124 JOHN BEARD. 
arch, according to Balfour, Marshall, and Van Wijhe. It 
gives rise to the superior oblique muscle, therefore the nerve 
of this muscle, the fourth nerve, must also belong to the man- 
dibular segment, as Van Wijhe insists. 
Further, if the first head cavity is morphologically of the 
same nature as the second and third head cavities, then the 
third nerve, which innervates the muscles derived from the 
first head cavity, is, a priori, of the same nature as the fourth 
and sixth nerves. 
Marshall himself regards the sixth nreve as a ventral root of 
the seventh nerve,! and says, “ Concerning the actual value of 
the sixth nerve, I see no reason to alter the opinion I pre- 
viously expressed, that the sixth nerve may be regarded as 
having the same relation to the seventh that the anterior root 
of a spinal nerve has to its posterior root.” 
We have also seen reason to believe that the fourth is a 
ventral root of the trigeminal nerve. And from all these facts 
we might fairly regard the third as also a ventral root. 
But further, the dorsal root of no other cranial nerve, if we 
except the third, innervates the structures arising out of a head 
cavity. The dorsal roots, so far as they are motor, only in- 
nervate those structures derived from the lateral muscle plates 
(Van Wijhe). 
According to Van Wijhe, the third nerve develops after the 
ciliary ganglion, and hence could not be its dorsal root. 
The third, at any rate, is an exceedingly fine nerve, and is 
much thinner than the ophthalmicus profundus; hence, if the 
third nerve be the dorsal root of the second segment, then 
the proximal stem of the nerve is thinner than one of its 
distal branches. Hence there seems to be no avoiding the 
conclusion, in which I[ agree with Krause and Van Wijhe, that 
the third is not the dorsal root of the ciliary ganglion, but is 
the ventral root of the second segment. 
Returning to the general schema of the development of the 
dorsal root of a cranial nerve, it is found that, so far as its 
development goes, the nerve of the second segment agrees with 
1 ¢ Segmental Value, &c.,’ pp. 42—44. 
