THE OSSEOUS SYSTEM. 375 



tlie frontal eminences, even after the ossific union of the two 

 portions of the frontal bone, the change of form of the lower 

 jaw (the greater distance between the coronoid processes and 

 the mental spine, the alteration in its curvature, the partial 

 removal and renewal of the alveolec), &c? We have already 

 seen, that in the other bones, also, something of the kind must 

 be presumed to take place, and, consequently, cannot hesitate 

 to admit it in the present case, although the particulars of the 

 process be unknown. That this process occurs in the interior 

 of the secondary bones has been already mentioned. The 

 formation of the diplot, which becomes more evident in the 

 tenth year, is to be referred to it, as is also that of the frontal 

 sinuses, and antrum Hir/hmorianum, which however does not 

 take place till a later period. 



I would further remark, that the secondary bones, so long 

 as they are in a growing state, are much more vascular than 

 afterwards, even exceeding, in this respect, the periosteal layers 

 of the other bones; on which account their medulla, contain- 

 ing the multi-nuclear, enigmatical bodies, already referred to, is 

 of a redder colour. The vessels enter these bones at innu- 

 merable points on the surface, and, in the different bones, run 

 in vertical or horizontal canals. The latter is the case in the 

 flatter bones, in which the vascular channels run principally in 

 the longitudinal direction of the osseous rays proceeding from 

 the primary point of ossification ; and the former, in con- 

 sequence of which the surface of the bone frequently presents 

 an exti*emely delicate, millepore-like aspect, occurs in the 

 thicker portions. A great many of these canals afterwards 

 become obliterated, or, at all events, very much contracted, 

 whence the surface of the bone is rendered smoother. 



In conclusion to these remarks on the development of the 

 bones, I will add a few words regarding their conditions at 

 different periods. Valentin noticed the cartilaginous rudi- 

 ments of the ribs in a human embryo 6'" long. That of the 

 cranium is distinctly recognisable in the sixth or seventh week, 

 as well as those of the vertebral zone and that of the extremi- 

 ties ; those of the extremities proper do not appear till later 

 (in the eighth or ninth week). Ossification commences as 

 early as the second mouth, first in the clavicles and lower jaw 

 (fifth to seventh week), then in the vertebra, the humerus, 



