110 
of the cord and continues cephalad through the 4th and 
Srd 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.—TuHE CrantaL 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 Gaskell 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 
