CHAP. XXIV.] PRINCIPAL PARTS OF THE BRAIN. 481 



5. But when distinct limbs appear in higher Eeptiles, and when 

 in Birds and Mammals the movements of more or less similar limbs 

 become increasingly volitional and independent of one another, two 

 additional results might be expected to follow the primary dequssa- 

 tion of the Optic Nerves (howsoever this may have been deter- 

 mined) : (a) those other ' sensory ' channels whose impressions are 

 most concerned in the instigation of limb movements wou'd also 

 tend to decussate, because it would be very essential that more or less 

 unilateral Tactile and Auditory Impressions should be brought into 

 relation centrally with Visual Impressions coming from the same 

 side of the body; (&) coincidently with the establishment of a 

 decussation of the ' sensory ' channels and especially those of the 

 Tactile Sense and common sensibility in animals accustomed to 

 perform unilateral voluntary movements, we might expect that 

 there would be a tendency to the establishment of an answering 

 cross-relation between the ' motor ' channels of the Oerebro- Spinal 

 System. Thus, that half of the Brain which has first received the 

 instigating sensorial impressions would be enabled to send forth the 

 motor stimuli both for the reflex and for the volitional movements 

 of limbs on one side of the body. And, if there is to be no separate 

 decussation for the channels of reflex and volitional motor incita- 

 tions respectively, such crossings of motor channels as we find 

 existing in the Medulla of Man and many other Vertebrates (i.e. 

 at the ' decussation ' of the Pyramids) would seem to be the only 

 natural arrangement. 



6. This more complete cross arrangement seems only to be 

 perfected to the extent indicated, in higher Mammals and in 

 Man. 



relation exists in Cuttlefishes ; secondly, because there is no evidence 

 to show that the * motor ' channels undergo any similar decussation 

 (though this hypothesis assumes its existence) in the lower limbless 

 Vertebrates, in whom the decussation of the optic tracts becomes 

 initiated ; and, thirdly, because the experience of workers with the 

 microscope tends to show the ease with which adaptation of the 

 movements of the hands to meet the case of a reversal of the optic 

 image involving as it does, moreover, a reversal of upper and 

 lower, as well as of lateral parts is brought about. This latter 

 reason helps to show that no important anatomical changes would 

 be needed, as Marshall seems to suppose, to meet the case of a mere 

 reversal of the optic images. 



I I 



