32 DR. A. MILXES MARSHALL. 



a true visceral cleft ; aud, therefore, the maxillary process ougKt 

 to be viewed as a separate arch, and not as a bud of the 

 mandibular. 



Finally, to return to the ramus dorsalis hypothesis of the 

 ophthalmic, it is worthy of note that " in the tadpole and some 

 Urodelos the fifth nerve gives a cutaneous branch to the dorsum 

 of the head.'"^ Is not this the true ramus dorsalis of the fifth? 

 Similarly in Elasmobraiichs there are branches of the fifth which 

 would appear to be true rami dorsales.^ 



The sixth nerve. — I have made some observations on the 

 development of the sixth nerve, which, though incomplete, I 

 think worth while recording here, inasmuch as I am not aware 

 that it has been treated of by any previous writer. 



rig. 25 represents part of a longitudinal section through the 

 head and neck of a five-day chick, the section being taken a very 

 little to one side of the median line. It passes through the in- 

 vesting mass (?" r) , the downward prolongation of the fore brain 

 to form the infundibulura {inf), the mouth with its pituitary 

 diverticulum {pit), the fore gut, and mandibular and hyoid arches. 



At VI a nerve is seen arising from the floor of the^iaid-jbrain 

 by a large number of slender roots, extending over a considerable 

 longitudinal extent. This nerve runs forward without any gan- 

 glionic en'argement in any part of its course, parallel to the floor 

 of the brain, and just above the investing mass. 



By tracing this nerve forwards it is found to run straight 

 to a muscle [re, fig. 26), which, from its relations to the third 

 and ophthalmic nerves, can be none other than the rectus externus, 

 or, as it would be more properly called, rectus ])ostcrior. Tiiis 

 nerve is, therefore, the sixth. Its course is perfectly straight, 

 and it gives off" no other branch. 



The sixth nerve difl'ers from all the other pre-auditory nerves 

 in several important points : 



1. It arises by a series of small slender roots instead of by 

 a single large ganglionic root. In this respect it resembles the 

 anterior spinal roots of the chick^, to which it closely corresponds 

 in position and relations. 



2. It is very much more slender than any of the others. 



3. It does not branch. 



4. Its direction is at right angles to the segmental nerves. 



5. It appears much later than the others. On this point I 

 cannot speak definitely, as I have failed to observe its early 



' Huxley, ' Eucyclopcedia Brilannica,' vol. i, art. " AiTi])liil)ia,"p. 707. 

 • Jackboii and Clarke, " Tlie Cranial Nerves of Ecliiiiorliiiius spiiiosus," 

 ' Journal of Anal, and I'liys.,' vol. x, p. 83, and plate vii, fig. 2, d d. 

 ' Loc. cit.., !>. 505. 



