52 GENERAL ANATOMY. 



by the absorption of the original foetal bone in the same way as the medullary 

 tube is formed, and the same is the case with the Haversian spaces above re- 

 ferred to as a sort of intermediate step between the Haversian canals and the 

 medullary spaces. Thus the distinction between the cancellous and compact 

 tissue appears to depend essentially upon the extent to which this process of 

 absorption has been carried, and we may perhaps remind the reader that in 

 morbid states of the bone inflammatory absorption effects exactly the same 

 change, and converts portions of bone naturally compact, into cancellous 

 tissue. 



The intra-membranous ossification is that by which the bones of the vertex 

 of the skull are entirely formed. In the bones which are so developed no 

 cartilaginous mould precedes the appearance of the bony tissue. The process, 

 though pointed out originally by Dr. Nesbitt, in the year 1736, was first accu- 

 rately described by Dr. Sharpey; .and it does not appear that subsequent ob- 

 servers have been able to add anything essential to his description. This is, 

 substantially, as follows: In the membrane which occupies the place of the 

 future bone a little network of bony spiculae is first noticed, radiating from the 

 point of ossification. When these rays of growing bone are examined by the 

 microscope, there is found a network of fine clear fibres (osteogenic fibres), 

 which become dark and granular from calcification, and as they calcify they 

 are found to inclose in their interior large granular corpuscles (the so-called 

 " osteoblasts" described above in the account of the intra-cartilaginous ossifi- 

 cation). These corpuscles at first lie upon the osteogenic fibres, so that the 

 corpuscles must be removed by brushing the specimen with a hair pencil in 

 order to render the fibres clear; but they gradually sink into areola3 developed 

 among the fibres. The areolse appear to be the rudiments of the lacunae, 

 the passages between the fibres form the canaliculi, and the osteoblasts are the 

 rudiments of the bone cells. As the tissue increases in thickness vessels shoot 

 into it, grooving for themselves spaces or channels, which become the Haver- 

 sian canals. 



The subperiosteal is in all essential respects identical with the intra-membran- 

 ous process of ossification. 



The Period of Ossification is different in different bones. The order of succes- 

 sion may be thus arranged (Kolliker) : 



In the second month, first, in the clavicle, and lower jaw (fifth to seventh 

 week); then, in the vertebrae, humerus, femur, the ribs, and the cartilaginous 

 portion of the occipital bone. 



At the end of the second, and commencement of the third month, the frontal 

 bone, the scapula, the bones of the forearm and leg, and upper jaw, make their 

 appearance. 



In the third month, the remaining cranial bones, with few exceptions, the 

 metatarsus, the metacarpus, and the phalanges, begin to ossify. 



In the fourth month, the iliac bones, and the ossicula auditus. 



In the fourth or fifth month, the ethmoid, sternum, os pubis, and ischium. 



From the sixth to the seventh month, the calcaneum, and astragalus. 



In the eighth month, the hyoid bone. 



At birth, the epiphyses of all the cylindrical bones, with the exception of the 

 lower epiphysis of the femur, and occasionally the upper epiphysis of the tibia; 

 all the bones of the carpus; the five smaller ones of the tarsus; the patella; the 

 sesamoid bones; and the coccyx, 1 are still ossified. 



From the time of birth to the fourth year, osseous nuclei make their appear- 

 ance also in these parts. 



At twelve years, in the pisiform bone. 



The number of ossific centres is different in different bones. In most of the 

 short bones, ossification commences by a single point in the centre, and pro- 



1 On the development of the coccyx, vide Coccyx. 



