159 



when compared with the other parts which we have examined ; and 

 they seldom acquire their ultimate magnitude till after the adult pe- 

 riod of our life. 



About the seventh month, the caverns and bony fibres, which 

 exist in the upper and under part of the cavity of the tympanum, 

 begin to be developed. They have, however, a different appear- 

 ance now from what they exhibit in the adult ; resembling, at this 

 period, rather the cellular structure of the cancellated tissue of 

 bones, than any thing else. It is not till a considerable time after 

 birth, that they assume the same appearance which they present in 

 the after periods of life. The bony fibres, which cross the bottom 

 of the tympanum, have a regularly arranged appearance in adults, 

 which is wanting in earlier life. 



The cavity of the tympanum, two months after conception, is 

 scarcely to be observed. The membrane of the tympany lies then 

 in contact with the internal side of the cavity. In the third month 

 it is more obvious ; and in the fourth has attained a considerable 

 extent at its upper part, where the opening of the future mastoid 

 cells is observed. We remark a very considerable difference between 

 the rapidity of the growth of the upper and under part of this cavity. 

 The upper portion grows so much more rapidly than the under, 

 that, although the former has, at the fourth month, a very consi- 

 derable size, the latter, until the fifth, is scarcely observable. The 

 slow growth of the lower portion would appear to be connected with 

 the development of the eustachean tube ; for it does not commence 

 till the growth of the tube commences; and, like it, grows then so 

 rapidly, that, at birth, it has acquired almost its permanent dimen- 

 sions. It is also the slow growth of the lower part of the tympanum, 

 which causes the horizontal direction of its membrane during the 

 earlier months of uterogestation ; and it is its gradual development 



