CYCLOSTO:\IES 



271 



binocular field is possible. The rectus muscles are largely blended 

 together and are inserted into the globe as a ring around the periphery 

 of the cornea ; the inferior oblique arises in common with the internal 

 rectus, and the superior oblique, identifiable only by its nerve-supply, 

 is inserted into the infero-tem}Doral quadrant of the globe. The nerve- 

 supply to the muscles corresponds to the scheme common to all 

 Vertebrates (including man) except that the sixth cranial nerve appears 

 to supply the inferior as well as the external rectus ; it may be, 

 however, that the trunk contains fibres derived from the third nucleus. 

 The most interesting feature, however, is the coknealis muscle, a 



Fig. 285. — The Cornealis Muscle of the Lamprey. 



The cornealis muscle, c, running horizontally outside the orbit on the 

 caudal aspect of the globe, showing its insertion into the cornea (Mallory"s 

 phospho-tungstic acid hsematoxylin) ( ; 44) (Katharine Tansley) (cf. Fig. 276). 



massive muscle arising outside the orbit on the caudal aspect and 

 inserting into the transparent dermal cornea (Tretjakoff, 191G) (Fig. 

 285) ; its function is accommodative, dra\\ing this element of the 

 cornea taut and, in so doing, flattening the scleral cornea, pressing the 

 lens backw ards towards the retina and thus rendering the normally 

 myopic eye {■ — 8 dioj^tres) ennnetropic or even hypermetropic. Unlike 

 man, the lamprey thus accommodates for distant vision. ^ An accom- 

 modative mechanism acting by deforming the globe from the outside 

 is among Vertebrates unique to the lamprey. 



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