Ill 



imsuorossfiil, Init it has artuallv rotardod knowlodo'c l)y 

 divortino- the energies of invest ig-ators into an nnprofitahle 

 channeL The work on the eranial nerves of the frog's 

 tadpole, published in 1895 hy 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 sjiinal nerves at all, and 

 another only partly so. 



One of the first re^nlts 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 Ihat 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." AVe 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 structuial relations into five fibre 

 systems, three of which are sensory and two motor. Each 

 svstem is delimited by a uniformity of peripheral ter- 

 mination and a special and characteristic origin in llie 

 biain, 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 (b)ne in the Plaice, to 

 work out the whole course of lln- nerves bv nunms ot serial 



