374 NORMAL HISTOLOGY. 



the free border of the lamina spiralis ossea to the outer wall of the cochlea, 

 where it is connected to an inward bulging of the periosteum and subperios- 

 teal tissue, called the spiral ligament. The lower of the two tubes thus 

 formed is the scala tympani and communicates, in the macerated skull, with 

 the tympanum through the round window. The upper, tube is subdivided 

 into two compartments by an exceedingly delicate partition, known as Reiss- 

 ner's membrane, which extends from the upper surface of the osseous lamina 

 near its outer end, obliquely upwards and outwards, to the external wall of 

 the cochlea. The compartment above this membrane is the scala vestibuli 

 and communicates with the perilymphatic space of the vestibule. The scalse 

 tympani and vestibuli communicate only at the apex of the cochlea through 

 the helicotrema. They contain perilymph and are brought into relation with 



Corti's membrane 



Ganglion spirale 

 Organ of Corti 



Scala restibull 



pan! 



Basilar membrane 



Ganglion spirale / \ \ ~~~~ Cochlear nerve In internal auditory canal 



Modiolus 



FIG. 422. Section of human cochlea passing through axis of raodiolus. X 12. 



the subarachnoid space through the aquaeductus cochleae. They are lined 

 by a delicate fibrous periosteum, usually covered on the surface which is in 

 contact with the enclosed perilymph by a single layer of endothelial plates. 

 In some localities, however, as on the tympanic surface of the basilar mem- 

 brane, the lining cells retain their primitive mesodermic character and never 

 become fully differentiated into endothelium. 



The third compartment, the ductus cochlearis, is triangular on cross- 

 section (Fig. 423), except at its ends, and bounded by Reissner's membrane 

 above, by the basilar membrane and a part of the osseous spiral lamina 

 below, and by the outer wall of the bony cochlea externally. Save for the 

 narrow channel, the canalis reuniens, by which it communicates with the 

 saccule, the cochlear duct is a closed tube and contains endolymph. It 

 begins below as a blind extremity, the c<zcum vestibulare, lodged within the 

 recessus cochlearis of the vestibule and, after making two and three-quarter 

 turns through the cochlea, ends above at the cupola of the cochlea in a 

 second blind extremity, the c&cum aipidare, which is attached to the cupola 

 and forms a part of the boundary of the helicotrema. 



Architecture and Structure of the Cochlear Duct. Reissner's membrane or 

 membrana vestibularis, the delicate partition separating the cochlear duct from the scala 

 vestibuli, begins on the upper surface of the lamina spiralis, about .2 mm. medial to 

 the free edge of the bony shelf, and extends at an angle of from 40-45 with the lamina 



