No. I.] • THE VERTEBRATE EAR. 83 



The essential elements of the organ of Corti are, then, — 



The hair cells — Percipient elements. 



The nerve cells and fibres — Transmitting elements. 



The supporting cells — Isolating and supporting elements. 



These three include the only structures that are necessary 

 for the performance of the physiological function of the cochlea, 

 notwithstanding previous physiological speculations with respect 

 to the necessity of a damping apparatus and of a vibrating 

 membrane composed of separable but fixed cords, because they 

 are the only stritctiires present zvhich can possibly have to do tvith 

 the auditory function, and they also represent the striLcttcres pres- 

 ent in the type organ, from zvhich they have descended. 



Concerning the nervous elements in the mammalian organ of 

 Corti, there is still much uncertainty as to the exact distribution, 

 the number, and terminations of the fibres of the cochlear nerve 

 entering the organ through the limbus spiralis. There is sub- 

 stantial agreement, however, as to the main facts of nerve supply 

 so far as course and general disposition of the nerve fibres of 

 what may be called the direct system. 



We know enough about the matter to entitle us to enter into 

 comparisons with the lower forms, more especially those which 

 have been investigated up to date, and whose organ of Corti 

 presents as many rows of hair cells as are found in man. In 

 those cases in which the rows are not numerically as strongly 

 marked as in man, the cochlear organ appears to have lagged 

 behind in its development. It is true some forms have a greater 

 number of spiral turns to the cochlea without at the same time 

 actually having larger cochlear organs, i.e. increased number of 

 percipient elements. For, manifestly, it is not sufficient in esti- 

 mating the number of hair cells in a row to simply count the 

 number of complete turns of the cochlear tube and reckon on 

 the basis of the number of cells counted in the spiral turn of 

 some one form chosen as type. For the length of the curves, 

 i.e. the openness or closeness of the spiral, involves the circum- 

 ference of the complete ring of the spiral, and hence the number 

 of cells contained in it. 



It is evident that greater isolation of the hair cells of a com- 

 pound sense organ, and thus of their percipient hairs, is ob- 

 tained by the elongation of a few rows of cells, with the scant 



