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Show 152 ON 'l'HE S'l'RUCTURE Ol!' 'l'HE SKUL!J. reason why certain incorreC't tatements are r p ated year after year in almost all handbool~s. . It is not the ca e that the external part of the pyramid of the petro al bone and the labyrinth ossify separately, nor is it true that os ification begins as a thin crust on the wall of the labyrinth; on the contrary, ossification commences in the whole thickn ss of the wall of the labyrinth; in sueh a mann r, how ver, that it appears externally sooner than internally, and the whol pyramid becomes ossified fi·om centres which n1ako their appearance fir 't npon the cartilaginous semicircular canals and the cochlea. The nnmber of these is, as has been rightly stated, tbr e-one on the first turn of the cochlea, and one on ach of the upp r and po::;tcrior semicircular canals, w bene , by d gree , the w holB pars peltrosa, together with the cartilaginous p ars 1nastoiclea, which is united with it, ossifies in a manner, tho d tail of which would not especially interest you. On the other hand, I do not agree in the statements that have been made a to th time at whjch this ossification arises. Neither in the third, nor i11 the fourth month, as is commonly tatod, is there a trace of ossification ; in fact, I have found the entire pyramid artilaginous in an embryo five inches long at the ighteenth w ck, or, in the middle of the fifth month. Only at the end of the fifth, and especially in the sixth month, do the o e u deposit commence, but these increase very rapidly. In the ixth n1onth, however, one meets with nothing but a b autiful r ticulated cartilage ossification, and, as yet, no incli ·ation of tru bon , which only arises, in the later month , from th perio t utn of the labyrinth and from the external perio t urn, whil t, c ntemp01aneously, the internal cartilage ossifi ation i reabsorb d and is replaced by a vascular true bone, which, by d gr es, be ·on1es finely spongy. The Modiolus and Lamina spiralis, in the ixth month, are still quite membranou , and only o ify at the end of fcetal life, without ever having been cartilaginou ." ~rhere is no doubt that th statement of Meckel, confirmed by IGHliker, that the periotic cartilage ossifies from three centres, is perfectly correct; there i no doubt, further, as Meckel, followed less clearly by Hallmann, has affirmed, that one of these centres gives rise to the future mastoid process; THE DEVELOPMEN'£ 01!, 'l'IIE ,IIUl\IAN SKULL. 153 but it is equally indubitable that Kerckringius' original statement is true, and may be readily verified in the dry skulls of fcetuses of the age be mentions. The beautiful series of human fcetuses presented by Mr. MacMurdo, in the Museum of the l~oyal College of Surgeons, enable one easily to reconcile the teachings of the older and the later observers, when taken in conjunction with the study of the same parts in wet preparations. Fig. 61, A, represents the periotic capsule of a human fcetus five and a quarter inches long. One ossification in the cartilage ( Op. 0.) is seen surrounding the fenestra rotunda (F. R.), and extending a little way upon the promontory. A second, very small, quadrate ossification (Pr. 0.) is situated at the outer end of the superior vertical semieircular canal, and apparently extends into the cartilaginous tegmen tympani. There is no other ossification in the cartilage than these two. As the upper part of the periotic mass in man answers to the front part, and as the lower part corresponds to the hind part of the same mass in the majority of the Vertebrata, I term the ossification on the superior vertical semicircular canal the pro-otic bone, that on the cochlea the opisthotic bone. In son1e dry fcetal skulls of this age the opisthotic ossification only is seen, just as it is described by Kerckringius, who seems not to have observed the pro-otic ossification at this period. The pro-otic ossification rapidly extends, as Meckel states, over the superior vertical semicircular canal (see Fig. 59, A, p. 144), and reaching its posterior end, it includes the front and upper part of the posterior vertical canal; while, from the outer end of the anterior vertical canal, or the primitive centre, a 1nass of bone extends backwards in the periotic cartilage and, in the dry skull, appears conspicuously immediately behind the edge of the squamosal. (Pr. 0., Fig. 61, B.) This part of it is, iu fact, that one of the "tria ossicula" of which Kerckringius says, "pyriform a, acutiore sui parte, squamoso annectitur." The opisthotic ossification likewise extends backwards and, its hinder extremity becon1ing apparent in the dry skull behind |
