234 THE HUMAN BODY 



set of short stiff hairs, and constitute the inner hair-cells, which are 

 fixed below by a narrow apex to the basilar membrane; nerve-fibers 

 enter them. To the inner hair-cells succeed the rods of Corti (Co, 

 Fig. 73), which are represented much magnified in Fig. 74. These 

 rods are stiff and arranged side by side in two rows, leaned against 

 one another by their upper ends so as to cover in a tunnel; they are 

 known respectively as the inner and outer rods, the former being 

 nearer the lamina spiralis. Each has a somewhat dilated base, 

 firmly fixed to the basilar membrane; an expanded head where it 

 meets its fellow (the inner rod presenting there a concavity into 

 which the rounded head of the outer fits); and a slender shaft 

 uniting the two, slightly curved like an italic /. The inner rods arc 

 more slender and more numerous than the outer, the numbers be- 



** 



FIG. 74. The rods of Corti. A, a pair of rods separated from the rest; B, a bit 

 of the basilar membrane with several rods on it, showing how they cover in the 

 tunnel of Corti; i, inner, and e, outer rods; b, basilar membrane; r, reticular mem- 

 brane. 



ing about 6,000 and 4,500 respectively. Attached to the external 

 sides of the head of the outer rods is the reticular membrane (r, 

 Fig. 74), which is stiff and perforated by holes. External to the 

 outer rods come four rows of outer hair-cells, connected like the 

 inner row with nerve-fibers; their bristles project into the holes of 

 the reticular membrane. Beyond the outer hair-cells is ordinary 

 columnar epithelium, which passes gradually into cuboidal cells 

 lining most of the membranous cochlea. The upper lip of the 

 sulcus spiralis is uncovered by epithelium, and is known as the 

 linibus lamince spiralis; from it projects the tectorial membrane 

 (t, Fig. 73) which extends over the rods of Corti and the hair-cells. 

 Function of the Cochlea. We have already seen reason to be- 

 lieve that in the ear there is an apparatus adapted for sympathetic 

 resonance, by which we recognize different musical tone colors; the 



