THE ORGANS OF SPECIAL SENSE. 



597 



(2) Medially the inner fibrous layer is fused with the perichondrium of a shelf -like 

 process of cartilage which later ossifies to form the bony spiral lamina (Fig. 

 514). By these two partitions the cochlear perilymphatic space is separated 

 into two spiral compartments which communicate only at the apex of the 

 cochlea. The larger of these compartments, the scala vestibuli, communicates 

 with the perilymphatic space around .the utricle and saccule. The wall separat- 

 ing the scala vestibuli and cochlear duct becomes thinned out to form the 



Scala tympan j 



Cartilage 



Cochlear duct 



Cartilage 



Scala vestibuli 

 (gelatinous tissue) 



Cochlear duct L / J 



Cochlear (spiral) ganglion . L J. ;$ s& 



Coch. nerve to organ of Corti . — 1 



Scala tympani . J 



Cochlear nerve . — 1 



Fibrous con. tis. ■ — a 

 Connective tissue 



Scala vestibuli 

 Perichondrium 



Vestibular membrane 

 Lat. wall of coch. duct 



Organ of Corti 



Fig. 514.— Section through the developing cochlea of a 90 mm. cat embryo. Bottcher. 



vestibular membrane (of Reissner). The smaller compartment, the scala 

 tympani, remains separated from the cavity of the middle ear by a thin mem- 

 brane which closes the fenestra cochlea (rotunda). In the wall between the 

 scala tympani and the cochlear duct the organ of Corti develops (see below). 

 A membrane, similar to that closing the fenestra cochleae, occurs between the 

 cavity of the middle ear and the utricle, closing the fenestra vestibuli (ovalis). 

 As alluded to above, the organ of Corti develops from the wall of the cochlear 



