AUDITORY ORGANS. 529 



as the fenestra ovalis, the bone is deficient in the Amphibia, 

 Sauropsida and Mammalia, and its place is taken by a mem- 

 brane ; while in Mammalia and Sauropsida a second opening, 

 the fenestra rotunda, is also present. 



These two fenestrae appear early, but whether they are 

 formed by an absorption of the cartilage, or by the nonchondri- 

 fication 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 addi- 

 tional 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 integu- 

 ment " 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. 



B. III. 34 



