72 MORPHOLOGY OF THE ORGANS OF VERTEBRATES. 



CO, 



raa 



jus 



FIG. 76. Membranous labyrinth of thrush 

 ( Turdus), after Retzius, from Wiedersheim. 

 aa, anterior ampulla; af, eighth nerve; /, 

 posterior ampulla; ca, anterior canal; te, ex- 

 ternal canal ; cp, posterior canal ; de, ductus en- 

 dolymphaticus ; /, lagena ; mn, macula neglecta; 

 ms, macula sacculi; /, macula utriculi ; //, 

 papilla lagenae; raa, nerve to anterior ampulla ; 

 rap, to posterior ampulla ; rb, basilar nerve ; 

 rec, recessus utriculi; rl, nerve to lagena; r>/, 

 nerve to macula neglecta ; s, sacculus ; sc, septum 

 conciatum ; sp, posterior utricular sinus ; ss, 

 superior utricular sinus ; iv, tegmentum vasculo- 

 sum ; u, utriculus 



FIG. 77. Section through the cochlea of a 

 cat. Bone, shaded. C, organ of Corti ; G, spiral 

 ganglion; N, nerve; SM, ST, SV, scalae 

 media, tympani, and vestibuli ; R, Reissner's 

 membrane. 



cartilage, but in all except 

 the lower vertebrates the 

 cartilage is finally replaced 

 by bone. The inner walls 

 of these capsules follow 

 more or less closely the 

 contour of the membra- 

 nous labyrinth, thus con- 

 stituting the skeletal 

 labyrinth, between which 

 and the membranous por- 

 tions is a space filled with 

 the perilymphatic fluid. 

 The walls of these cap- 

 sules are perforated in- 

 ternally for the passage 

 of nerves, etc., while on 

 their lateral surfaces, in 

 all groups above amphibia, 

 are two openings, the 

 fenestra ovalis and the 

 fenestra rotunda (the lat- 

 ter crossed by mem- 

 branes), through which 

 sound waves pass to the 

 parts described. 



In the mammals the 

 skeletal labyrinth follows 

 very closely the membra- 

 nous portion, and in one 

 part these structures need 

 a further description. 

 That part called the la- 

 gena in the lower verte- 

 brates is greatly devel- 

 oped here, and is drawn 

 out and coiled in a spiral, 

 which is accompanied, 



