508 EMBRYOLOGY. 



on both sides of the closing membrane as depressions of greater or less 

 depth ; an inner one on the side toward the pharyngeal cavity, and 

 an outer one which is surrounded by ridges of the first and second 

 visceral arches. 



The inner depression, which is called canalis or sulcus tubo-tym- 

 panicus (pharyngo-tympanicus), is located, like the spiracle, between 

 trigeminus and acustico-facialis. It becomes the middle ear, and i> 

 enlarged by an evagination that is directed upward, outward, and 

 backward. The evagination inserts itself between the labyrinth and 

 the place of closure of the first visceral cleft, and takes the form of 

 a laterally compressed space, which is now to be distinguished as 

 tympanic cavity from the tubular remnant of the sulcus tympanicus, 

 or Eustachian tube. Its lumen is very small, especially in the case 

 of advanced embryos of Man and Mammals, its lateral and median 

 walls being almost in immediate contact. This results chiefly from 

 the fact that there is present beneath the epithelial lining of the 

 middle ear a richly developed gelatinous tissue. The latter still 

 encloses at this time structures, the auditory ossicles and the 

 chorda tympani, which later come to lie, as it were, free in the 

 tympanic cavity. 



The tympanic membrane also is now in a condition very unlike 

 that which it afterwards acquires. The history of its formation is 

 by no means so simple as was formerly supposed. For it is not 

 derived exclusively from the narrow closing membrane of the first 

 visceral cleft ; the neighboring parts of the first and second mem- 

 branous visceral arches also participate in its production. The 

 embryonic tympanic membrane is therefore at first a thick con- 

 nective-tissue plate, and encloses at its margins the auditory ossicles, 

 the tensor tympani, and the chorda tympani. The reduction in the 

 thickness of the tympanic membrane takes place at a late period, 

 simultaneously with an increasing enlargement of the tympanic 

 cavity. Both are brought about by shrinkage of the gelatinous 

 tissue, and by an accompanying growth of the mucous membrane 

 lining the tympanic cavity. Wherever the gelatinous tissue disapj N 

 the mucous membrane takes its place, inserting itself between the 

 individual ossicles and the chorda tymj ani. which thus come to 

 lie apparently free in the tympanic cavity. In reality, however, 

 they lie outside of it, for they continue to be clothed on all sides by 

 the growing mucous membrane, and are connected with tlit* wall of 

 the tympanic cavity by mans of folds of that membrane (malleus- 

 told, incus- fold, etc.), in much the same manner as the abdominal 



