REPARATION OF BONE. 173 



the course of five or six weeks, or less, in young persons. At that period, 

 however, the contiguous surfaces of the bone itself are not cemented by bony 

 union ; and the formation of the permanent callus occupies some months ; 

 during which the provisional callus is gradually absorbed, and the continuity 

 of the medullary canal is thus restored, in the manner in which it was first 

 established. Mr. Gulliver has remarked that, when the broken portions of 

 bone form an angle, there is quite a Distinct centre of ossification in the new 

 matter ; from which that portion of it is ossified, that lies between the sides 

 of the angle; thus forming what has been termed an accidental callus, and 

 giving support to the two portions of the shaft, in a situation which is exactly 

 that of the greatest mechanical advantage. Though for some time quite uncon- 

 nected with the old bone, it soon becomes united to the regular callus. This 

 instance proves, that continuity with previously-formed bone is not absolutely 

 requisite for the production of new osseous structure ; although the process 

 is decidedly favoured thereby. 



207. The reparation of Bone, after disease or injury, seems to take place 

 upon a plan essentially the same as that of its first formation. A plastic or 

 organizable exudation is first poured out from the neighbouring blood-vessels ; 

 and thus forms a sort of bed or matrix, in which the subsequent processes 

 take place. The next stage, in young animals, is the formation of a true car- 

 tilaginous substance, exactly resembling their temporary cartilages; and this 

 is gradually converted into bone, in the manner in which those cartilages are 

 consolidated in the first instance. In older animals, however, the new struc- 

 ture appears to be rather of a membranous character; and the ossifying pro- 

 cess would therefore correspond rather with that by which the normal in- 

 crease of their bones is effected. Mr. Tomes states* that he has examined 

 various cases of fracture of the neck or shaft of the femur, in which union 

 had not been effected, in consequence of the patient's advanced age; and that 

 he found in these no intervening cartilage, and but a scanty amount of con- 

 densed areolar tissue. In this latter, traces of an attempt at repair may be 

 generally found, in the presence of osseous matter in granules or granular 

 masses; but in these there is no arrangement of tubes or bone-cells of definite 

 character; indeed, such osseous masses are generally small, and are deficient 

 in density, owing to the want of union between the individual granules. 



208. The Teeth are nearly allied to Bone in structure ; and in some of the 

 lower Vertebrata, there is an actual continuity between the bone of the jaw, 

 and the teeth projecting from it, notwithstanding that the latter form part of 

 the dermal skeleton, whilst the former belongs to the neural or internal. In 

 Man and the higher animals, however, there is an obvious difference in their 

 structure ; as in their mode of development. These subjects have lately re- 

 ceived much attention; and the practical importance of an acquaintance with 

 them, renders it desirable that they should be here treated somewhat fully. 

 The Teeth of Man, and of most of the higher animals, are composed of three 

 very different substances ; Dentine (known as ivory in the tusk of the Ele- 

 phant), Enamel and Cementum or Crusta Petrosa. These are disposed in 

 various methods, according to the purpose which the Tooth is to serve: in 

 Man, the whole of the crown of the tooth is covered with Enamel ; its root or 

 fang is covered with Cementum; whilst the substance or body of the tooth is 

 composed of Dentine. In the molar Teeth of many Herbivorous animals, 

 however, the Enamel and Cementum form vertical plates, which alternate 

 with plates of Dentine, and present their edges at the grinding surface of the 

 tooth ; and the unequal wear of these substances, the Enamel being the 

 hardest, and the Cementum the softest, occasions this surface to be always 

 kept rough. 



* CyclopeetUa of Anatomy and Physiology, vol. iii , p. 857. 



15* 



