SENSE ORGANS. 



The sacculus is always con- 

 nected with the ductus endolym- 

 phaticus, and it gives off behind 

 an outpushing known in the lower 

 vertebrates as the lagena. In the 

 mammals this lagena becomes 

 greatly developed, and forms the 

 scala media of the cochlea de- 

 scribed below. 



As long as the otic vesicle re- 

 mains a simple sac, it bears on its 

 surface a single patch of sensory 

 epithelium ; but with differentiation 

 of parts, the epithelium becomes cor- 

 respondingly divided into a num- 

 ber of maculae (the sensory cells 

 of which bear short sense hairs) 

 and cristae (provided with long 

 hairs). In the lampreys, where 

 there is no sacculus, there are but 

 three of these patches, a crista 

 in each ampulla and a macula in 

 the vesicle. In other forms there 

 are three cristae, and at least one 

 macula in the utriculus, two in the 

 sacculus, and one in the lagena. 



These parts, derived from the 

 ectoderm, form the membranous 

 labyrinth. It is filled with a fluid, 

 the endolymph, in which are otoliths 

 or particles of calcic carbonate, 

 sometimes of microscopic size, but 

 in the teleosts forming 'ear 

 stones' of considerable magnitude. 



The membranous labyrinths are 

 protected by the otic capsules de- 

 scribed in connection with the 

 skull. These are laid down in 



FIG. 74. Diagram of the mem- 

 branous labyrinth. A, anterior 

 canal; A A, anterior ampulla; 

 AE, external ampulla ; AP, 

 posterior ampulla ; C, utriculo- 

 saccular canal ; D, ductus endo- 

 lymphaticus; E, external (hori- 

 zontal) canal ; P, posterior canal ; 

 R, recessus utriculi ; S, saccus 

 endolymphaticus; SA, sacculus; 

 U, utriculus. 



FIG. 75. Ear of Myxine, after 

 Retzius. a, ampullar crista ; ap, 

 posterior ampulla ; c, semicircular 

 canal; ca, anterior ampulla; d, 

 ductus endolymphaticus ; m, ma- 

 cula; n, nerves; s, sacculus endo- 

 lymphaticus. 



