x THE FORE-BRAIN 611 



lobe (Fig. 296) was destroyed on both sides in dogs, it produced 

 a disturbance of hearing which he termed, psychical deafness, its 

 characteristic being that although the animal hears, i.e. has 

 auditory sensations, it has lost the perceptions and memory of the 

 auditory images perceived in its previous life. This is a more 

 correct interpretation of the effects described by Ferrier as due to 

 destruction of the. upper temporal convolutions ; the monkey was 

 not deaf, for it reacted to a sudden sound, but it did not respond 

 to calls nor to friendly addresses. 



Munk's psychical deafness is a transient phenomenon, which 

 gradually disappears, so that after a few days the operated can 

 hardly be distinguished from the normal animal. But if the 

 whole of the temporal lobe is destroyed on both sides by sub- 

 sequent operations, the psychical deafness is transformed into 

 absolute and permanent deafness, which Munk terms cortical 

 deafness. 



Our experiments with Tamburini (1879), and particularly 

 those with Seppilli (1885), brought out new and interesting facts. 

 They proved that the auditory centre cannot be restricted to the 

 limits laid down by Ferrier, nor those assumed by Munk. It 

 spreads more or less beyond the confines of the temporal lobe : 

 above, towards the parietal and occipital region ; behind, towards 

 the gyrus hippocampi, and mesially, towards the cornu Ammonis. 



Unilateral extirpation of the auditory sphere causes bilateral 

 disturbance of hearing, principally in the ear of the opposite side. 

 When the effects of extirpation of the auditory sphere on one side, 

 e.g. the right, have subsided", and the opposite auditory sphere is 

 then destroyed, not only is auditory disturbance produced on the 

 right, but the deafness of the left ear which had disappeared 

 returns. This fact was unmistakable in six dogs under our own 

 observation. Here we have experimental proof that the cerebral- 

 ward paths that come from the cochlear nuclei undergo in- 

 complete decussation like the optic nerves ; and that neither the 

 crossed paths nor the direct are* related to distinct portions of the 

 auditory centres, but both spread more or less uniformly through- 

 out these centres. 



The effects of more or less extensive extirpations of the 

 auditory sphere consist in a more or less grave affection of hearing, 

 which never amounts to complete deafness. This auditory dis- 

 turbance is transitory and due to the shock of the operation ; as 

 it disappears, the signs of partial psychical deafness appear more 

 and more clearly, as seen by the animal's failure to appreciate the 

 value of sounds, noises, and calls, although it shows signs of hear- 

 ing them. 



Bilateral extirpation of the auditory centres produces more 

 serious effects, even when incomplete. At first the disturbance of 

 hearing may amount to total deafness ; but this soon becomes 



