106 BOXE. 



describes them as springing originally from the cells which compose the walls of 

 the blood-vassels. Ostoclasts are found in connection with the roots of the milk 

 teeth where these are undergoing absori^tion to make way for the iJennanent set : 



and cells precisely similar occur 

 in various situations quite apart 

 from any hard tissue, and in 

 siich situations have long been 

 known as '• giant-cells " (Riesen- 

 _,-!Sw>.. :-:\ (3B&;;.s»v»-s^g zellen, Yirchow). "\Miether in 



<f?^?'^ ■ / '% ?-'^'^f^W these cases also they are con- 



^( ■:: J ■■ • ".W cerned in any way with absorp- 



ife'S'Xj^-' "'"" vM tion is unknown. 



The changes of shape which 

 the bones imdergo in the pro- 

 cess of growth, as well as any 

 m /^ changes which may occur in 



Fig. 63.— Three Ostoclasts from Abs:)rption them in adult life, are all pro- 



bURKACES OF (iROWOG JjONE. 400 DiAMETERS ^„„„ 1 ,•„ +!,„ „„ 



,,,..,,., , diTceu m the same manner as 



the increase of size — that is to 

 a, with thickened striated border. gay, not by interstitial growth 



and expansion of the sub- 

 stance of the bone in one direction more than in another, but by a deposition 

 of new bone by osteoljlasts at some parts and a simultaneous absorption by 

 ostoclasts at others ; whilst in other places again neither absorption nor deposi- 

 tion is occmi-ing— just as a modeller corrects his work by plastering the clay 

 on at one jiart whilst scraping it away at another part.* 



From the foregoing account of the development of bone, it is evident that a 

 great portion of a long bone is formed independently of cartilage. It appears there- 

 fore reasonable to consider the pre-existence of that tissue as not being a necessary 

 condition of the ossific process, and to regard the precursory cartilage of the 

 foetal skeleton in the light of a temporary substitute for bone, and also as afford- 

 ing, as it were, a mould of definite figmre and of soft but yet sufficiently consistent 

 material in which the osseous tissue may be at first deposited and assume a suit- 

 able form. In fact the cartilage-cells are not ossified, and, as to the slender walls 

 of the primary areolas formed by calcification of the intercellular cartilaginous 

 matrix, most of them are. in a long bone, swept away by absolution, in the 

 excavation of the medullary canal ; so that they can only remain — coated, how- 

 ever, and obscured by secondary laminated deposit — in the cancellar structure of 

 bones which begin to ossify in cartilage. f 



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 subjects 

 that belong to special anatomy. It may, however, be here remarked in general, 

 that the commencement of ossification does not in all cases follow the order in 

 which the bones appear in their soft or cartilaginous state. The vertebra;, for 

 instance, appear as cartilages before there is any trace of the clavicle, yet 

 ossification begins in the latter sooner than in any other bone of the skeleton. 

 The time when it commences in the clavicle, and consequently the date of the 

 first ossification in the skeleton, is referred by some to the seventh week of 



* For special details of this modelling process as it is met with in tlie different bones 

 'if tlic skeleton, the reader is referred to KoUiker's memoir : Die uormale ResoriJtion des 

 Knocheugewebes. Leipzig, 1873. 



t Nesbitt, in 1736, maintained that the cartilage is "entirely destroyed ;" he there- 

 fore considered it to be a mere temporary substitute ; but the steps of the process of 

 intracartilaginous ossification as now traced with the aid of the microscope were unknown 

 to him. Tlie view stated in the text, together with most of tlie facts adduced in support 

 of it, was announced by Sharpey in the fifth edition of this work in 1846, but notwith- 

 standing the compreliensive researches of liruch, by wliich ho was led to the same opinion 

 (Denks. d. Schweitz. naturf. Gesclls. 1852), it met witli little notice, and probaljly less 

 assent, until the subject was treated of in a special memoir by the late H. ]Muner (Zeits. 

 fur wissensch. Zool. vol. ix., ISoS), to whom the doctrine in its modern shape is 

 now commonly ascribed. 



