OUR BEST FEATURES 



on the plate, the retina immediately proceeds to 



" televisualize " its images through the myriads of 



nerve fibers covering its surface. After passing 



through many microscopic relay and "booster" 



stations the disturbances pass along a vast cable 



route known as the optic nerve. Instantly 



reaching their first main destination, the visual 



cortex of the brain, the visual currents now incite 



millions of repercussions which are flashed and 



reflashed to the relay stations and great central 



systems in many parts of the brain, where they 



set off many triggers that control the secretory 



activities of glands or the contractility of muscle 



fibers. 



The foregoing description holds in a general way 



for the eye of vertebrates of all grades from fish to 



man, the vertebrate eye, except in degenerate 



forms, being extraordinarily constant in its main 



features. Hence the basic features of the human 



eye date back to the beginnings of the vertebrates 



and are fully exemplified in such primitive forms 



as the sharks (Fig. 99) . The six eye muscles of the 



human eye (Fig. 98) likewise date back at least 



to the shark-like stage. Here again the shark is 



vastly nearer to man in the essential features of its 



191 



