Chap, xii.] INTERNAL EAR. 469 



the hyoid and mandibular arches (the hyomandibular 

 cleft) becoming useless as an organ of respiration has 

 been seized upon by the ear ; in Mammals, parts of the 

 mandibular arch that lies in front, and of the hyoid 

 arch that lies behind the cleft, are adapted to the use of 

 one and the same organ. 



Internal ear. This, which is the essential 

 portion, as those just described are the accessory 

 parts, of the organ of hearing, consists primarily of 

 the so-called membranous labyrinth, formed by the 

 sacculus, utriculus, and semicircular canals ; as we 

 ascend the scale we find a bony labyrinth fashioned 

 around this membranous one ; the space between them 

 contains lymph, and is known as the perilymphatic 

 space or cavity, while the fluid within the membranous 

 labyrinth is the endolymph. 



The simplest stage obtains in the Myxinoid round- 

 mouths, where there is only a single semicircular 

 canal, at the base of which there is, on either side, a 

 swelling or ampulla. In this, as in the underlying 

 vestibule, special auditory cells are developed, which 

 are supplied with filaments from the auditory nerve. 

 In the other division of the round-mouths, that is, in 

 the lampreys, there are two semicircular canals, and 

 the vestibule into which they open has two blind 

 diverticula arising from it, in each of which auditory 

 cells are developed. In all the remaining Vertebrata 

 there are three semicircular canals. 



In the Elasmobranch fishes, and in the Dipnoi, no 

 bony labyrinth is formed around the membranous, but 

 a promise, as it were, is offered by the excavation of 

 the cartilage around the labyrinth, in a form not un- 

 like the membranous internal ear ; in the bony fishes 

 the labyrinth is protected by bone, but there is no 

 proper bony labyrinth. A further point of advance is 

 to be observed in the Ganoidei and Teleostei, for in 

 them, as in Chimsera, the gradual insinking of the ear 



