154 



or middle, the cavity of the tympanum properly so called, and its 

 bones. 



At the termination of the third month after conception, we can 

 first remark the rudiments of the external auditory tube. Two thirds 

 of the circle of its root are^ at this period, formed by a small bone, not 

 thicker than a hair, curved, so as to describe a semicircle ; and the 

 remainder, by the roots of the mastoid and zygomatic processes. Its 

 greater edge is convex, but the smaller is grooved, for the reception 

 of the circumference of the membrane of the tympanum. Its thick- 

 ness increases, and it gradually forms a larger portion of a cir- 

 cle ; at first connected by a membrane to the parts, on which it 

 lies. About the fifth month after conception, its bony union 

 commences at its extremities ; and, soon after, throughout its 

 entire extent. Bone is now gradually deposited, along its concave 

 or lesser edge ; in such a way as to form the nonarticular part of 

 the glenoid cavity, and afterwards the auditory process. In this 

 manner, the lower and anterior part of the auditory tube is completed ; 

 and, by the gradual development of the zygomatic and mastoid 

 processes, the upper and back parts are formed. 



The diameter of the bony circle, which I have described as con- 

 stituting the root of the auditory tube, is equal to the diameter of the 

 tube itself, at any period of life ; hence, it is only in length, tliat this 

 tube encreases after birtli. The period, at which the auditory tube 

 gains its ultimate length, varies considerably ; but, I believe, it will 

 generally be found, about the 14th year, a? long as at any after period; 

 unless we except the portion, which is formed by the root of the 

 mastoid process, and this is not perfected so soon. 



I have been induced, from observing- a close connexion between 

 the growth of this tube and the organs of mastication, to consider the 

 bony ring, first formed, in which the groove for the membrane of the 



