THE SEGMENTAL VALUE OP THE CRANIAL NERVES. 159 



degree of certainty ; and the views of different writers on the point 

 are far from being in harmony with one another. 



In a former section of this paper we have established the fact that 

 the sixth is an independent nerve throughout the vertebrate sub- 

 kingdom. It always supplies the rectus extemus muscle of the eyeball, 

 and may supply other parts as well ; thus, in reptiles it supplies the 

 retractor muscle of the bulb of the eye, and in Batrachia the suspensor 

 muscle of the bulb and the muscles of the nictitating membrane. 1 In 

 all cases it is a purely motor nerve. Indeed, if we omit the eleventh 

 and twelfth pairs, which are not constant cranial nerves, the sixth is 

 not only the most purely motor cranial nerve, but the only exclusively 

 motor one throughout the vertebrate series. 



Its point of origin from the brain in adult vertebrates is also a 

 remarkable and constant one. It arises from the under surface of the 

 medulla, very close to the mid-ventral line, and vertically below, or 

 more usually slightly posterior to the common root of origin of the 

 seventh and eighth nerves. In some cases the root may be in front 

 of that of the seventh nerve. The root is always slender, and devoid 

 of ganglion cells. 



Concerning the development of the sixth nerve, we unfortunately 

 know but little. At the fifth day in the chick, 2 and at a correspond- 

 ing stage in the clog-fish 3 , it has been detected and described, its 

 appearance and relations being practically identical in the two cases. 

 It is a slender nerve, with no ganglion cells at any point in its length, 

 arising from the ventral surface of the hind-brain, below the seventh 

 nerve, by a number of small slender roots, and running forward to the 

 rectus extemus muscle, in which it ends. The roots are, from the 

 earliest period at which the nerve can be recognised, close to the median 

 ventral line (fig. 7, VI), and some distance below the root of the seventh 

 (fig. 7, VII), from which they are from the start perfectly distinct. So 

 far as can be inferred from negative evidence, the sixth nerve appears 

 to develop later than the seventh and other segmental nerves. 



From the above account it is clear that the sixth has no claim 

 whatever to segmental rank, inasmuch as it distinctly fails to answer 

 any one of the tests of such rank laid down on page 133. It does 



1 Stannius, Handbuch der Zootomie, Zweite Auflage, Zootomie der Amphibien, 1856, p. 150. 



2 Marshall, "Development of Cranial Nerves of Chick," Quart. Joum. of Micros. Science, 

 Jan. 1878, pp. 23—25. 



3 Marshall, " Head Cavities and Associated Nerves of Elasmobranchs," Quart, Joum. of 

 Micros. Science, Jam 1881, pp. 89—93. 



