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Show 150 ON THE S'rHUOTURE OF TilE SKULL. annulus. The ossification begins above, descends posteriorly, and, after a ring has been formed in this 1nanner, extends forwards. "At the same time arises a proper centre of ossification, completely separated from this, at the ext rnal end of the superior vertical canal. "After this, a third little scale is produced, opposite about the 1niddle 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 sen1icircular canal, with the exception of its lower concave surface, is o sified. Simultaneously ossification is continued from its inner nd over the inner surface of the petrous bone, SUlTound 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 longation of t.ho bone forming the upper vertical semicircular canal : thi is continued backwards, from without and b low, round the horizontal semicircular canal. At least, I could di cover no proper osseous centre for this canal, a.nd 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 51 of the work cited, that "the o seous labyrinth is at first perfectly separate from th bony rna s 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 ari e independently of the osseous substance of the petrous bon ." How Meckel arrived at thi" conclusion I do not know ; but it is certainly erroneous, and it has been the means of ereating a great deal of unsound speculation as to the ossified labyrinth being a something distinct from th proper par s p etrosa. It is further singular that, in this passage, Meckel not only, as I have said above, makes no reference to I\erekringius, but that he does not attempt to r £ r the region · of the pars petrosa 'rHE DEVELOPMEN'r OF TilE II OMAN SKULL. 151 and mastoidea to their separate origins. This is the more reJnarkable as, in his well-known paper on the " Ossification of the Vertebral and Cranial Bones" (Meckel's" Archiv,"] 815), p. 636, he states expressly that the mastoid process arises from a special centre. Possibly the ornission arose from Meckel's supposing that the exterior of the periotic mass is developed distinctly from the proper bony labyrinth. Hallmann, in his well-known work, "Die Vergleiehende Osteologie des SchHifenbeins" (1837), does not cite the account A given by Meckel, and does not really in1prove upon the views of Kerckringius. "In man, after, in the first place, the squamosal and then the annulus tympanicus are formed, the os petros1JJm and mastoideum is still a common cartilage, which fills, externally, the gap between the squamosal, the parietal, the supra-occipital and the ex-occipital. When, in the fourth month, the cochlea and a part of the semicircular canals, viz., the upper canal and the anterior crus of the external canal, already consist of porous bony substance, while the ossification of the posterior canal (and probably of the posterior crus of the external canal) has not proceeded so far ; the pars mastoidea appears as a single or double nodule of the size of a millet-seed, which is deposited upon the arch of the posterior canal, contributes to its ossification, and now soon spreads over the whole cartilage, the four neighbouring bones growing towards it. In Nos. 2543 and 9420 of the Berlin Museum, the insertion of this nodule upon the petrous bone is quite distinct. This osseous centre appears in the dry skeleton as an oval nodule, which I could easily scratch offwithout injuring the canals, which proves that it arises as a separate part." Lastly, Kolliker, in his recently published "Entwickelungs Geschichte" (1861 ), sums up the present state of our knowledge respecting the ossification of the periotic cartilage as follows (p. 320) :- "The ossification of the labyrinth does not appear to have been investigated since the time of Cassebohm ('Tract. de Aure Hum.,' I-Ial. et Magdeb., 17:14 and 1 725) and J. Fr. Meckel (' I-landb. d'Anat.' iv. p. 42, et seq.), which seems to be the |