256 TRANSACTIONS LIVERPOOL BIOLOGICAL SOCIETY. 



sections. Microscopic work either on the brain or the 

 peripheral nerves only is inadequate, and dissection, as a 

 means of research, has but a very doubtful value. The 

 only fish which has been thoroughly investigated according 

 to the component theory is Menidia — in an important 

 work published recently by C. J. Herrick, who truly 

 remarks : " Until each component can be isolated and 

 treated as a morphological unit, and then unravelled in its 

 peripheral courses through the various nerve roots and 

 rami — until this is possible, no further great advances 

 in cranial nerve morphology can be looked for even among 

 the lower vertebrates, still less in man." 



The five systems of fibres which variously compose 

 the cranial nerves of the Plaice are as follows : — 



1 . General Cutaneous or Somatic Afferent System. — 

 These fibres, which undoubtedly correspond to the 

 cutaneous fibres of the spinal nerves, are derived from 

 continuations of the dorsal horns of the spinal cord, which 

 form two longitudinal bundles in the medulla known as 

 the spinal vth tracts. These fibres in the Plaice leave the 

 brain by the roots of two cranial nerves only — the vth and 

 the xth. In the former case their ganglion is the Gasserian 

 ganglion, in the latter the jugular ganglion. The 

 cutaneous fibres in the facial nerve are distinctly derived 

 from those of the fifth. The fibres of this system are 

 distributed generally to the skin, and do not end in any 

 specialised dermal sense organs. Hypertrophy of this 

 system produces a corresponding hypertrophy of its centre 

 in the central nervous system, as witness the remarkable 

 lobes at the anterior extremity of the spinal cord of 

 Prionotus (Morrill). 



2. Somatic Efferent System.— Represented by the 

 heavily myelinated eye muscle nerves (iii., iv. and vi.). 

 This system is of course largely present in the so-called 



