142 PROFESSOR MARSHALL. 



pression to it ; for after referring to Schlenmi and d' Alton's observa- 

 tions, lie says :— " Offenbar ist hier ein Theil der Wurzeleleniente des 

 N. oculoruni motorius, so wie auch die Wurzel des N. abducens, in 

 die Bahn des N. trigeminus, iibergetreten," * and remarks that it is 

 quite possible that a very careful examination of the nerve-roots would 

 show that the abducens has really an independent root of origin. 



The point at issue is an important one, and must be clearly stated. 

 When we find two nerves — the third and fifth, which in the great 

 majority of vertebrates are independent of one another both in origin 

 and distribution — in certain forms, as the lampreys, arising from the 

 brain independently and normally, but becoming united together at 

 some point or other of their course, so that it is no longer possible 

 from mere anatomical observation to say with certainty to which of 

 the two a given branch belongs, are we to infer, as is done tacitly or 

 explicitly by many writers, 2 that the condition shown by the lamprey 

 is the more primitive one, and represents an intermediate stage in the 

 process by which the eye-muscle nerves gradually emancipated them- 

 selves from their parent nerve — the fifth — and attained ultimately 

 the complete independence they show in the great majority of existing 

 vertebrates 1 Or, on the other hand, are we to infer that the indepen- 

 dent origin of the third nerve is primitive, and that its connection 

 with the fifth, when, as in the lamprey, it does occur, is a secondarily 

 acquired one 1 To my mind there can be no doubt whatever that the 

 latter is the correct explanation ; and the chief reasons that lead me 

 to think so are the following : — 



la) Though we know of instances — notably in the case of the vagus 

 — of nerves originally distinct and independent gradually becoming 

 fused, and then this fusion getting thrown back to a very early de- 

 velopmental stage ; yet we know of no established case of a branch 

 attaining independence, and acquiring the character of a distinct 

 nerve. 



(6) Supposing it were possible for such a process to occur, it would 

 certainly be very surprising if, as in the supposed case of the third nerve, 

 the process of differentiation should commence at the proximal end, 

 and that there should be a stage in which the roots were independent 

 and the two nerves still fused distally. 



(c) There are very strong reasons, which we shall discuss later on, 



1 Stannius, Das peripherische Nervensystem der Fische, p. 18. 



s Cf. the authors mentioned above, and especially the passage quoted from WiedersheiiH 

 on p. 139 above, 



