THE HEAD AND NECK 



361 



in the dogfish, for example, the walls of the somites become 

 thickened by the formation of muscle-fibres, and the contained 

 ccelomic cavity is obliterated. The premandibular somite 

 wraps round the optic nerve from behind, and becomes divided 



Fig. 168. — Reconstructions showing stages in the conversions of the first 

 three somites into the extrinsic eye-muscles in a dogfish. 



A to E, successive stages, ab, abducens ; exr, external rectus muscle ; 

 /, facial nerve ; hy, hyoid or 3rd somite ; infr, inferior rectus muscle ; 

 inob, inferior oblique muscle ; inr, internal rectus muscle ; m, mandibular 

 or 2nd somite ; oc, oculomotor nerve ;" opn, optic nerve ; p, profundus 

 ophthalmicus nerve ; pa, trochlear nerve ; pm, premandibular or 1st 

 somite ; ros V and VII, superficial ophthalmic branches of trigeminal and 

 facial nerve ; suob, superior oblique muscle ; sur, superior rectus muscle ; 

 tr, trigeminal nerve. 



into four pieces. The two dorsal portions are the internal and 

 superior recti muscles, the two ventral portions are the inferior 

 oblique and the inferior rectus muscles. The mandibular 

 somite grows forwards above the premandibular and gives 



