones of fishes, in 

 which there may not even be osteoblasts present within a bone at any 

 period of the life of the animal ; the bones being in reality nothing but 

 absolutely homogeneous or laminated plates of a matrix which has calci- 

 fied throughout. The matrix in this case, as in all the others, has been 

 deposited by the action of connective-tissue cells and vessels, and both of 

 these may be observed in the vicinity, but lying external to the bone 

 matrix. In other cases an elaborate reticular calcifying matrix is devel- 

 oped within cartilage without the presence of vessels. The most singular 

 type of this is that met with in the vertebral centra of sharks, where the 

 radiating and concentric fibres of the calcifying matrix arise between the 

 cartilage cells which formed the primary or embryonic vertebral body. 

 The radiating fibres, in this case, may be traced as continuations into the 

 fibrous connective tissues investing the vertebral column. This matrix is 

 homogeneous, highly refringent and its origin may be traced in the em- 

 bryo directly to the membrana elastica externa of authors. 

 • The non-vascular character of all the tissues of animals immediately 

 ihvolved in calcification has been thus well established. The only vessels 

 wliich can be identified as actually perforating as the minutest canals 

 either bone matrix or cartilage are the canaliculi from the lacuute in which 

 tlie bone and cartilage cells lie. The comparatively coarse capillaries of 

 the Haversian systems are remnants of the erosive and constructive pro- 

 cesses which took place when the bone was built up during ontogeny. 

 They become narrower and more constricted as life advances, and the 

 bones become more solidified. It is therefore obvious that the processes of 

 metabolism are here normally at a very low ebb so far as they affect the 



