THE EAR. 



76 



on each side of the posterior cerebral vesicle, just above the 

 end of the second visceral cleft. The mouth of the involution 

 soon closes, and a shut sac results. The sac enlarges, and, by 

 a remarkable series of changes, its upper part becomes (ordi- 

 narily) converted into three semicircular canals — the anterior 

 and posterior vertical, and the eaternal or horizontal canals 

 of the membranous labyrinth. The body of the sac remains, 

 lor the most part, as the vestibule ; but a csecal process, which 

 eventually becomes shut off from the vestibule, is given oft 

 downward and inward, toward the base of the skull, and is 

 the rudiment of the scala media of the cochlea. This may be 

 called the membranous cochlea. 



In the anomalous vertebrate, Amphioxus, no ear has yet 

 been discovered. The Hag [Myxine) has only one, and in the 

 Lampreys (Petromyzon) there are only two, semicircular ca- 

 nals ; but, in fishes in general, all three are developed, and it 

 is a question whether the cochlea is not also represented. 



In fishes, the periotic cartilage and its ossifications enclo.se 

 this membranous labyrinth, externally, and present no merely 

 membranous gaps, or fenestrcB, toward the first visceral cleft, 

 or the space which represents it. 



But in higher Vertebrata {Amphibia, 8auropsida, Mam- 

 m.alia), in which the membranous labyrinth is always enclosed 

 within a complete bony periotic capsule, the outer wall cf 

 this capsule invariably remains unossified over one or two 

 small oval areas, which consequently appear like windows wH h 

 membranous panes, and are termed ihe fenestra ovalis and the 

 fenestra rotunda. 



The fenestra ovalis is situated in that part of the periotic 

 mass which bounds the chamber containing the membranous 

 vestibule externally ; and it is always found that, when both the 

 proOtic and the opisthotic bones exist, they contribute nearly 

 equal shares to the formation of its boundaries. In fact, the 

 fenestra ovalis is situated in the line of junction of these two 

 bones. The fenestra rotunda, on the other hand, is below 

 i\ie fenestra ovalis, and lies altogether in the opisthotic. It 

 forms part of the outer wall of the cavity in which the mem- 

 branous cochlea is lodged. 



In the Sauropsida and Mammalia, this membranous coch- 

 lea, become flattened and bandlike, and its communication 

 with the vestibule obliterated, is lodged in a conical cavity, in 

 such a manner as to divide that cavity into two portions, 

 called scalce, which only communicate at their apices. The 

 base of the one scala, called scala vestibuli, opens into the 



