THE ORGANS OP THE OUTER GERM-LAYER. 459' 



ophthalmicus of the trigeminus and, tis motor root, the oculo- 

 motorius. The second segment is supplied by the remaining part 

 of the trigeminus and the trochlearis, the latter having a ventral 

 origin. The dorsal roots of , the third (and fourth?) segments are 

 represented by the acustico-facialis, the ventral roots by the 

 abducens. The fifth segment possesses only the exclusively sensory 

 glossopharyngeus, which arises from the neural ridge. The segments 

 from the sixth to the ninth inclusive are innervated by the vagus and. 

 the hypoglossus, the former of which represents a series of dorsal 

 roots, the latter a series of ventral ones. 



According to Wijhe's account, notwithstanding the general agree- 

 ment, there still exists a considerable difference between the innervation 

 of the cephalic segments and that of the trunh-segments. For in the 

 head the ventral, motor roots (oculomotorius, trochlearis, abducens,. 

 hypoglossus) supply only a part of the musculature — the eye- 

 muscles and certain muscles that run from the skull to the pectoral 

 girdle ; that is to say, muscles which, as has already been stated, are 

 developed out of the cephalic segments. Other groups of muscles, 

 which arise from the lateral plates of the head, are innervated by 

 ,the trigeminus and facialis, which have a dorsal origin. Thus the 

 dorsal roots of the nerves in the head would be distinguished from 

 those in the trunk by the important fact that they contain motor as 

 well as sensory fibres. Bell's law would consequently possess a very 

 limited application for the head-region of Vertebrates, and would 

 have to be replaced by the following law, formulated by Wijhe :— 



" The dorsal roots of the head-nerves are not exclusively sensory,, 

 but also innervate the muscles that arise from the lateral plates, not,, 

 however, those from the primitive segments (somites)." 



" The ventral roots are motor, but innervate only the muscles of 

 the primitive segments (somites), hot those of the lateral plates." 



In view of this fundamental difference, I desire to express a doubt 

 whether there are not after all enclosed in the facialis and trigeminus 

 parts which are established as ventral roots, but have hitherto been 

 overlooked, as in the beginning all the ventral roots in the brain 

 (see Balfour) were overlooked. 



According to Eabl the nerves of the posterior part of the head only — 

 glossopharyngeus, vagns, accessorius, and hypoglossus — can be compared with 

 the type of spinal nerves ; the nerves of the anterior part of the head, on the 

 contrary, — the olfactorius, opticus, trigeminus, together with those of the eye- 

 musoles and the acustico-facialis, — belong in a separate category, just as the 

 rour most anterior head-segments do. 



