388 



Dr. W. H. Gaskell. Structure, Function, [Feb. 23, 



two groups, viz., (1) those which arise from the mid-brain and hind- 

 brain, i.e., Ill, IV, Y m , VI, VII, all of which are at present, i.e., in 

 man, motor, but possess a degenerated posterior root and ganglion ; 

 and (2) those which arise from the med. oblongata, viz., IX, X (in 

 part), XI (in part), XII, Y s (in part), which are characterised not 

 by the loss of any component part, but by the scattering of the dif- 

 ferent components : a scattering which bears an intimate connexion 

 with the making good of the loss of the sensory elements of Group I. 



Finally, certain points connected with the question of the segmental 

 value of the cranial nerves other than those already discussed are 

 worthy of mention. The question of the segmental arrangement of 

 any nerves may as far as their distribution is concerned be considered 

 in a twofold light ; 1st, the evidence for any segmental arrangement 

 of somatic parts, as, for instance, of somatic muscles ; and 2nd, 

 evidence for any segmental arrangement of splanchnic parts, such as 

 visceral clefts and arches, and the visceral muscles formed from the 

 walls of such clefts. 



In order to compare cranial nerves with spinal nerves we must 

 compare structures of the same kind ; if the segmental arrangement 

 is based in the one case on the formation of myotomes, then we must 

 search for the corresponding myotomes in the other, if on visceral 

 arches, then we must argue upon the basis of visceral arch formation 

 throughout. Now, van Wijhe has pointed out that the cranial 

 muscles are derived from two groups of muscle segments, (1) a set 

 of myotomes corresponding to the segmented muscle plates through- 

 out the animal which form the muscles I have called non-ganglionated 

 somatic, and (2) a set of lateral plates of mesoblast, lining the walls 

 of the different visceral and branchial cavities, which give rise to the 

 group of muscles which I have called non-ganglionated splanchnic. 

 These latter muscles are unknown segmentally except in the head, 

 they probably form in the trunk, as will be shown immediately, the 

 diaphragm and trans versus abdominis muscles. 



Now van Wijhe, on the strength of his embryological researches, 

 divides the head of the Selachians into nine segments, with the follow- 

 ing arrangement of muscles and nerve supply. 



