The meatus auditorius externus is formed at the region of a shallow depression where the closure of the first visceral cleft takes place. It is in part formed by the tissue surrounding this depression growing up in the form of a wall, and Moldenhauer believes that this is the whole process. Kölliker states however that the blind end of the meatus becomes actually pushed in towards the tympanic cavity.
The tympanic membrane is derived from the tissue which separates the meatus auditorius externus from the tympanic cavity. This tissue is obviously constituted of an hypoblastic epithelium on its inner aspect, an epiblastic epithelium on its outer aspect, and a layer of mesoblast between them, and these three layers give rise to the three layers of which this membrane is formed in the adult. During the greater part of fœtal life it is relatively very thick, and presents a structure bearing but little resemblance to that in the adult.
A proliferation of the connective tissue-cells in the vicinity of the tympanic cavity causes in Mammalia the complete or nearly complete obliteration of the cavity during fœtal life.
The tympanic cavity is bounded on its inner aspect by the osseous investment of the internal ear, but at one point, known as the fenestra ovalis, the bone is deficient in the Amphibia, Sauropsida and Mammalia, and its place is taken by a membrane; while in Mammalia and Sauropsida a second opening, the fenestra rotunda, is also present.
These two fenestræ appear early, but whether they are formed by an absorption of the cartilage, or by the nonchondrification of a small area, is not certainly known. The upper of the two, or fenestra ovalis, contains the base of a bone, known in the Sauropsida and Amphibia as the columella. The main part of the columella is formed of a stalk which is held by Parker to be derived from part of the skeleton of the visceral arches, but its nature is discussed in connection with the skeleton, while the base, forming the stapes, appears to be derived from the wall of the periotic cartilage.
In all Amphibia and Sauropsida with a tympanic cavity, the stalk of the columella extends to the tympanic membrane; its outer end becoming imbedded in this membrane, and serving to transmit the vibrations of the membrane to the fluid in the internal ear. In Mammalia there is a stapes not directly attached to the tympanic membrane by a stalk, and two additional auditory ossicles, derived from parts of the skeleton of the visceral arches, are placed between the stapes and the tympanic membrane. These ossicles are known as the malleus and incus, and the chain of the three ossicles replaces physiologically the single ossicle of the lower forms.
These ossicles are at first imbedded in the connective tissue in the neighbourhood of the tympanic cavity, but on the full development of this cavity, become apparently placed within it; though really enveloped in the mucous membrane lining it.
The fenestra ovalis is in immediate contiguity with the walls of the utricle, while the fenestra rotunda adjoins the scala tympani.
Hunt (No. [391]) holds, from his investigations on the embryology of the pig, that “the Eustachian tube is an involution of the pharyngeal mucous membrane;” and that “the meatus is an involution of the integument” while “the drum is formed by the Eustachian tube overlapping the extremity of the meatus.” Urbantschitsch also holds that the first visceral cleft has nothing to do with the formation of the tympanic cavity and Eustachian tube, and that these parts are derived from lateral outgrowths of the oral cavity.
The evolution of the accessory parts of the ear would be very difficult to explain on Darwinian principles if the views of Hunt and Urbantschitsch were correct; and the accepted doctrine, originally proposed by Huschke (No. [389]), according to which these structures have originated by a ‘change of function’ of the parts of the first visceral cleft, may fairly be held till more conclusive evidence has been brought against it than has yet been done.