202 FOUNDATIONS OF BIOLOGY 



and SCLEROTIC COATS, the IRIS, and the VITREOUS HUMOR 

 all of which contribute to the make-up of the eye-ball. The 

 eye of Vertebrates is an optical apparatus which may be com- 

 pared roughly with a camera. Light waves which pass 

 through an outer transparent protective coating and an 

 opening (PUPIL) in a regulating diaphragm (iris) reach the 

 lens and are brought to a focus on the retina. The sensory 

 stimulation thus brought about is transmitted by the optic 

 or second cranial nerve to the brain. 



A broad survey of the sense organs of Vertebrates 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, and the 

 brain in particular, is considered. And so we must look 

 chiefly to the cumulative influence of the sensory stimuli 

 themselves for the underlying factor 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 im- 

 plies. 



