526 



PRINCIPLES OF GENERAL PHYSIOLOGY 



and set it in vibration. A structure of this nature exists in the cochlea of the 

 internal ear of higher vertebrates and is known as the " basilar membrane.' 1 It is 

 true that it does not consist of separate strings, but as it only possesses tension in 

 a transverse direction and not longitudinally, it is only capable of periodic vibration 

 in the one direction. Its transverse measurement is of a regularly increasing magni- 

 tude from the base to the apex of the cochlea and the whole organ is coiled into 

 a spiral. The nerve fibres, by means of a complex structure, the organ of Corti (see 

 Fig. 166), are stimulated when that particular element of the basilar membrane 

 which resonates to a given note is set into vibration by it. This is, in brief, the 

 theory proposed by Helmholtz and further details may be found in his book 

 " Tonempfindungen" (1863). In the sixth edition, the description of the theory will 

 be found on p. 232. 



Other theories have been suggested, such as the " sound pattern" theory, in which 

 the basilar membrane is supposed to vibrate as a whole, but with " nodes," or 

 lines of rest, in different places according to the pitch of the note. None, however, 

 seems to agree with the general facts of the structure of the organ of Corti as 



Linibub 



Membrana 

 tectoria 



Outer hair 



Inner Blood Basilar 

 rod vessel membrane 



Outer Cells of 

 rod Deiters 



FIG. 166. ORGAN OF CORTI OF MAN. Magnified. 



(Retzius. Schafer's " Essentials of Histology," Fig. 635.) 



well as that of Helmholtz does. This theory has, moreover, recently received 

 a striking confirmation in the experiments of Yoshii (1909). Guinea pigs were 

 exposed to the sound of a particular note on an organ pipe or siren for thirty to 

 forty days in succession. Local degenerations were then found to have been 

 produced in the organ of Corti. These degenerations were in different places 

 according to the note made use of, and were transverse, not longitudinal. It is 

 true that the degeneration extended somewhat on both sides of the actual region 

 corresponding to the tone itself, but Helmholtz's theory of resonance would not 

 exclude the possibility of neighbouring portions of the membrane being, to some 

 extent, also set in vibration and it is clear that the nature of the experiments of 

 Yoshii could scarcely afford evidence as to the minimal intensity of sound necessary 

 to cause resonance of a very limited element of the membrane. Secondary changes 

 were also found, in the experiments quoted, in the nerve fibres and ganglion cells 

 belonging to the particular region of the organ of Corti affected by the sound ; none 

 in the tympanic membrane nor in the transmitting structures of the middle ear. 



The hair cells are supposed to act as transmitters of the vibrations of the 

 membrane, possibly after these vibrations have been magnified by the structures 

 forming the pillars of the arch of Corti, which rest on the membrane. 



The tympanic membrane is interesting physically. It is so formed, by its shape, 



