OSSIFICATION IN CONNECTIVE TISSUE. 271 



Some of the osteoblasts are involved in the ossifying matrix, and remain as the 

 corpuscles of the future bone, the spaces enclosing them being the lacunae. It is 

 supposed that the canaliculi, which are at first short, are afterwards extended by 

 absorption, so as to anastomose with those of neighbouring lacunae. 



. It is believed by many histologists that the fibrillated ground-substance of bone is formed 

 not outside the cells in an intercellular substance, but by a direct conversion of the proto- 

 plasm of some of the osteoblasts into bony tissue. If this were the case, there ought to be 

 some indication in the formed osseous substance of the cell-areas of which it was made up, 

 but nothing of the kind has been shown to exist. There should moreover often be observed 

 osteoblasts which are only partly converted into bony substance, but this has also never been 

 described. And if as some suppose, the peripheral part of each osteoblast becomes converted 

 into osseous substance, while the central part and nucleus remain as the corpuscle within a 

 lacuna, the osteoblasts would have to be originally far larger than the permanent lacunae, 

 which is certainly not the case. The view in question is similar to that which supposes 

 ordinary connective tissue to have a like origin, and appears to rest more upon theory than 

 on actual observation of the stages of the developmental process. 



Meanwhile, the meshes of the bony network, which were occupied as we have 

 seen by one or more blood-vessels, and by numerous osteoblasts, become diminished 

 in extent, and the bone at the same time increased in thickness by the deposit upon 

 the original trabeculae of irregular bony laminae and trabeculee, some of the osteo- 

 blasts remaining, and forming the corpuscles and lacunas as before. The interstices 

 of the bony spongework thus becomes gradually narrowed, containing one or more 

 blood-vessels surrounded by osteoblasts. 



At a later stage the increase in thickness takes place by successive depositions of 

 bony lamellae under the periosteum, a concentric deposition occurring at the same 

 time on the walls of the vascular channels. But since the growth in thickness of a 

 membrane-bone takes place in exactly the same manner as that of one of the long 

 bones, which will be fully described in a subsequent page, the reader is referred to 

 the account of the process there given. 



It may be observed that the appearance of the ossifying membrane-bone in the 

 shape of a network of trabeculas seems to be determined by the pre-existence of a 

 vascular network in the embryonic tissue. The new bone everywhere makes its 

 appearance in the spots which are furthest from the vessels, and the bony network 

 everywhere alternates with the vascular network. At the edges of the advancing 

 bone the spicules which prolong it pass between, and avoid the capillary blood- 

 vessels, which are thus left in the bays between the spicules : the divergent bunches 

 of osteogenic fibres which prolong the adjacent spicules complete the enclosure of 

 the blood-vessel. 



After a time the membrane-bone extends so as almost to come into contact with 

 the neighbouring bones. But as long as growth continues, there always remains in 

 the situation occupied afterwards by the sutures a vascular connective tissue with 

 numerous osteoblasts. This is continually on the increase, but as fast as it grows, 

 the osteogenic fibres and the osseous spicules extend into it from the young bones 

 on either side. At length, however, when these have attained their full dimensions, 

 the growth of the intermediate tissue ceases, and it becomes completely invaded by 

 the bone on either side, with the exception of the narrow and irregular line of 

 suture, which may eventually itself become more or less obliterated. 



From a morphological point of view, the membrane-bones, especially those of the skull, are 

 probably to be regarded as the modified remains of an integumental skeleton which is 

 extensively developed in some of the lower vertebrata, and which had in all probability as its 

 phylogenetic precursor a formation of dentinous. cutaneous spines. Even the membrane- 

 formation in connection with the cartilage-bones may have originated in the same manner. 



