NEAL: NERVOUS SYSTEM IN SQUALUS ACANTHIAS. 233 



which develops in precisely the same way as do ventral (medullary) 

 spinal nerves, possesses a much elongated motor nucleus in the ven- 

 tral horn of the medulla, and innervates pre-otic (possibly also in the 

 embryo post-otic) musculature (muse, rectus posterior). These facts 

 seem significant in dealing with the question of the primitive metameric 

 relations of this nerve. 



At a stage when the embryo has a length of 17 mm. (78-80 somites) 

 the ramus opthalmicus superficialis V (Plate 4, Fig. 20; compare Fig. 21) 

 appears as a fibrillar nerve with peripheral nuclei extending from the 

 Gasseriau ganglion just dorsal to the point of exit of the fibres of the 

 r. ophth. profundus V, and passing anteriorly close to the ectoderm 

 below the r. ophthalmicus superficialis VII. The relations of these two 

 ophthalmic nerves are therefore such that they have usually been re- 

 garded as of the same morphological value, i. e. as rami cutauei dorsales 

 of nerves V and VII respectively. Yet an interesting relation of proto- 

 plasmic processes from the r. ophth. sup. V with the myotome of the 

 second somite, such as is represented in Plate 8, Figure 60, has been 

 called to my attention by Miss Piatt. Since at this stage of develop- 

 ment the fibres of the trochlearis have not appeared, the inference would 

 seem w^arranted that motor impulses may have primitively passed to this 

 myotome (muse, obliquus superior) through the fibres of the r. ophth. 

 sup. V. Such a supposition, however, is greatly diminished in force, 

 and in my opinion rendered untenable, by the fact that in embryos of 

 19 mm. — therefore before the fibres of the trochlearis are in connection 

 with the m. obliquus superior — the r. ophth. sup. V shows no longer 

 connection with this muscle (Figure K). The fibres of the anterior root 

 (portio minor) of the trigeminus nerve may now be traced from their 

 origin through the Gasserian ganglion into the mandibular ai'ch, where 

 they give otf fibres both to the muscles of the arch and to the skin of its 

 anterior and lateral surface. The fibres appear in large part motor. 

 Since this is the only motor branch of the V, it would follow that the 

 posterior root (portio major) includes chiefly, if not entirely, sensor 

 fibres. It would moreover follow that encephalomere III is chiefly, if 

 not wholly, connected with motor fibres, which may be traced forward to 

 a considerable distance in it to the neuroblasts in the lateral horn, with 

 which they are in connection, while encephalomere IV has chiefly sensory 

 fibres in connection with it. Mitrophanow's ('93, p. 178) evidence is, 

 however, considerably at variance w'ith that just stated. He finds that 

 in an embryo Squalus of 18 mm. ''la racine du nerf trijumeau est large 



