28 DR. A, MILNES MARSHALL. 



regret, inasmuch as the fourth nerve is in some respects the most 

 remarkable of all the cranial nerves. It is the only one arising 

 in the adult from the dorsal surface of the brain ; and since ail 

 the other nerves arise primitively from the neural ridge, i.e. from 

 the mid dorsal line, it might be argued that the fourth nerve is 

 the only one wliich has preserved its primitive connection with 

 the dorsal surface. It is very difficult, however, to conceive 

 how the fourth nerve could so preserve its relations while the 

 nerves immediately in front of and behind it lose their attach- 

 ment and shift down nearly to the mid ventral line. It is just 

 possible that the fourth nerve arises primitively from the con- 

 striction between the raid and hind brains, at which point the 

 primitive relations undergo coroparaiivelj/ little change, and 

 which is compurativeli) unaffected by the rapid growth of the 

 other parts. I am not at all disposed, however, to adopt this 

 view, inasmuch as my investigations tend very strongly to prove 

 that all the nerves arise primitively from the widest parts of the 

 dilated vesicles, whether of the brain or cord, and never from the 

 intervening constrictions. 



In the adult chick the fourth nerve runs parallel to, and 

 dorsad of, the ophthalmic branch of the fifth, from which it is 

 separated by the rectus superior. The muscle it supplies — the 

 obhquus superior — is, at its origin in the chick, as in the skate, 

 the mod anterior of all the eye-muscles. Now, since it arises 

 posteriorly to the third nerve, and supplies a muscle in front of 

 those supplied by the third, it must cross this latter nerve, as 

 indeed it is readily seen to do in the adult chick. Therefore, if 

 the origin of the fourth nerve in the adidt is its primitive origin, 

 then the third and fourth nerve cannot be morphologically equi- 

 valent ; and if the third nerve is a true cranial segmental nerve, 

 in favour of which we have seen that evidence of a very strong 

 nature exists, then the fourth nerve cannot possibly be one also. 

 It is impossible for two segmental nerves to cross one another in 

 the manner in which the third and fourth nerves do. 



Such suggestions are perhaps, in the absence of direct embryo- 

 logical evidence, not of much weight, but I have thought it worth 

 while to record them, because it is of very great importance to 

 determine, by any method we can, the relations of the eye-muscle 

 nerves to tiie other cranial iierves and to the head segments. 



The fifth nerve. — At twenty-four hours the neural ridge has ex- 

 tended backwards from the mid brain a certain distance down the 

 hind brain (lig. 5, m). By the twenty-ninth hour the hind brain 

 is divitled by a series of slight constrictions (cf. figs. 9 and 10) 

 into a series of vesicles, of which tlie most anterior one is the 

 largest. At its centre or widest part the neural ridge (fig. 9, m) 



