ABSTRACTS OF TECHNICAL ARTICLES 689 



further if, in response to a sound, the basilar membrane vibrates more 

 violently in one place than in another, the stimulation of the nervous 

 tissue is greatest where the vibration is most violent. 



2. The basilar membrane does vibrate in response to sound and does 

 so differently at different frequencies. It is easily shown by an ele- 

 mentary theory of mechanics that all bodies of whatever nature, 

 whether solids, diaphragms, membranes, rods or bodies of fluid, behave 

 in this fashion. Theoretically, it is possible to describe a body which 

 vibrates the same at all frequencies, but such a body is never found 

 experimentally. This leads to the conclusion that the basilar mem- 

 brane where the nerve terminals are situated is quite capable of per- 

 forming an analysis of a kind of sound. 



3. The vibration of the basilar membrane resulting from sound is 

 greatest at the proximal end for high frequencies and at the distal end 

 for low frequencies. In order to arive at this conclusion Helmholtz 

 depended on purely mechanical considerations, which for any one 

 familiar with this type of philosophy is fairly satisfactory. Histo- 

 logical examination of ears known to have lowered acuity in certain 

 frequency ranges have shown this to be the case. 



4. In the normal ear there is only one spot which vibrates sensibly 

 in response to one pure frequency in the cochlea. This thesis is quite 

 well established by measurements on masking of one pure tone by 

 another, in which case it is found that one sound masks another more 

 effectively when the frequency of the second is nearer the first. 



5. The only sensible functioning connections between nerve cells of 

 the spiral ganglion, either direct or indirect, through branching of the 

 peripheral axones at the organ of Corti, are confined to near neighbors. 

 This thesis is also established by the physical data on masking. 



6. The minimum detectable change of pitch corresponds to a shift 

 along the basilar membrane of the vibrating spot for a distance equal to 

 the space occupied by a definite number, approximately constant, of 

 ganglion or hair cells. 



With these points taken for granted it is possible to describe the 

 mechanism of hearing in its broader aspects and to calculate to an 

 approximation the actual position on the basilar membrane at which 

 different frequencies stimulate it and to calculate also the extent of 

 the stimulating spot for each frequency. 



Automatic Power Plants for Telephone Offices}^ R. L. Young and 



R. L. LuNSFORD. The nature of power requirements for telephone 

 offices is discussed, with emphasis on continuity of service. Auto- 



1" In abridged form in Elec. Engg., June, 1931. In complete form, Bell Tel. Sys. 

 Mo7iograph B-561, May, 1931 and complete with discussion in Trans. A. I. E. E., 

 October, 1931. 



