118 



LOCOMOTION. 



[CHAP. v. 



F ig.z\, 



principally by their transparency around each nucleus (fig. 30, 

 a. a). 



In the vicinity of the point of ossification for example, in 

 one of the long bones), a singular change is observed. The cells 

 are seen to be gradually arranging themselves in linear series, 

 which run down, as it were, towards the ossifying surface. The 

 appearance they present on a vertical section is represented in 

 fig. 30. At first their aggregation is irregular, and the series 

 small (b.b'.^j but, nearer to the surface of ossification, they form 

 rows of twenty or thirty. These rows are slightly undulated, 

 and are separated from one another by the intercellular substance. 

 The cells composing them are closely applied to one another, 

 and compressed, so that even their nuclei seem in many in- 

 stances to touch: the nuclei themselves are also flattened, and 

 expanded laterally. 



The lowest row dip into, and rest in 

 deep narrow cups of bone, formed by the 

 osseous transformation of the intercellular 

 substance between the rows. These cups 

 are seen by a vertical section in fig. 30, c. c ., 

 and by a transverse section on the level of 

 the ossifying surface in fig. 31. As ossifi- 

 cation advances between the rows, these 

 cups are of course converted into closed are- 

 olse of bone, the walls of which are lamelli- 

 form, and at first extremely thin. 



Immediately upon the ossifying surface, 

 the nuclei, which were before closely com- 

 pressed, separate considerably from one 

 another by the increase of material within 

 the cell s. The nuclei likewise often enlarge 

 and become more transparent ; a condition 

 first pointed out to us by Mr. Tomes, but not present in fig. 31, 

 which was taken from a preparation that had been immersed in 

 spirit. The changes now enumerated may be conveniently con- 

 sidered to constitute the first stage of the process, which extends 

 only to the ossification of the intercellular substance. In this stage 

 there are no blood-vessels directly concerned. 



The areolse or minute cancelli, when first formed, contain only the 

 rows of cells which they have enclosed. It is remarkable, that, when 

 the cartilage is torn from the bone, it usually carries with it one or 

 two layers of these cancelli, or a little more than is represented in 



Horizontal section at the ossi- 







