138 BIOLOGICAL LECTURES. 



region the nerves find their exit between the vertebrae, and so 

 they should do in the cranial region, the number of such seg- 

 mental nerves being of course one less than the number of 

 vertebrae recognized in the head. So those who followed Oken 

 arranged all the cranial nerves in two groups, while those who 

 thought they saw an additional or nose vertebra, collected the 

 nerves in three divisions. Here must be enumerated the labors 

 of Stieda, Johannes Miiller, Stannius, and others. 



Stannius, however, took another set of structures, the gill 

 clefts, into consideration, and although he admitted only three 

 groups of these nerves (since he recognized four vertebrae) he 

 still says that " the number of the branches of each cranial 

 nerve, and the number of spinal-like (segmental) cranial nerves 

 is determined not so much by the number of cranial nerves as 

 by that of the visceral arches." 



To Gegenbaur is usually given the credit of building upon 

 this foundation, but we must not forget that in 1869 Huxley 

 claimed that no part of the skull was vertebral in its nature, 

 and that not only skeletal, but all structures should be invoked 

 in settling the question. It would not do to make the bones 

 the basis, and work everything else to fit. So, dismissing the 

 bones, Huxley took cranial nerves and gill clefts as his basis. 

 Behind the ear, the nerves split above each cleft, and send a 

 nerve to each of its margins. Here the relationships are clear, 

 and the segments can readily be distinguished. In front of 

 the ear, however, there is more difficulty. The facial nerve 

 splits in the same way, above what is known in sharks as the 

 spiracle, a cleft which persists in man as the Eustachian tube. 

 For the next nerve in front, there is at first sight no cleft, but 

 Huxley advanced the suggestion, often attributed to Dohrn, 

 that the mouth had been formed by the coalescence of a pair 

 of gill slits, and upon this supposition, this nerve, the trigemi- 

 nal, was partly brought into harmony. But not all of it; there 

 are branches which go farther forward, and for a cleft for these 

 the orbito-nasal fissure of the embryo was suggested. Still 

 another cleft still farther in front was advocated, so that Hux- 

 ley recognized in the head nine segments, — four in front of the 

 ear, and five behind. 



