222 STRUCTURE OF THE COCHLEA. [BOOK in. 



what seems an important fact, namely that the basilar membrane 

 especially that outer part of it which reaches from the foot of the 

 outer rods to the spiral ligament, increases in length from below 

 upwards, except at the very top. 



At the top of the spiral the organ of Corti becoming rapidly 

 less conspicuous conies suddenly to an end ; the inner and outer 

 hair-cells and rods of Corti suddenly stop, and with them the fibres 

 of the cochlear nerve stop also ; the blind end of the canalis coch- 

 learis is lined merely with epithelial cells of a simple character, 

 the continuation of the cells of the spiral groove, of the cells of 

 Claudius, and of the other cells lining the canal ; the vascular band 

 is continued a little way beyond the organ of Corti and then comes 

 to an end too, and by thus ending indicates its functional connec- 

 tion with that organ. 



At the other, lower extremity or beginning of the spiral, the 

 organ of Corti similarly ceases, and the blind end or " cup " beyond 

 the canalis reunions, is, like the extreme top, lined by a simple 

 epithelium. 



Vertebrates below mammals do not possess an organ of Corti 

 strictly so called ; in the rudimentary cochlea of birds and reptiles 

 a basilar membrane and hair-cells are present; but the rods of 

 Corti are absent. As we pass in review the features of the mem- 

 branous labyrinth in various vertebrates from the lowest upwards, 

 we find the several parts becoming more and more distinct from 

 each other; the saccule becomes more and more separate from 

 the utricle and the semicircular canals, and the cochlea, which is 

 at first a process of the saccule, becomes more and more distinct 

 from it. 



839. It may be worth while to call attention to the follow- 

 ing data concerning the cochlea of man which have been obtained 

 by careful partial measurements and calculations. 



Length of the Canalis Cochlearis 35 mm. 

 Length of the organ of Corti 3 3 '5 mm. 

 Kadial width of the Basilar Membrane 

 (measured from the entrance of the 

 nerve fibres to the spiral ligament) 

 In the Basal whorl of spiral -21 mm. 

 Middle '34 mm. 

 Topmost '36 mm. 

 Number of perforations for nerve fibres 4000. 

 Inner Hair-cells 3500. 

 Inner Rods of Corti 5600. 

 Outer 3850. 

 Outer Hair-cells (in 4 rows) 12000. 

 Fibres of the Basilar Mem- 

 brane 24000. 



