THE INTERNAL EAR IN VERTEBRATES 589 



amphibia, where it is related to one of the experimental auditory 

 organs mentioned above. In reptiles and birds it again sinks 

 into insignificance ; in monotremes and possibly other mammals * 

 it appears for a moment in the embryo ; in the adult it has gone. 



Another interesting organ that disappears (at least function- 

 ally) in the mammals is the lagcna. This chamber with its 

 otolith organ is first separated off from the sacculus among the 

 sharks. It is a conspicuous object in the labyrinth of bony fish. 

 In amphibia and most reptiles it still holds its own against the 

 encroachment of the growing pars basilaris which intervenes 

 between it and the sacculus. In birds it has become a mere 

 terminal appendage of the now preponderant pars basilaris. It 

 is still present as a sense organ in adult monotremes but in 

 other mammals it persists merely as the non-nervous tip of the 

 cochlear canal — a functionless vestige. 



Other reptilian characters may be recognised in peculiarities 

 of the perilymph scalae and will be referred to again later. 



Stripped of these surviving relics, the mammalian cochlea 

 very closely resembles that of man. Differences occur in the 

 length of the cochlear canal and in its mode of coiling 2 but in all 

 essentials there is great uniformity and this is nowhere more 

 apparent than in the detailed structure of the sense organ — the 

 organ of Corti. 3 



This organ, which is absolutely characteristic of the ear of 

 mammals, has an extremely elaborate and definite construction 

 into which it is needless to enter now. It must suffice to em- 

 phasise certain essential peculiarities. 



The cells that compose this sense organ have an absolutely 

 regular disposition. The sensory hair cells are set in parallel 

 rows from end to end of the cochlea. 



All the elements — the sensory hair cells, the supporting 

 cells, the " Pillars of Corti " — increase regularly in both number 

 and size from the base of the cochlea to the apex. A similar 

 increase is noticeable in the size of the tectorial membrane that 

 floats above the sense organ and in the breadth of the basilar 

 membrane and therefore in the length of the transverse cords 

 of which it is composed. 



1 Alexander,/^ Denkschr., Bd. VI. Th. 2, 1904; Stutz, Morph. Jahrb. 44, 

 1912. 



* Gray, The Labyrinth of Mammals, vol. i. 1907, 22. 



3 Kolmer, Arch. mikr. A fiat. 70, 1907, p. 695, and 74, 1909, p. 259. 



