SARGENT : THE OPTIC REFLEX APPARATUS OF VERTEBRATES. 131 



Introduction. 



This research treats of a single functional unit in the central nervous 

 system of vertebrates, its origin, history and function. The structures 

 involved have been studied in many widely varying types, progressing 

 from the more primitive and simpler forms to the higher and uiore com- 

 plex. But the methods of comparative anatomy have not alone been 

 relied upon. The evidences of development and degeneration have also 

 been of value. An endeavor, too, has been made to attain that phylo- 

 genetic perspective which reveals necessity and environment at work in 

 producing and modifying organic structures. It is in the beginnings of 

 things that their secrets are laid bare. The subject of the present study 

 is no exception, for the more primitive organisms present a simplicity of 

 structure that makes possible an understanding of the mechanism of this 

 apparatus that could not otherwise be attained. The optic reflex apjmra- 

 tus is shown to be a primitive structure, probably traceable to invertebrate 

 ancestry, and is followed in its variations through the vertebrate series. 



Aided by physiological experiment, an attempt has been made to 

 interpret the structures described, in the belief that to the search for the 

 more evident phenomena should be added that for mechanisms and 

 causes. For structure autl function are correlative, as are cause and 

 effect, and our understanding of any organic structui-e is largely depend- 

 ent upon our ability to interpret it in functional terms. The present 

 study reveals the existence of a highly specialized nervous mechanism, 

 by which, as the result of optical stimuli, motor reactions are by reflex 

 activity brought abuut in the shortest jjossible time, thus accounting for 

 the ' fliglit reflex ' of the lower vertebrates and the quick reaction to an 

 ocular sense of danger in the higher animals. 



In the spring of 189'J, while studying the central nervous system of 

 teleosts, my attention was first attracted to a fibre or rod-like structure 

 lying in the lumen of the canalis centralis. This was first seen in 

 sagittal sections of the spinal cord of the trout (Salvelinus fontinalis). 

 It could easily be traced from section to section through the canal and 

 the brain ventricles. This rod had a very regular cylindrical form, took 

 certain stains sharply, and had a uniform diameter (jf 3 micra. Further 

 study revealed its presence in all species of vertebrates examined. 

 Though unmentioned in the text-books and more recent literature of the 

 nervous system, I found it referred to in the older papers as Eeissner's 

 fibre, and generally alluded to as an artifact. 



