THE ORGANS OF THE OUTER GERM-LAYER. 



501 



gelatinous tissue, a large space filled with perilymph, which is indicated 

 in the diagram, fig. 282, in black. Here and there, however, 

 connective-tissue cords remain running from one layer of connective 

 tissue to the other, and serving as bridges for the nerves and blood- 

 vessels which are distributed to the semicircular canals. 



Finally, a last alteration takes place in the cartilaginous envelope 



\ 



Fig. 282. Diagrammatic representation of the whole organ of hearing in Man, from WIEDKKSHEIM. 



Outer air: M, <!/, auricle; Mac, meatus auditorius externus ; 0, its wall; Mt, membrana 

 tympani. Middle car: Ct, Ct, cavum tympani ; O l , its wall; SAp, sound-conducting 

 apparatus, which i.s drawn as a simple rod-like body in place of the auditory ossicles ; the 

 place f corresponds to the stapedial plate, which closes the fenestra ovalis ; Tb, tuba 

 Eustachii ; Tb 1 , its opening- into the pharynx; 0", its wall. Inner car: the bony labyrinth 

 (KL, KL') for the most part cut away ; S, sacculus ; a, b, the two vertical membranous and 

 osseous semicircular canals ; S.e, D.e, saccus and ductus endolymphaticus, of which the 

 latter is divided at 2 into two arms ; Cp, cavum perilyinphaticum ; Cr, canalis reunions ; 

 Con, membranous cochlea, which produces at + the vestibular cceciun ; Con 1 , bony cochlea ; 

 Sv and St, scala vestibuli and scala tympini, which at * communicate with each other at the 

 cupula tenninalis (Ct) ; D.p, ductus perilymphaticus, which arises from the scala tympani 

 at (I and opens out at D.p 1 . The horizontal semicircular canal is not specially designated, 

 but is easily recognisable. 



by its becoming converted into bone-substance by endochondral 

 ossification. Thus the membranous semicircular canals are enclosed 

 in the bony semicircular canals (fig. 282 a and 1> KL], which are 

 enlarged reproductions of the former. 



Corresponding changes (iig. 282) are also accomplished in the 

 periphery of the utriculus and sacculus (#), and lead to the formation 

 of (1) a perilymphatic space (Cp), which is in communication with 



