SEGMENTS OF TRIGEMINAL NERVE-GROUP 277 
obliteration of the muscular arch; the nerve on each side, following 
the shifts of the muscle, naturally took up the position of the original 
muscular arch, and so formed the trochlear nerve, with its dorsal 
crossing. This explanation of Firbringer’s was associated by him 
with movements of the median pineal eyes, the length of their nerve, 
according to him, even yet indicating their previous mobility. This 
assumption is not, it seems tome, necessary. The length of the nerve 
is certainly no indication of mobility, for in Limulus and the scorpion 
group the nerve to each median eye is remarkably long, yet these 
eyes are immovably fixed in the carapace. All that is required is a 
pair of dorso-ventral muscles belonging to the segment immediately 
following the group of segments represented by the oculomotor nerves, 
the fibres of which should cross the mid-dorsal line at their attach- 
ment; for, seeing that the lateral eyes were originally so near this 
position, it follows that such muscles might form part of the muscular 
group belonging to the lateral eye without having previously moved 
the pineal eyes. In fact, Fiirbringer’s explanation requires as starting- 
point that the pair of muscles which ultimately become the superior 
oblique should have the exact position of the pair of dorso-ventral 
muscles in the scorpion, called by Miss Beck the anterior dorso- 
plastron muscles (63), which I have named the oblique muscles. 
Here, and here only, do we find an interlacement, across the mid- 
dorsal line, of the fibres of attachment of the muscles on the two sides, 
in consequence of which this pair of muscles is described by her as 
forming an arch encircling the alimentary canal and dorsal vessel. 
If, then, as I have previously argued, the primitive plastron formed a 
pair of trabecule, and the nervous system grew round the alimentary 
canal, such an arch would encircle the tubular central nervous system 
of the vertebrate. 
Still more striking is this pair of muscles (63) in Phrynus (Fig. 
108), where we see how the arch formed by them almost touches 
the posterior extremity of the supra-cesophageal brain-mass, crossing, 
therefore, over the beginning of the stomach region of the animal. 
The angle formed by the arch is much more obtuse than that formed 
in Scorpio, so that an actual crossing of the muscle-fibres has taken 
place at the point of attachment to the carapace. Also, only the part 
nearest the carapace is muscular, the rest forming a long tendinous 
prolongation of the plastron wall (the primordial cranium), as seen in 
the figure. 
