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THE ORIGIN OF VERTEBRATES 



peripheral organ, no marked difference occurs in the arrangement 

 of the nerves issuing from the trigeminal motor centre, no new 

 nerves are formed to supply the new muscles, but every motor nerve- 

 fibre and the motor cell from which it arises increases enormously in 

 size, and these giant nerve-fibres thus formed split into innumerable 

 filaments corresponding with the proliferation of the muscular elements. 

 The clue, then, to the origin of the suctorial apparatus and of the 

 nature of the original organs supplied by the trigeminal is afforded 

 in this case, as in all other similar inquiries, by the central nervous 

 system and its outgoing nerves. Here is always the citadel, the 

 fixed seat of government, here is ' headquarters,' from which the 

 answers to all our inquiries must originate. 



The Trigeminal Nerve of Ammoccetes. 



Striking is the answer. In Fig. 114, Miss Alcock has drawn the 

 distribution of the trigeminal nerve as traced by her through a series 



Diet. pslbf. 



Fig. 114. — Distribution op Trigeminal Nerve in Ammoccetes. 



ps. br., pseudo-branchial groove; met., nerve to lower lip, or metastomal nerve; /., 

 nerve to tongue ; tent., nerve to tentacles. The mandibular and internal maxil- 

 lary nerves are coloured red ; the purely sensory nerves to the external surface 

 are coloured black. 



of sections. It arises, as is well known, from two separate ganglia, of 

 which the foremost gives rise to a purely cutaneous nerve, the oph- 

 thalmic nerve, and the hindmost to three nerves, the most posterior 

 of which is purely cutaneous and passes tailwards over the ventral 

 branchial region, as shown in the figure ; the other two nerves, both 



