CRANIAL NERVES. 



8S5 



the cervical segmental nerves, there is also, arising separately from the cord, the spinal 

 accessory nerve, which, although reckoned with the cranial nerves, is in greatest part of spinal 

 origin. This belongs to a group, more developed amongst the cranial nerves, which is 

 designated by Gaskell splanchnic non-ganalionated efferent, but as it is distributed to skeletal 

 muscles, it is better termed lateral somatic efferent. The somatic efferent fibres in this region 

 are therefore again divided into a menial set, which arise from the cells of the fore part and 

 inner side of the anterior horn of the grey matter of the cord, and a lateral xrt derived from 

 cells placed at the outer part of the anterior horn. It is uncertain whether the latter fibres 

 are represented in the lower parts of the cord, but Gaskell places the phrenic nerve and the 

 branches supplying the transversalis abdominis muscle in this group ; if present, they also run 

 in the anterior roots of the spinal nerves. 



CRANIAL NERVES. The attempt to determine the morphological position of the cranial 

 nerves is beset with difficulties, and any classification that is based on the facts at present 

 known with regard to their development, connections, and functions can only be regarded as 

 tentative. Assuming that they include the representatives of a series of segmental nerves, the 

 arrangement is profoundly modified, on the one hand by the separation of the constituent 

 portions of some of these nerves so as to form independent trunks, and on the other by the 

 coalescence of elements belonging to different segments in a single stem. In many cases also 

 parts of the typical segmental nerves are suppressed, either not being formed in the embryo, 

 or disappearing more or less complete!}' in the course of development. So great indeed is the 



STATIONARY GANGUON 



SOMATIC AFFERENT 

 TERAL SOM. EFFT. 

 MESIAL SOM. CFFT. 



ANCHNIC AFFERENr 



Fig. 240. SCHEME OP THE HYPOTHETICAL CONSTRUCTION OP A SEGMKNTAL NERVE, BASKD MAINLY ON 



GASKELL'S CLASSIFICATION OF THE CONSTITUENT FIBKES. (G. D. T.) 



change that it may be affirmed that no cranial nerve, according to the usual enumeration, 

 corresponds exactly to a single complete segmental nerve, such as is met with in the dorsal 

 region for example. 



In considering the cranial nerves from this point of view the first two pairs are generally 

 set aside as occupying an exceptional position, but evidence is not wanting that these have in 

 some respects a similar origin to other sensory nerves. According to van Wijhe and Hoffmann 

 the olfactory nerves are developed at the anterior margin of the neuropore, at a spot where 

 the floor of the neural groove or canal passes into the epidermis. This would correspond to 

 the foremost part of the ganglion-groove of His, the forerunner of the neural or ganglion 

 crest, at the place where it passes from one side to the other, following the margin of the 

 medullary plate, and is not irreconcileable with the statement of Milnes Marshall that the 

 olfactory nerves are derived from the foremost part of the neural crest. The optic nerve, 

 including the retina, is developed as a hollow outgrowth of the neural tube, and Hoffmann 

 states that in the elasmobranch the dorsal portions of the other cranial nerves are formed in 

 the same way, their ganglion-rudiments at first enclosing a cavity which is continuous with 

 the medullary canal, so that the retina would correspond to a specially developed root-ganglion, 

 and the optic nerve to a dorsal root. The relations of these nerves to the head segments, 

 however, have not been ascertained, and at all events for the present they cannot be grouped 

 amongst the segmental nerves. In the following observations therefore only the ten nerves 

 from the third to ohe twelfth inclusive are dealt with. 



Constitution of the cranial nerves. Four of these nerves, namely, the twelfth, the 

 sixth, the fourth, and the third, arise from large-celled nuclei forming a group which appears 



