SEA-FISHERIES LABORATORY. 255 



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 theoiy 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. Each 

 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 



