82 THE ANATOMY OF VEETEBRATED ANIMALS. 



catmls — the anterior and posterior vertical, and the external 

 or horizontal canals oi the membranous labynnth. The body 

 of the sac remains, for the most part, as the vestibule; but a 

 caecal process, which erentually becomes shut oiF from the 

 vestibule, is given off downwards and inwards, towards 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 anomaloiis vertebrate, Amphioxus, no ear has yet 

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

 the Lampreys (Petromyzon) there are only two, semicii-cular 

 <3anals ; but, in fishes in general, all three are developed, 

 and it is a question whether the cochlea is not also repre- 

 sented. 



In fishes, the periotic cartilage and its ossifications in- 

 close this membranous labyi-inth, externally, and present 

 no merely membranous gaps, or fenestrce. towards the first 

 visceral cleft, or the space which represents it. 



But in higher Vertebraia [Amphibia, Sauropsida, Mam- 

 malia), in which the membranous labyiinth is always in- 

 closed within a complete bony periotic capsule, the outer 

 wall of this capsule invariably remains unossified over one 

 or two small oval areffi, which consequently appear like 

 windows with membranous panes, and are termed the 

 fenestra ovalis and the fenestra rotunda. 



The fenestra ovalis is sitiiated 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 pro-otic and the opisthotic bones exist, they contribute 

 nearly equal shares to the formation of its boundaries. In 

 fact, the foiestra ovalis is situated in the line of junction of 

 these two bones. The fenestra rotunda, on the other hand, is 

 below thefenestra ovalis, and lies altogether in the opisthotic. 

 It forms part of the outer wall of the cavity in which the 

 membranous cochlea is lodged. 



In the Sauropsida and Mammalia, this membranous 

 cochlea, become flattened and bandlike, and its communi- 

 cation with the vestibule obliterated, is lodged in a conical 

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



