THE INTERNAL EAR. 



865 



and are called the inner and outer rods of Corti. They are placed on the basilar 

 membrane, at some little distance from each other, but are inclined toward each 

 other, so as to meet at their opposite extremities, and form a series of arches 

 roofing over a minute tunnel, the tunnel of Corti, between them and the basilar 

 membrane, which ascends spirally through the whole length of the cochlea. 



The inner rods, some 6000 in number, are more numerous than the outer 

 ones, and rest on the basilar membrane, close to the labium tympanicum ; they 

 project obliquely upward and outward, and terminate above in expanded 

 extremities which resemble in shape the upper end of the ulna, with its sigmoid 

 cavity, coronoid and olecranon processes. On the outer side of the rod, in 

 the angle formed between it and the basilar membrane, is a nucleated mass of 

 protoplasm ; while on the inner side is a row of epithelial cells (inner hair-cells), 

 surmounted by a brush of fine, stiff, hairlike processes. On the inner side of these 

 cells are two or three rows of columnar supporting cells, which are continuous with 

 the cubical cells lining the sulcus spiralis internus. 



The outer rods, numbering about 4000, also rest by a broad foot on the 

 basilar membrane ; they incline upward and inward, and their upper extremity 

 resembles the head and bill of a swan : the back of the head fitting into the con- 

 cavity the analogue of the sigmoid cavity of one or more of the internal rods, 

 and the bill projecting outward as a phalangeal process of the membrana reticu- 

 laris, presently to be described. 



In the head of these outer rods is an oval portion, where the fibres of which 

 the rod appears to be composed are deficient, and which stains more deeply with 

 carmine than the rest of the rod. At the base of the rod, on its internal side 

 that is to say, in the angle formed by the rod with the basilar membrane is a 

 similar protoplasmic mass to that found on the outer side of the base of the inner 

 rod ; these masses of protoplasm are probably the undifferentiated portions of the 

 cells from which the rods are developed. External to the outer rod are three or 



Fio.465. Longitudinal section of the cochlea, showing the relations of the scalse, the ganglion spirale, etc. 

 S. V. Scala vestibuli. S. T. Scala tympani. S. M. Scala media. L. S. Ligamentum spirale. G. S. Ganglion 

 spirale. 



four successive rows of epithelial cells, more elongated than those found on the 

 internal side of the inner rod. but, like them, furnished with minute hairs or cilia. 

 These are termed the outer hair-cells, in contradistinction to the inner hair-cells 

 above referred to. There are about 12,000 outer hair-cells, and about 8500 inner 

 hair-cells. 



The hair-cells are somewhat oval in shape ; their free extremities are on a 

 level with the heads of Corti's rods, and from each some twenty fine hairlets 



55 



