482 THE FUNCTIONAL RELATIONS OF THE 



7. A cross arrangement of sensory channels would seem to be 

 less essential in the case of Taste and Smell than for either of the 

 other kinds of ingoing impressions, first, because the organs subser- 

 vient to these endowments are situated more in the middle line of 

 the body than either of the others; and, secondly, because impressions 

 of Taste and Smell are perhaps less immediately provocative than 

 those of other senses, of unilateral limb movements. The nerves 

 of Taste being, however, bound up with or forming part of two 

 nerves of common sensibility (the Fifth and the Glosso-pharyngeal) 

 they, as it were, follow the lead of the nerve trunks to which they 

 belong, and decussate with them. But in regard to Olfactory 

 channels, it is, as matter of fact, notable that they are the only 

 ones in which no decussation is known to occur, either in the lower 

 animals or in Man. The Olfactory Centres of the two hemispheres 

 are, however, very amply connected by means of commissural 

 fibres principally gathered together in, and for the most part con- 

 stituting, the ' anterior commissure.' 



Thus, in brief, the writer's view is this: That the cross relation 

 between the halves of the Brain and the body may have been 

 initiated in some Fishes in a quasi-accidental manner, and that 

 in the first stage of its existence it was, and still is, represented 

 only by the decussation of the Optic Tracts ; that in higher animals 

 possessed of well-formed limbs reflex and volitional movements of 

 those of one side are very often evoked in response to unilateral 

 sensory stimuli, so that in such creatures there would be a dis- 

 tinct advantage if other sensory channels, by decussating, were to 

 be brought into relation, at their central termini, with those of 

 the Yisual Sense ; finally, the same influences, whatever they may 

 be, which determine this additional sensory decussation, would 

 lead to an establishment of the equally necessary sequential de- 

 cussation of the motor channels for the limbs. The cross arrange- 

 ment of sensory and motor channels met with in Man and higher 

 Mammals is, therefore, to be regarded as an almost necessary 

 sequence, from the point of view of the evolution theory, of a 

 primary and perhaps quasi-accidental decussation of the Optic 

 Tracts in Fishes. 



