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the length of the chain of bones is accommodated to the increasing 

 breadth of the cavity of the tympanum, without undergoing any 

 alteration in their magnitude. 



The labyrinth, or second division of the auditory Cavities, consists 

 of the cochlea, vestibule, and semicircular canals. The tube, which 

 conducts the auditory nerve to the labyrinth, may also be here 

 considered. 



About the second month after conception, we can observe the 

 rudiments of the cochlea, vestibule, semicircular canals, and inter- 

 nal auditory tube, excavated in cartilage, though they are all 

 remarkably small ; but, before the conclusion of the third month, 

 their dimensions are considerable, and their form well defined. In 

 the course of the fourth month, ossification commences, generally, 

 first in the cochlea, and afterwards, successively in the vestibule, in- 

 ternal auditory tube, and semicircular canals ; those parts which are 

 nearer to the point of the petrous portion of the bone ossifying sooner 

 than those near the base. About the termination of the sixth 

 month, all the cavities of the labyrinth are formed of bone ; except 

 a small portion of the semicircular canals, corresponding to the 

 base of the petrous portion of the temporal bone. At birth their os- 

 sification is complete, and they all appear to be then as large as at 

 any future period of life. 



The substance of the petrous portion of the temporal bone, which 

 encases and surrounds these several cavities, does not grow so ra- 

 pidly as the cavities themselves ; hence it exhibits several peculiari- 

 ties in form, prior to birth, and for some years afterwards. During 

 VOL. xm. z 



