SUMMARY OF THE EYE-MUSCLE NERVES. 237 



mony to the doctrine that the eye-muscle nerves grow 

 directly out from the brain. If this be true, I see no 

 reason why a given motor nerve should not grow out 

 either above or below some other structure, depending 

 upon the peripheral relations of its end-organ with refer- 

 ence to that structure. 



Finally, Allis in his account of the elasmobranchs 

 (p. 522) says: "These different relations of the oculo- 

 motorius to the internal and superior recti, in elasmo- 

 branchs, are due to and are caused by the gradual shifting 

 from before backward of the origins of all the recti 

 muscles, and also of the place of exit of the oculomotorius 

 from the cranium. As a result of this shifting the inter- 

 nal and superior recti, at their origins, either traverse, or 

 are traversed by, the issuing nerve." That is, he invokes 

 a principle to account for the diverse relations of nerve 

 and muscle in elasmobranchs, which, if applied more 

 broadly, might weaken the phylogenetic value of some of 

 his other cases. 



It seems to me, therefore, that Allis' phylogenetic table 

 is based largely upon inconclusive data, and that the 

 various arrangements which he diagrams are of cenoge- 

 netic origin rather than phylogenetic significance. In the 

 case of Menidia the deviations from what Allis regards as 

 the typical piscine arrangement can be easily explained 

 mechanically by the great size of the eyes and the con- 

 sequent crowding of the recti muscles far backward. 



VI. — Summary of Section 9. 



The sub-cranial, or eye-muscle canal is very highly 

 developed, running back under the cranium for its entire 

 length to the caudal end of the basioccipital bone. Cor- 

 related with the large eyes, the eye-muscles are highly 

 developed. The recti, except the rectus inferior, arise 

 in the sub-cranial canal. The obliqui arise in an anterior 

 eye-muscle canal in the internasal cartilage. All of the 

 eye-muscles have some smaller muscle fibres which usually 



