v. 9 PARTS OF THE TRIGEMINAL 153 



muscles, and leaves the orbit for the skin of the snout. In higher 

 vertebrates the nasociliary nerve and the long ciliary, innervating the 

 eyeball, represent the profundus, while the rest of the ophthalmic 

 nerve of mammals corresponds to the superficial ophthalmic of 

 elasmobranchs. 



The relations of these nerves to other structures shows that the 

 so-called trigeminal nerve really includes the dorsal roots of two 

 segments combined. The profundus can be traced back in develop- 

 ment to a nerve that is obviously the dorsal root of the first somite, 

 of which the oculomotor nerve is the ventral root. Indeed it may be 

 noticed that the profundus and oculomotor partly join, at the ciliary 

 ganglion. The ramus ophthalmicus profundus and the oculomotor 

 nerve thus constitute the dorsal and ventral roots of the first or pre- 

 mandibular somite, whose corresponding branchial arch is presumably 

 the trabecula cranii (p. 147). The dorsal root does not show the full 

 structure of a branchial nerve, presumably because there is no gill-slit. 

 The profundus represents only the dorsal branch of a typical branchial 

 nerve, innervating the skin. 



The ramus ophthalmicus superficialis, and the maxillary and mandi- 

 bular branches together constitute the dorsal root of the second pro- 

 otic somite, whose ventral root is the trochlear nerve. The correspond- 

 ing gill arch is the mandibular (palato-pterygo-quadrate bar and 

 Meckel's cartilage), whose gill-slit we have suggested has been in- 

 corporated with the edge of the mouth. The trigeminal nerve shows 

 considerable similarity to a branchial nerve, its maxillary branch repre- 

 sents the pre-trematic and the mandibular the post-trematic ramus, 

 while the ophthalmicus superficialis is the dorsal branch to the skin. 

 There is no pharyngeal branch. An anomalous feature of the trige- 

 minal is that it contains sensory fibres whose cells of origin lie within 

 the brain (mesencephalic root). These fibres are probably proprio- 

 ceptors from the masticatory muscles and eye-muscles. The latter run 

 from the eye-muscle nerves to join the trigeminal. 



The dorsal root of the third segment, whose ventral root is the 

 abducens, includes the whole of the facial and also the auditory nerve. 

 The facial is a large mixed nerve in the dogfish. Its ophthalmic branch 

 runs to the snout, carrying mainly fibres for the organs of the lateral 

 line system that lie there. A large buccal branch supplies sensory 

 fibres to the mouth and a palatine branch joins the trigeminal. A small 

 prespiracular branch carries sensory fibres from the skin in front of the 

 spiracle, and the main portion of the nerve continues behind the 

 spiracle as the hyomandibular nerve, dividing up into motor branches 



