In 1881 Ibsen1 published the first description of the evagination of the periotic tissue into the otic capsule back of the stapes which Siebenmann2 in 1890 called the "fossula post fenestram." Ibsen saw this in 23 of 60 new-born children whose ears he studied. He said that there is a funnel-shaped evagination from the vestibule back of the stapes. In his figure 6 he showed projecting into it a diverticulum of the utriculus which he called the "canalis membranaceus caecus." In a few cases he could insert a human hair into the opening from the vestibule to the middle ear. Ibsen credited Cassebohm3 with the first description of this structure. From a careful review of Cassebohm's works, such credit does not seem to be due him. While his figure 13 showed the region posterior to the stapes, he did not picture the fossula. In his text he mentioned the entrance
BAST TH. DEVELOPMENT OF THE OTIC CAPSULE: IV. THE FOSSULA POST FENESTRAM. Arch Otolaryngol. 1938;27(4):402–412. doi:10.1001/archotol.1938.00650030413002
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