254 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



of the cord and continues cephalad through the 4th and 

 3rd ventricles to the anterior end of the optic lobes," 

 where it passes into the brain tissues. It is not a single 

 fibre, but a collection of axis cylinders, and is therefore a 

 fibre tract. Some of the fibres in the tract originate in 

 cells situated at the posterior extremity of the central 

 canal, and pass forwards to the tectum opticum. Others 

 originate in cells in the tectum opticum and pass back- 

 wards as far as the " posterior canal cells." The tract, 

 therefore, contains fibres coursing in two opposite direc- 

 tions. According to Sargent this unique apparatus forms 

 a " short circuit between the visual organs and the muscu- 

 lature, and has for its function the transmission of motor 

 reflexes arising from optical stimuli.'' It is most highly 

 developed in active fish, and is entirely absent in the blind 

 vertebrates of the cave fauna. 



2.— The Cranial Nerves (Fig. 23). 



In spite of the fact that the cranial nerves of Fishes 

 have been more or less investigated for about two and a 

 half centuries, it is only within the last few years that our 

 knowledge of them has assumed a form likely to be at all 

 lasting. Although these results were made possible as 

 long ago as 1811 by the enunciation of Bell's law, and 

 although this law was very ingeniously developed and 

 applied to Fishes in 1849 by Stannius, who has never 

 received due credit for his work, it was only in the 

 eighties that Graskell stated his " four root theory " of the 

 spinal nerves, which showed that there were represented 

 in each spinal nerve four kinds of fibres instead of the two 

 assumed by Bell's law. 



The attempt to strictly apply the four root theory to 

 the cranial nerves of lower vertebrates has not only been 



