160 



afterwards, by pressing outwards the lower part of the bony circle, 

 that incases this membrane, which produces its oblique direction 

 observable at birth. 



I have not been able to observe the rudiments of the bones of the 

 tympanum, before the beginning of the third month after concep- 

 tion. They are then in a cartilaginous state, and it is not till the 

 termination of this month, that their conversion into bone com- 

 mences ; but their ossification then advances so very rapidly, that, 

 about the end of the sixth month after conception, they have at- 

 tained their permanent dimensions, and only differ from the same 

 bones in the adult, in being a little less compact iii their texture. 



As these bones grow so much more rapidly than the cavity of the 

 tympanum, in which they are contained, until the sixth month after 

 conception, they appear to be closely enveloped by its lining mem- 

 brane, and are almost every where in contact with its parietes ; but, 

 after this period, the capacity of the cavity, compared with the mag- 

 nitude of the bones, becomes greater, and they are gradually re- 

 moved to a distance from its sides. 



There is a remarkable change observed to take place in respect 

 to the direction of the long leg of the incus and its relations to the 

 handle of the malleus, destined to enable these bones, which do not 

 increase in magnitude after the sixth month, to accommodate them- 

 selves to the progressive development of the cavity of the tympa- 

 num. Until the termination of the fourth month after conception, 

 the long leg of the iucus, and the handle of the malleus are parallel 

 and in contact. After this period, the long leg of the incus is gra- 

 duallly removed from the handle of the malleus; and that ex- 

 tremity of it, which is connected with the orbicularis, directed in- 

 wards; so that they form, when the cavity of the tympanum is fully 

 developed, a considerable angle with each other. I3y this change 



