SEGMENTAL BILATERAL SYMMETRY 237 



figure was the cause of the great decussation of the sensory 

 tracts occurring in the Fish and higher vertebrates in the 

 region of the medulla, and also of the spreading out of these 

 to reach the cortex of the brain ? Might it not also explain 

 the minor decussation at intervals all up the cord, as 

 suggested in Fig. 76 ? 



From this figure it is clear that a painful impression 

 at, say, R would be equivalent to attraction towards A, 

 the continuation of the line from R, and the proper motor 

 response would be an extra strong drive of the fins on the 



Fig. 77. — Diagram of the crossed motor tract which causes 

 the human eyes to move towards a source of attraction on 

 the right. 1, left external rectus ; 2, left internal rectus 

 muscle ; 3, right internal rectus ; 4, right external rectus ; 

 5, 6, nuclei of left and right 3rd nerves ; 7, 8, tracts from 6th 

 nerve to opposite 3rd nerve ; 9, 10, nuclei of left and right 6th 

 nerves ; 11, 12, left and right cortical centres. A motor 

 impulse from 11 would cross over to 10 and cause 2 and 4 to 

 contract, the eyes thus moving to the right where the attract- 

 ing object was situated. There is really double decussation 

 here. (After Swanzy.) 



same side as R ; the motor impulse coming down a crossed 

 motor tract. If R were a pleasant impression, the reverse 

 would be the case. 



With regard to sight sensations, in order that the eyes 

 turn towards the source of light a crossed motor tract would 

 also be necessary as the complement of the crossed sensory 

 optic tract, and the next figure shows how this occurs in 

 the human brain. 



Crossed motor tracts here produce the expected result. 

 But if we take the response of fin-movement to bring the 

 body towards the source of light attraction, it becomes 



