THE EAR 395 



spiralis. Thus there is a groove (the sulcus spiralis), the upper 

 and lower lips of this sulcus being called respectively the labium 

 vestibulare and tympanicum. From the latter the membrana 

 basilaris extends to the outer wall, along the latter attach- 

 ment forming the ligamentum spirale. Above the limbus 

 to the outer wall stretches another membrane, Reissner's. 

 The space below the osseous lamina and the membrana basilaris 

 is the scala tympani; above the membrane of Reissner is the 

 scala vestibuli; and that space bounded by the two membranes 

 and the outer wall of the cochlea is known as the scala media, 

 or canal of the cochlea, which ends at the apex of the cochlea 

 in a blind pointed extremity, and opens below into the saccule, 

 as described above. Between the two membranes mentioned 

 a third stretches across in the scala media to the outer wall. 

 This is called the membrane of Corti, or membrana tectoria. 

 Between the membrana basilaris and the last-named mem- 

 brane is a space which contains the organ of Corti. 



The organ of Corti lies on the basilar membrane. The central 

 part is composed of two rows of peculiarly shaped cells called 

 the rods of Corti, outer and inner. These rods meet above 

 by their extremities, and enclose an angular tunnel between 

 them and the basilar membrane, the zona arcuata. The inner 

 rods run close to the labium tympanicum, and along their 

 inner side is a series of epithelioid cells continuous with the 

 cubical epithelium of the sulcus spiralis. These present a 

 row of short, stiff hairs, forming a sort of brush. External 

 to the outer rods are several rows of similar cells. These are 

 called the outer and inner hair cells. 



The reticular lamina is a delicate structure composed of 

 small segments called phalanges arranged side by side and 

 separated by holes, through which the hairs of the outer hair 

 cells project. The whole organ thus described is covered by 

 the membrane of Corti (membrana tectoria). 



The arteries of the internal ear are the auditory branch of 

 the basilar, the stylomastoid branch of the posterior auricular, 

 and branches occasionally from the occipital. The first named 

 divides into a cochlear and a vestibular branch. 



The auditory nerve divides at the bottom of the internal 

 auditory meatus into a superior and an inferior branch. The 

 former divides into branches, which are distributed to the 

 utricle and to the ampullae of the superior and external semi- 



