194 JOHN BEARD. 



nerve, if it does take place, occurs, by Marshall's admission, to a 

 greater extent than in the case of the other cranial nerves, a point 

 which is surely of some importance, there are other objections which 

 cannot, I think, be ignored. Marshall's views have also been contested 

 by Van Wijhe, for whose reasons the reader is referred to his oft- 

 quoted work on the nerves of the Elasmobranchii. 



In any discussion as to the nature of the third nerve the morpho- 

 logy of the head cavities is bound to have an important place. The 

 second or mandibular head cavity undoubtedly gives rise to the 

 superior oblique muscle (fig. 12, A.c >2 ). On this point I can fully 

 confirm Van Wijhe. 



This fact alone ought to dispose of the fourth nerve, which Marshall 

 considers as part of the nerve of the second segment — that is, as part 

 of the third nerve. The mandibular head cavity arises from the meso- 

 blast plate of the mandibular arch, according to Balfour, Marshall, and 

 Van Wijhe. It gives rise to the superior oblique muscle, therefore 

 the nerve of this muscle, the fourth nerve, must also belong to the 

 mandibular segment, as Van Wijhe insists. 



Further, if the first head cavity is morphologically of the same 

 nature as the second and third head cavities, then the third nerve, 

 which innervates the muscles derived from the third head cavity, is, 

 a priori, of the same nature as the fourth and sixth nerves. 



Marshall himself regards the sixth nerve as a ventral root of the 

 seventh nerve, 1 and says : " Concerning the actual value of the sixth 

 nerve, I see no reason to alter the opinion I previously expressed, that 

 the sixth nerve may be regarded as having the same relation to the 

 seventh that the anterior root of a spinal nerve has to its posterior 

 root." 



We have also seen reason to believe that the fourth is a ventral root 

 of the trigeminal nerve. And from all these facts we might fairly 

 regard the third as also a ventral root. 



But further, the dorsal root of no other cranial nerve, if we except 

 the third, innervates the structures arising out of a head cavity. The 

 dorsal roots, so far as they are motor, only innervate those structures 

 derived from the lateral muscle plates (Van Wijhe). 



According to Van Wijhe, the third nerve develops after the ciliary 

 ganglion, and hence could not be its dorsal root. The third, at any 

 rate, is an exceedingly fine nerve, and is much thinner than the ophthal- 

 1 'Segmental Value, &c.,' pp. 42— 44. 



