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in the adult, extremely hard and compact. In the fourth month 

 after conception, the pyramid is visible ; and, about the fifth, tlie 

 canal, which lodges the muscle of the stapes, is ossified. This 

 canal is, in the foetus, comparatively much nearer the stapes, than in 

 the adult. 



Until the middle of the fourth month after conception, neither 

 the eustachean tube, nor the canal for the reception of tlie internal 

 muscle of the malleus, are to be observed. The bony circle, which 

 forms the rudiment of the auditory tube, crosses the place, where 

 they are to be developed ; and their first appearance corresponds 

 with the period, when the circle is united by bone to the parts on 

 which it lies. About the end of the fifth month after conception, 

 their ossification commences. At first, there is no partition between 

 these canals ; but ossification, when it has begun, advances so ra- 

 pidly, that they are almost fully formed at birth. 



About the fourth month after conception we remark, at the upper 

 and back part of the cavity of the tympanum, an opening of con- 

 siderable size ; which, in the foetus at birth, leads into a cavity, 

 found between the lower part of the squamous portion of the tempo- 

 ral bone, and the upper part of the petrous portion. This hole and 

 cavity lodge the short leg of the incus, and are the rudiments of the 

 mastoid cells, and their opening into the cavity of the tympanum. 

 One cannot say that, at birth, there are any of the mastoid cells 

 formed. This opening, which I have mentioned as seated at the 

 upper and back part of the cavity of the tympanum, is of consider- 

 able size, and leads into a deep and wide fossa ; but this fossa is not 

 divided, as it will be in the adults, into a number of cells by bony 

 partitions ; nor does it lead downwards into any cells in the situa- 

 tion, in which the mastoid process is afterwards to be developed. 

 The mastoid cells are to be considered as very slow in growth, 



