COORDINATION 217 



must be noted. A camera is focussed by altering the distance 

 between the lens and the film, whereas the eye is focussed by 

 changing the curvature of the lens. Thus light waves passing 

 through the conjunctiva, cornea, and an opening (pupil) in a 

 regulating diaphragm (iris) reach the lens and are focussed on 

 the retina. The sensory stimulation of the rods and cones of the 

 retina thus brought about is transmitted by the optic nerve to 

 the brain. The brain itself interprets the nerve impulses and com- 

 poses — sees — the picture. How this is done, nobody knows. 

 (Fig. 151.) 



A broad survey of the sense organs of Vertebrates from the low- 

 est to the highest impresses one with the fact that, taken by and 

 large, the improvements, though considerable, are not so marked 

 as one might expect when the great development of the nervous 

 system, particularly the brain, is considered. The brain increases 

 enormously in volume and complexity from Fish to Man. In many 

 Fishes it seems to be little more than a slight modification of the 

 anterior end of the spinal cord, while in the Frog the brain and 

 cord weigh about the same. But the human brain weighs about 

 three pounds, nearly fifty times as much as the cord, and com- 

 prises many billions of nerve cells. So it would seem that we must 

 look to the general influence of the sensory stimuli themselves for 

 the underlying factors in the development of the brain during its 

 long evolutionary history — the brain, in turn, being enabled to 

 make more out of the same stimuli and create in Man the higher 

 mental life with all that it implies. It is, indeed, an appalling 

 thought that all human mental states are represented by a few 

 thimblefuls of cells constituting the cerebral cortex. 



