92 Dr. Dundas Grant [April 27, 



ears to these cortical ceutres, the internal ear itself, and the tympanic 

 mechanism by which the sonorous vibrations are received. 



A sensation of sound is appreciated by the brain, and the parts of 

 the cortex allocated for this function have been specified by experi- 

 ment and by observation of disease. By kind permission of Colonel 

 Mott the lecturer was able to show on the screen the photograph 

 of a brain in which the auditory cortical centres were destroyed by 

 disease on the two sides successively. 



The case, briefly stated, was that of a woman in whom, owing to 

 disease — blocking of certain arteries — the auditory centre of the left 

 side was destroyed. The result was not loss of hearing but loss of 

 the power of understanding words and of naming things. The 

 corresponding area on the other side gradually took on these functions 

 and she recovered the understanding for words ; she got married 

 and the birth of her first child was unfortunately followed by a 

 blocking of vessels on the right side ; the destruction of the two 

 auditory centres led to complete deafness and she ultimately died. 

 Colonel Mott was able to obtain possession of this instructive specimen, 

 and his description of it is now a "classic" in neurology. 



Certain tracts of nervous tissues can be traced from the auditory 

 nerve-endings in each internal ear running to both cortical centres. 

 This is why destruction of one centre alone is not sufficient to cause 

 deafness. 



The auditor'ij nerves themselves take their rise in certain sensory 

 nerve-cells in the cochlea of the internal ear^ and these cells are 

 bathed by a liquid which is set in motion by the stapes, the inner- 

 most of the chain of bones attached to the tympanic membrane, 

 whenever this membrane moves. The sensory nerve-cells are fur- 

 nished with certain hairs which are disturbed or displaced by each 

 vibration of the liquid (endolymph) in which they stand.* The 

 auditory cells rest on a membrane made up of transverse fibres 

 which are longest at the upper part of the cochlea and shortest at 

 the base. Helmholtz taught that the fibres corresponded to particular 

 tones, and that each entered into sympathetic vibration when the 

 appropriate tone was sounded, very much as when we utter a sound 

 in front of an open pianoforte with the dampers raised by the " loud " 

 pedal. 



The organs of hearing are exposed to injury from various 

 causes : — 



First, and very obviously, direct mechanical damage by bullets, 

 fragments of shells or othe/ missiles actually striking the hearing 

 passages, the middle or internal ear, or possibly the nervous strands 

 connecting the internal ear with the brain. The active structures in 



* In a work recently published by Sir Thomas Wrightson since this lecture 

 was delivered, the mode of this vibration is minutely studied, and his views 

 receive confirmation from Prof. Arthur Keith's anatomical investigatioES. 



