SYMPATHETIC SYSTEM 183 



the wall of the neural tube opposite the ganglion, and immediately to the mesial 

 side of its entering dorsal root (fig. 174). The group of cells therefore belongs to 

 the lateral column. The descending root appears later, and is derived from a mass 

 of neuroblasts lying in close relationship to the oculo-motor and trochlear nuclei 

 in the floor of the mid-brain. The sensory root arises from a large ganglion, a 

 derivative (in part at least) of the ganglion-crest (see below). It is situated 

 opposite the pontine flexure, and in the fourth week has become connected by 

 a single dorsal root with the neural tube, while peripherally it is already 

 provided with three primary branches ophthalmic, maxillary, and mandibular 

 (figs. 173, 175). The ganglion in an early phase is said by Giglio-Tos to be 

 connected with the surface ectoderm (placode). The ganglion becomes the 

 Gasserian ganglion. The various subsidiary ganglia (ciliary, Meckel's, otic, and 

 sub-maxillary) arise like sympathetic ganglia (see below) in connection with the 

 several branches of the nerve. 



In some lower forms the trigeminal shows traces of a composite character, and it has hence been 

 suggested that it represents the union of more than one segmental nerve. In a human embryo 

 of the seventeenth day Giglio-Tos has described a complicated origin for the Gasserian ganglion. 

 He believes, in the first place, that the rudiment is primarily connected with the mes- 

 encephalon, and that the nerve becomes displaced backwards. He recognises three neural 

 ' proganglia,' three epibranchial ' proganglia ' connected with surface placodes, and three 

 ' pronerves ' '. P. cellular strands between the neural and lateral ganglia. This complex mass fuses 

 into the single ganglion generally described as the rudiment of the dorsal root. 



From the foregoing account it will be seen that ectodermic placodes have been described in 

 connexion with all the cerebral nerve-ganglia in the human embryo ; it must be left an open 

 question, however, whether the ganglia which are related to these placodes have not an origin 

 from the surface ectoderm. 



The trochlear is springs from a tract of neuroblasts situated in the isthmus. 

 They occupy both the mesial and lateral portions of the basal lamina (His, 1904). 

 The fibres take a dorsal course in the reticular zone to the roof of the isthmus 

 (future valve of Vieussens), where they cross and, again emerging, pass round the 

 wall of the mid-brain to their muscle (superior oblique of the eye). 



The trochlearis presents the special and puzzling peculiarities first, that though a ventral 

 nerve it emerges from the dorsal aspect of the neural tube ; and second, that it crosses with its 

 fellow to form a dorsal commissure. These facts have not received a satisfying ontogenetic 

 explanation. 1 



The oculomotorius springs from a ventral and mesial tract of neuroblasts 

 in the mesencephalon, and is generally pronounced a mesial or somatic nerve. 

 The root passes off from the ventral aspect of the neural tube just in the cephalic 

 bend, and passes backwards in its course to the rudiment of the ocular muscles, 

 which it supplies. 



It has no ganglion-rudiment, but in some forms the nerve-path is beset with nuclei, which have 

 been regarded as such, and the ciliary ganglion has been sometimes considered as arising by an 

 aggregation of the outwandering elements on this nerve instead of from the Gasserian ganglion 

 of the fifth. 



DEVELOPMENT OF THE SYMPATHETIC SYSTEM 



The problem of the origin of the sympathetic is only a part of the larger one of 

 the origin of the peripheral nerves. There are two chief views regarding the source 

 of the ganglion-cells. According to the one (Remak, Kolliker, Paterson), they are 

 mesodermic ; according to the other (Balfour, His Sr., His Jr., and many others), 

 they are ectodermic in origin. The first view is based (Paterson) on the inde- 

 pendent appearance in birds and mammals of an unsegmented strand of mesoderm 



1 For a general discussion of the question, see Fiirbringer, ' Morphologische Streitfragen,' Morpho- 

 logisches Jahrbuch, xxx., 1902. 



