Tt] 
unsuccessful, but it has actually retarded knowledge by 
diverting the energies of investigators into an unprofitable 
channel. The work on the cranial nerves of the frog’s 
tadpole, published in 1895 by Strong, distinctly proved 
this, for he showed that, for example, there were three 
systems of sensory fibres in the cranial nerves of the larval 
frog, one of which must be considered characteristic of the 
head and not represented in the spinal nerves at all, and 
another only partly so. 
One of the first results of Strong’s work was to show 
that the old system of classifying the cranial nerves of 
Fishes into ten formal pairs was essentially unsatisfactory, 
and that attention should be concentrated rather on the 
various definite systems of nerve fibres characterised by 
their structure, central origin and peripheral distribution, 
than on those heterogeneous collections of nerve rami 
‘known as the “cranial nerves.” We must, however, in 
the meantime adhere to the old classification, until suffi- 
cient work has been carried out on the new lines to justify 
a revision of the cranial nerves, and to ensure for its 
findings some permanent value. 
The new theory of the cranial nerves is known as the 
“component theory.” It takes advantage of the fact that 
the fibres forming them, and omitting the olfactory and 
optic nerves and the sympathetic, which present problems 
of an altogether special nature, fall by reason of their 
functions and certain structural relations into five fibre 
systems, three of which are sensory and two motor. Hach 
system is delimited by a uniformity of peripheral ter- 
mination and a special and characteristic origin in the 
brain, and each system may appear in a variable number 
of cranial nerves as a component of those nerves. It is 
therefore indispensable, as we have done in the Plaice, to 
work out the whole course of the nerves by means of serial 
