ON THE STllUCTURE AND DEVKLOPMENT OF BONE. 



139 



layer after layer of osteogenic fibres in a concentric manner 

 upon the wall of the space. Some of the osteoblasts, in their 

 containing lacunae, remain between the lamellae as the corpuscles 

 of the future bone, and since the lacunae are at first of con- 

 siderable size they give the appearance of distinct gaps in the 

 osteogenic substance, which thus appears to form a network 

 upon the walls of the spaces which are to become the vertical 

 Haversian canals. This concentric deposit and the correspond- 

 ing appearances just noticed commence already upon the sides 

 of the advancing spicule. Hence the osteogenic substance was 

 described by Sharpey as " spreading out at the sides of the 

 trabeculae and encroaching on the intervening space in form of a 

 bright trellis-work." In reality it is formed here in the same 

 way as elsewhere, namely, in the form of straight or curved, 

 stiff fibres applied to each other layer upon layer, and their 

 interstices occupied by a homogeneous substance which, as 

 well as the fibres, soon becomes infiltrated with calcareous 

 deposit.' 



b. Periosteal Ossification of a Long Bone. — The mode of 

 advance of the ossific process in the periosteum of a long bone 

 is precisely similar to that seen in the skull-membranes. The 



Part of section of developing tooth of young rat, showing the mode of 

 deposition of the dentine. Highly magnified, a. Outer layer of fully 

 formed dentine ; I), uncalcified matrix (pdontogen') with a few nodules 

 of calcareous deposit ; c, odontoblasts, with processes extending: into 

 the dentine ; d, pulp. The section was stained with carmine, which 

 colours the uncalcified matrix, but not the calcified part. 



• It is interesting in comparing the development of dentine with that of 

 bone to note that in this case also the hard substance is preceded by a soft 

 material deposited in layers, and becoming subsequently infiltrated by cal- 

 careous deposit in the form of globules (much larger than those of osseous 

 development), which for the most part disappear as the intermediate 

 ground-substance becomes calcified. The main difference in the two pro- 

 cesses is to be found in the fact, that in dentine there are no fibres deve- 

 loped in the ground -substance. Since the soft material in which the 

 calcareous deposit of bone takes place is termed " osteogen," that in which 

 the calcification of dentine proceeds might be termed "odontogen." 



