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 

 Fiirbringer, 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 Fiirbringer, 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 



