THE SEGMENTAL VALUE OF THE CRANIAL NERVES. 129 



eye-inuscle nerves, remarking that any attempts to homologise them 

 with spinal nerves "encounter insuperable difficulties on account of 

 their peculiar origin, their absence of ganglia, and their exclusive 

 distribution to the muscles of a sensory apparatus, which are in no way 

 comparable with the muscles of the vertebrae." The remaining nerves, 

 however, Stannius deals with in a very complete aud masterly manner. 

 He considers that the fifth, seventh, ninth, and tenth nerves are each 

 equivalent to a spinal nerve, and compares in detail both the roots of 

 origin and the branches of these nerves with those of the spinal nerves. 



Stannius was also the first to point out the very important relations 

 of the ventral branches of these segmental cranial nerves to the 

 visceral arches. In the essay quoted above he shows how each visceral 

 arch is supplied by two branches belonging to different nerves, one 

 running along its anterior border, and one along the posterior. He 

 points out how the first branchial arch is supplied along its anterior 

 border by the glossopharyngeal nerve, and along its posterior by the 

 vagus ; how the remaining branchial arches are supplied by the vagus, 

 each arch by branches from separate stems ; how the hyoid arch is 

 supplied in front by the hyoidean branch of the facial nerve, and 

 behind by the anterior branch of the glossopharyngeal ; how the 

 mandibular arch has the mandibular branch of the trigeminal nerve 

 along its anterior border, and along its posterior the anterior branch 

 of the facial, which he identifies as the chorda tympani of Aves and 

 Mammalia ; and finally, how the upper jaw is supplied by the 

 ophthalmic and maxillary divisions of the fifth nerve. 



He concludes this portion of his treatise with the following very 

 suggestive sentence : — " Hence it follows that the number of the 

 ventral branches of each cranial nerve, and the number of the 

 spinal-like (segmental) cranial nerves is not determined so much by 

 the number of the skull-vertebra? as by that of the visceral arches." 1 



In thus stating that the number of segmental cranial nerves was 

 no longer to be determined by preconceived ideas concerning the 

 composition of the skull, but by direct examination of the nerves 

 themselves, Stannius rendered an invaluable service to morphology. 

 Had he, indeed, gone one step further; had he been able to completely 

 disabuse his mind of this notion of skull- vertebras which was exercising 

 so pernicious an influence on the zoologists of the day, he would have 



1 Stannius, op. oit, p. 131 



