SEGMENTS OF TRIGEMINAL NERVE-GROUP 2 J J 



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 Fiirbringer'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 to me, 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 trabecular, 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-o?sophageal 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. 



