334 THE BODY AT WORK 



contact with external objects than they are elsewhere. The 

 efficiency of the sense-organs of the head nose, eye, and ear 

 depended upon their remaining compact. Progress in animal 

 life, as we understand it the rise from lower to higher forms 

 has depended upon increasing integration of the body and 

 co-ordination of its functions. The nervous system is the 

 agent which has accomplished this unification. Each step in 

 advance has depended upon the provision of more nerve- 

 tissue for the lacing together of the various parts. We have 

 seen already (p. 329) how intimate is the union of receptors 

 and effectors of every kind via the spinal cord and brain. 

 The overwhelming predominance in the direction of action of 

 the nose, the eye, and the ear has led to the accumulation in 

 their vicinity of the ever-increasing grey matter. The cerebral 

 hemispheres, or " great brain," are pouched outgrowths from 

 the first pair of ganglia directed towards the olfactory pits. 

 The original eyes bore a similar relation to the second pair of 

 ganglia the epithet " original " implying that the eyes which 

 we now use are not the organs with which our prevertebrate 

 ancestors saw. First one of the original eyes disappeared, and 

 then the other. The vestige of the second is still to be seen in 

 the " pineal body " which is found on the dorsal side of the 

 brain of every vertebrate animal in a mammal deeply hidden 

 in the cleft between the cerebrum and cerebellum. In place of 

 the pineal eyes two other sense-organs have specialized as eyes. 

 They are constructed on a different plan, being, to put it shortly, 

 pineal eyes turned inside out ; for whereas in the pineal eyes, as 

 in most of the eyes of invertebrate animals, the rods and cones, 

 which are the cells of the retina sensitive to light, are directed 

 forwards towards the lens, the rods and cones of our permanent 

 eyes are directed away from the source of light. This change 

 has made it possible to provide more abundantly for their 

 nutrition, and hence a greater power of discriminating separate 

 points in space and of distinguishing colours is conferred upon 

 them. The substitution of other sense-organs for the original 

 eyes has complicated the pictures which are presented to us by 

 a brain in its successive stages of growth ; but it does not pre- 

 vent us from recognizing the general plan. Probably the 

 secondary eyes, like their predecessors, belonged to a pre-oral 

 segment. The sense-organs of a segment behind the mouth 



