468 



HISTOLOGY 



The mesenchyma immediately surrounding the entire system of ducts 

 becomes mucoid in appearance, and cavities lined with mesenchymal 

 epithelium are formed within it. They contain a tissue fluid called peri- 

 lymph. Around the semicircular ducts the perilymph spaces are so large 

 that the tissue between them is reduced to strands as shown in Fig. 473 ; 

 these are sometimes called ligaments. The perilymph spaces around the 

 semicircular ducts are irregularly arranged and communicate with one 

 another at various points; they connect also with the perilymph cavities 

 of the vestibule, which is the central part of the internal ear, from which 

 the semicircular, cocblear and endolymphatic ducts proceed outward. 

 All of these structures are surrounded by spaces, connecting with those of 

 the vestibule which enclose the sacculus and utriculus. At the distal 



Modiolus. 



Bone. 



Scala vestibuli. 



of the nervus 



acusticus. 



Meatus acusticus interims. 



FIG. 474. HORIZONTAL SECTION OF THE COCHLEA OF A KITTEN. X 8. 



The winding ductus cochlearis, x, crossed the plane of section five times. Above it in every case is the 

 scala vestibuli, and below it is the scata tymponi. 



end of the endolymphatic duct, the spaces communicate with those of 

 the cerebral arachnoid, and the perilymph mingles with cerebro-spinal 

 fluid. 



Around the cochlear duct the perilymph spaces form a single tube. 

 Starting from the vestibule, it ascends to the cupola, following the windings 

 of the cochlear duct, to which it is closely applied. It is known as the 

 scala vestibuli (i.e., "staircase of the vestibule," from which it passes out) 

 At the apex of the cochlea it turns and becomes the descending scala 

 tympani, which ends blindly at the base of the cochlea, close against 

 the wall of the tympanum. The two scalae bear a constant relation to the 

 coils of the cochlear duct. If the cochlea is so placed that its apex is 

 upward, the scala vestibuli is always found on the upper side of the 

 duct, and the scala tympani on the lower side, as shown in Fig. 474. In 

 the body, the apex of the cochlea is directed forward and outward. 



The temporal bone develops from the mesenchyma surrounding the 



