224 VISCERAL ANATOMY. 



or Labium Vestibulare, a lower, or Labium Tympanicum, with a furrow 

 between them, called the Sulctts Spiralis Internus. 



Membrane of Reissner,^ extends from the edge of the lamina spiralis to 

 the outer cochlear wall, forming the vestibular wall of the membranous 

 cochlea. Its outer insertion is the Angulus Vestibularis. 

 Membrana Basilaris,* is the tympanic wall of the membranous cochlea, 

 extending from the labium tympanicum of the lamina spiralis to the outer 

 cochlear wall, where its insertion is called the Ligamentum Spirale* or 

 Muscle of Todd and Bowman. On it rests the Organ of Corti* cov- 

 ered by the 



Membrana Tectoria* 1 extends parallel to the membrana basilaris from 

 the labium vestibulare to the outer cochlear wall, where, according to 

 some authorities, it is inserted, according to others it is free and rests 

 directly on the organ of Corti, perhaps to act as a damper to its 

 vibrations. 



Describe the Organ of Corti. It is a peculiar arrangement of cartilagi- 

 nous rods and epithelial hair cells, which is supposed to constitute the essential 

 organ of hearing. 9 It lies on the membrana basilaris 3 and is covered by the 

 membrana tectoria. 11 The 



Rods of Corti, are arranged in two rows, resting, by their pedestals, on the 

 p IG 1I2 membrana basilaris, and uniting with each other 



by their heads, so as to form an arched tunnel all 

 along the membrane. Number of rods in inner 

 row about 6,000, in outer row 4,500. Average 

 diameter of the rods, from ^-^^ to -y^^-p inch. 

 Head-plates, are attached to the heads of the inner 

 rods, for the reception of the rods of the opposite 

 row. 



Lamina Reticularis,^ a perforated delicate membrane which extends from 

 FIG. 113. the articulations of 



the rods outwards to 

 the external wall of 

 the cochlea. 



Auditory Cells,' 2 ' are 

 epithelial structures 

 covering the inner 



surfaces of the walls and the floor of the membranous cochlea. Those 

 which cover the inner rods by a single row number about 3,300, are nu- 

 cleated, covered with tufts of cilia, and called the Inner Hair-cells. On 

 the outer rods are 3 or 4 rows of similar cells, numbering about 1 8,000, 

 and named the Oiiter Hair-cells. 



