276 THE ORIGIN OF VERTEBRATES 
exception, for orbits of the same character have since been discovered, 
as is seen in Rohon’s figure (Fig. 14). In Fig. 110, C, I give an 
outline of the frontal part of the head-shield of a Cephalaspid, in 
which I have drawn the eye-muscles as in the other two figures. 
Although all the members of the Osteostraci possess large lateral 
eyes towards the centre of the head-shield, the other group of ancient 
fishes, the Heterostraci, are characterized by the presence of lateral 
eyes far apart, situated on the margin of the head-shield on each 
side (cf. Fig. 142, 0, p. 350). 
So, also, on the invertebrate side, the lateral eyes of Pterygotus and 
Slimonia are situated on the margin of the prosomatic carapace, while 
those of Eurypterus and Stylonurus are situated much nearer the 
middle line of the prosomatic carapace. 
Next comes the question of the superior oblique muscle and the 
trochlearis nerve. Why does this nerve (n.IV. in Fig. 106, C and D) 
alone of all the nerves in the body take the peculiar position it 
always does take? The only suggestion that I know of which 
sounds reasonable and worth consideration is that put forward by 
Firbringer, which is an elaboration of the original suggestion of 
Hoffmann. Hoffmann suggested in 1889 that the trochlearis nerve 
represented originally a nerve for a protecting organ of the pineal 
eye, which became secondarily a motor nerve for the lateral eye as 
the pineal eye degenerated. Fiirbringer differs from Hoffmann in 
that he considers that the nerve was originally a motor nerve, and 
was not transformed from sensory to motor, yet thinks Hoffmann’s 
suggestion is in the right direction. 
He points out that the crossing of the trochlearis is not a crossing 
of fibres between two centres in the central nervous system, but may 
be explained by the shifting of the peripheral organ, i.e. the muscle, 
from one side to the other, and the nerve following this shift. Con- 
sequently, says Firbringer, the course of the nerve indicates the 
original position of the muscle, and therefore he imagines that the 
ancestor of the superior oblique muscle was a muscle the fibres of 
which were attached in the mid-dorsal line, and interlaced with those 
of the other side, the two muscles thus forming an arch through 
which the nervous system with its central canal passed. Then, for the 
sake of getting a more efficient pull, the crossing muscle-fibres became 
more definitely attached to the opposite side of the middle line, and 
finally obtained a new attachment on the opposite side, with the 
