234 THE EVOLUTION OF CONTINUITY 



cross over to the right. In both cases the remainder continue 

 down the cord anteriorly, but at intervals give off fibres 

 to run down the opposite side, and this goes on till finally 

 decussation or crossing is complete. Thus it is that the 

 right side of the brain controls the movements of the left 

 side of the body, and the severance of the right pyramid 

 would cause left-sided paralysis ; the reverse occurring if 

 the left pyramid were severed. 



In the last figures, it will be noticed that the lines of 

 attracting force which call for deviation into the straight 

 pass through the near side of the brain to the side of the 

 body whose fins are called on to act strongly ; and there 

 is some reason for supposing that the habitual crossing of 



Fig. 73. — The main directions taken by light-waves passing 

 to the brain of the developing primitive Fish, o, o, objects 

 straight ahead to all purposes ; o', o', objects situated well 

 to the right or left ; b, the brain ; I, I, direction of light-waves. 



force-lines from before backwards was a factor in deter- 

 mining crossed nerve-tracts. But the brain is not an exposed 

 organ, and force-waves have to pass to the brain along 

 what are called sensory paths or nerve-tracts. No matter 

 where the source of the waves is situated, these require a 

 sensory path to conduct them to the brain cortex, from 

 whence motor impulses pass down to the fins, and this has 

 to be understood with respect to preceding figures. 



While the bilaterally symmetrical brain was taking 

 shape, sensory conducting paths leading to it would also 

 be developing, and these may, for our purpose, be divided 

 into three main classes : (1) those conducting waves from 

 sources situated somewhere in front of the brain ; (2) from 



