43G THE TYMPANIC CAVITY. 



the tympanic cavity causes in Mammalia the complete or nearly 

 complete obliteration of the cavity during foetal 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 Mam- 

 malia, and its place is taken by a membrane ; while in Mammalia 

 and Sauropsida a second opening, the fenestra rotunda, is also present. 



These two fenestra 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 b^ne, known in the Sauropsida and 

 Amphibia as the columella. The main part of the colnmella 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 

 uf 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 011 tlie 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 integu- 

 ment" while " the drum is formed by the Eustachian tube overlapping the 

 extremity of the meatns." Urbantschitscli 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 <~-ar 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 Husclike 

 (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. 



Tuilicata. The auditory organ of the Tunicata (fig. o(H>) is 



