THE VISUAL APPARATUS. 143 



CHAPTER VIII. 



SOMATIC AFFERENT DIVISION. THE VISUAL 

 APPARATUS. 



The general and special cutaneous systems serve for the recep- 

 tion of stimuli from the external world due to mechanical contact 

 or pressure or to vibrations in fluids. Another set of stimuli of 

 the greatest importance to the vertebrate animal, namely those 

 given by light, seem to common observation not to affect these 

 cutaneous organs. As a matter of fact, it has been shown that 

 light stimuli do affect the endings of general cutaneous nerves 

 in such a way as to produce characteristic reactions. If a light 

 of suitable intensity be allowed to fall upon a frog whose eyes 

 have been removed, the animal will turn its head toward the source 

 of light and jump toward it. If the frog's skin also be covered 

 from the light no such reaction takes place. Other amphibia, 

 some fishes and reptiles are influenced by light which falls on the 

 skin alone. In the case of ammocoetes, which lives buried in 

 the mud, the skin of the tail is more sensitive to light than any 

 other part of the body, including the eyes. This condition is 

 useful to the animal, since it burrows head foremost and the sen- 

 sitiveness of the tail ensures that it shall completely bury itself. 



While light must be reckoned as one of the most important 

 factors in the external world influencing the organism, it is evident 

 that the free nerve endings in the skin are not an adequate means 

 for the perception of light. It is beyond the province of this 

 work to inquire how light has influenced the organism so as to 

 produce an organ for its perception. It is, however, the business 

 of comparative morphology to consider whether the organism 

 responded to external influences by producing an entirely new 

 structure or whether the organ produced was a modification of 

 some structure already existing, and if the latter, what was its 

 probable mode or course of evolution. The attempt to answer 



