THE CENTRAL NERVOUS SYSTEM 86 1 



There is evidence that the posterior corpora quadrigemina and 

 the mesial geniculate body form an inferior relay on the route 

 between the fibres of the auditory nerve and the temporal cortex. 

 There are indications that within the auditory area so-called 

 ' musical centres ' exist that is, an orderly arrangement of the 

 cell-bodies of the neurons that have to do with the perception of 

 pitch, so that a limited lesion may cause deafness to notes of 

 a particular pitch when it is situated on one part of the area, 

 and deafness to notes of a different pitch when it is situated else- 

 where (Larionow). 



Centre for Smell. As to the position of the centre for smell, 

 direct experiment on animals cannot teach us much, for if the 

 outward tokens of visual and auditory sensations are dubious 

 and fluctuating, still more is this the case with the signs of 

 sensations of smell. A further source of fallacy is the fact that 



FIG. 365. SENSORY AREAS OF MESIAL SURFACE OF HUMAN BRAIN. 

 The front of the brain is towards the right. 



other sensations than those of smell are caused by stimulation 

 of the mucous membrane of the nose. Substances like ammonia, 

 for example, affect entirely the endings of the trigeminus, which 

 is the nerve of common sensation for the nostrils. Pathological 

 and clinical evidence would be of great value, but it is as yet 

 scanty, and of itself indecisive. Some cases of epilepsy have 

 been reported in which the attack was heralded by smells for 

 which there was no objective cause. At autopsy the uncinate 

 gyrus was found diseased. So far as it goes, such evidence 

 supports the view derived from the anatomical connections 

 of the olfactory tracts, that the centre for smell is situated 

 in the uncinate gyrus on the mesial aspect of the temporal lobe, 

 for the olfactory tract may be traced into this region. In animals 

 with a very acute sense of smell, this gyrus is magnified into a 



