150 ON THE STRUCTURE OF THE SKULL. 



annulus. The ossification begins above, descends posteriorly, 

 and, after a ring has been formed in this manner, extends 

 forwards. 



" At the same time arises a proper centre of ossification, 

 completely separated from this, at the external end of the 

 superior vertical canal. 



" After this, a third little scale is produced, opposite about 

 the middle of the internal vertical semicircular canal. 



" At the same time ossification extends swiftly backwards 

 and downwards from the first piece, so as to give rise to the 

 floor of the labyrinth. 



" The second piece increases in size still more rapidly, so 

 that, soon, the whole vertical semicircular canal, with the ex- 

 ception of its lower concave surface, is ossified. Simultane- 

 ously ossification is continued from its inner end over the inner 

 surface of the petrous bone, surrounds the internal auditory 

 meatus, penetrates into it, and so forms the floor of the cochlea. 



"The horizontal semicircular canal begins to ossify, on its 

 outer side, in the fifth month, by elongation of the bone form- 

 ing the upper vertical semicircular canal: tins is continued 

 backwards, from without and below T , round the horizontal semi- 

 circular canal. At least, I could discover no proper osseous 

 centre for this canal, and it seems merely to become inclosed by 

 the increase of the first and second." 



All this is accurate, but, unfortunately, Meckel goes on to 

 say, at page 5 1 of the work cited, that " the osseous labyrinth is 

 at first perfectly separate from the bony mass of the petrous bone 

 which surrounds it, is developed earlier than it, and is provided 

 with quite a smooth surface, though the two lie close together ;" 

 and that " the bony labyrinth arises independently of the osseous 

 substance of the petrous bone." 



How Meckel arrived at this conclusion I do not know ; but 

 it is certainly erroneous, and it has been the means of creating 

 a great deal of unsound speculation as to the ossified labyrinth 

 being a something distinct from the proper pars petrosa. 



It is further singular that, in this passage, Meckel not only, 

 as I have said above, makes no reference to Kerckringius, but 

 that he does not attempt to refer the regions of the pars petrosa 



