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A TEXTBOOK OF PHYSIOLOGY 



membrane. The upper spiral section, or staircase, is called the scala 

 vestibuli, the lower one the scala tympani. The scala vestibuli 

 begins at the fenestra ovalis, and ascends to the top of the 

 whorl. Here it connects by way of an opening in the lamina spiralis 

 the helicotrema with the scala tympani, which descends to the 

 fenestra rotunda. A membrane that of Re issuer cuts off a part 

 of the scala vestibuli, the scala media, membranous cochlea, or cochlear 

 canal. This is bounded by the basilar membrane below, and ends 

 blindly at the top of the cochlea. Like all the membranous structures, 

 it is filled with endolymph; the other two staircases, being bony, are 

 filled with perilymph. In section, the scala media is triangular. 



FIG. 378. SECTION (Low POWER) THROUGH PART OF COCHLEA, SHOWING 

 MEMBRANOUS CANAL OP THE COCHLEA. (After Retzius.) 



A, Basilar membrane; B, rods of Corti; C, hair cells; D, fibres of auditory nerve; 

 E, tectorial membrane; F, membrane separating off membranous canal of 

 cochlea; G, wall of cochlea. 



Within this membranous cochlea, or scala media, is contained 

 the organ of Corti, the receptor mechanism for hearing. The 

 organ of Corti is set upon the upper aspect, or in respect to its 

 position in the head, the anterior aspect of the basilar membrane. 

 It runs almost the whole length of the cochlear canal, and consists 

 of a set of elongated, cylindrical, rod-like cells, the outer and 

 inner rods fixed by an expanded base to the basement membrane. 

 These meet at their upper ends like the beams of a sloping roof, the 

 outer cells fitting into a socket in the inner cells. The inner rod may 

 be compared in shape to the ulna, and the outer to a swan's neck, 

 head, and beak. From the head of each rod a flattened process 

 projects, the process of the inner cell overlapping that of the outer, 

 and giving, when seen from above, an appearance like the keyboard 

 of a piano. The inner rods are about half as numerous again as the 

 outer, two outer rods thus fitting into three inner ones. It is con- 

 jectured that there are about 6,000 inner and 4,000 outer rods. Intern- 



