152 ON THE STRUCTURE OF THE SKULL. 



reason why certain incorrect statements are repeated year after 

 year in almost all handbooks. It is not the case that the 

 external part of the pyramid of the petrosal bone and the 

 labyrinth ossify separately, nor is it true that ossification begins 

 as a thin crust on the Avail of the labyrinth ; on the contrary, 

 ossification commences in the whole thickness of the wall of the 

 labvrinth ; in such a manner, however, that it appears exter- 

 nally sooner than internally, and the whole pyramid becomes 

 ossified from centres which make their appearance first upon the 

 cartilaginous semicircular canals and the cochlea. The number 

 of these is, as has been rightly stated, three — one on the first 

 turn of the cochlea, and one on each of the upper and posterior 

 semicircular canals, whence, by degrees, the whole pars petrosa, 

 together with the cartilaginous pars mastoidea, which is united 

 with it, ossifies in a manner, the details of which would not 

 especially interest you. On the other hand, I do not agree 

 in the statements that have been made as to the time at which 

 this ossification arises. ^Neither in the third, nor in the fourth 

 month, as is commonly stated, is there a trace of ossification ; 

 in fact, I have found the entire pyramid cartilaginous in an 

 embryo five inches long at the eighteenth week, or, in the 

 middle of the fifth month. Only at the end of the fifth, and 

 especially in the sixth month, do the osseous deposits commence, 

 but these increase very rapidly. In the sixth month, however, 

 one meets with nothing: but a beautiful reticulated cartilage 

 ossification, and, as yet, no indication of true bone, which onlv 

 arises, in the later months, from the periosteum of the labyrinth 

 and from the external periosteum, whilst, contemporaneously, 

 the internal cartilage ossification is reabsorbed and is replaced 

 by a vascular true bone, which, by degrees, becomes finely 

 spongy. The Modiolus and Lamina spiralis, in the sixth month, 

 are still quite membranous, and only ossify at the end of fastal 

 life, without ever having been cartilaginous." 



There is no doubt that the statement of Meckel, confirmed 

 by Kolliker, that the periotic cartilage ossifies from three 

 centres, is perfectly correct ; there is no doubt, further, as 

 Meckel, followed less clearly by Hallmann, has affirmed, that 

 one of these centres gives rise to the future mastoid process ; 



