282 BONE OR OSSEOUS TISSUE. 



similar cells occur under pathological conditions in various situations apart from any 

 hard tissue, and have long been known as " giant-cells " (Riesenzellen, Virchow). 



The changes of shape which the bones undergo in the process of growth, as well 

 as any changes which may occur in them in adult life, are all produced in the same 

 manner as the increase of size that is to say, not by interstitial growth and 

 expansion of the substance of the bone in one direction more than in another, but 

 by a deposition of new bone by osteoblasts at some parts and a simultaneous absorp- 

 tion by ostoclasts at others ; whilst in other places again neither absorption nor 

 deposition is occurring just as a modeller corrects his work by laying clay on at 

 one part whilst removing it at another. 1 



Since during the growth of bones their shape is becoming continually altered, it 

 follows that in nearly all bones during growth there are parts of the bone which are 

 in process of absorption, and others which are in process of more active deposition 

 than the rest. In most of the long bones, towards their ends, absorption is 

 generally taking place at one side, and deposition on the opposite side. The 

 former process may, and probably does, proceed to such an extent, that the 

 endochondral bone may be laid bare or even partially absorbed, but after a while, 

 when the absorption has ceased at any part, re-deposition may take place, the 

 ostoclasts being replaced by osteoblasts, and successive circumferential lamellae 

 being deposited by these. 



A large amount of variation is met with in the different bones of the skeleton in 

 the relative extent to which they are formed in cartilage and in the sub-periosteal 

 tissue respectively. Whereas in some, such as the shafts of the long bones of the 

 limbs, the endochondral bone is almost entirely removed, as we have seen, and 

 periosteal bone substituted for it ; in others, such as the bodies of the vertebras, and 

 the epiphyses of the long bones, a much larger proportion of the adult bone has had 

 an endochondral formation. In one or two bones or parts of bones again, which 

 may be said to have typically an intramembranous origin, cartilage may, according 

 to Kassowitz, become developed under the periosteum at certain places, and the 

 continuation of the ossification may occur in this secondarily developed cartilage. 

 This is said to be the case with the clavicle, the foundation of which is laid in 

 membrane, but which is found at a later period to have cartilaginous ends ; and also 

 with the halves of the lower jaw-bone, which develops cartilaginous ends both 

 towards the symphysis and towards the articular and coronoid processes, these 

 cartilaginous ends being altogether distinct from the cartilage of Meckel, which at 

 those parts is unconnected with the jaw-bone, although at another place (in front) 

 it is involved in the ossification of the maxilla (see vol. ii., p. 78). Kassowitz has 

 described similar cartilaginous developments in connection Avith the sub-periosteal 

 tissue at the tuberosity of the radius and the spine of the scapula. They are merely 

 an extension of the process which normally goes on at the ossification groove 

 (p. 279). 



The time of commencement of ossification in the different bones, as well as 

 the number and mode of conjunction of their centres of ossification, are treated 

 of in the Descriptive Anatomy (vol. ii.). 



Regeneration of bone. In the reunion of fractured bones, osseous matter 

 (often preceded by a new formation of cartilage), is formed between and around the 

 broken ends, connecting them firmly together ; and when a portion of bone dies, a 

 growth of new bone very generally takes place to a greater or less extent, and the 

 dead part is thrown off. The importance of the periosteum in the process of repair 

 is shown by the fact that if a portion of periosteum be stripped off, the subjacent 



1 For special details of this modelling process as it is met with in the different bones of the skeleton, 

 the reader is referred to Kblliker's memoir ; Die normale Resorption des Knochengewebes. Leipzig, 

 1873 ; and to a paper by Kassowitz (Die normale Ossification, &c.) in Strieker's Med. Jahrb, 1879 1880. 



