182 THE SPINY DOGFISH— THE SKATE 



into the cartilage of the skull . Cut away the cartilage above the eye 

 until the superior ophthalmic branch of the trigeminal nerve (V) is 

 exposed. This ophthalmic nerve represents the fused ophthalmic 

 branches of the trigeminal nerve (V) and the facial nerve (VII). Trace 

 this branch from its origin over the eye muscles to the snout. Follow 

 the pathetic, which passes under the ophthalmic, to the superior 

 oblique eye muscle. 



(iii) Cut away the cartilage further down between the eye and the brain, 

 and note the optic nerve (II) passing straight from the brain to 

 the eye. 



(iv) Note the upward cartilaginous projections of the upper jaw between 

 the superior ophthalmic branch and the sides of the skull. 



(v) Make out the following eye muscles : — 



(a) The two oblique muscles, arising close together at the anterior 



part of the orbit and inserted widely apart on the anterior 

 half of the eye, the superior oblique on the dorsal surface and 

 the inferior oblique on the ventral surface. 



(b) The four recti muscles, arising close together at the posterior 



part of the orbit and inserted mostly on the posterior half of 

 the eye, the superior rectus on the dorsal surface, the inferior 

 rectus on the ventral surface, the posterior rectus or rectus 

 extemus on the posterior surface, and the anterior rectus or 

 rectus internus, which passes behind the eye and between the 

 two oblique muscles, and which is inserted on the anterior 

 surface of the eye. 



(vi) The oculomotor nerve (III), arising from the under-surface of the mid- 

 brain, passing through the wall of the skull, and supplying the follow- 

 ing eye muscles : the inferior oblique, the superior and the inferior 

 and the anterior rectujs. 

 (vii) The inferior ophthalmic or ophthalmicus profundus branch of the 

 trigeminal nerve (V), passing over the eye under the superior rectus 

 and the superior oblique muscles, and going forwards to the snout. 

 [This branch is absent in Scyllium.'] Trace both the ophthalmic 

 branches back to their origin. Raise the eye and note, passing along the 

 floor of the orbit, the maxillary branch of the trigeminal nerve (along 

 with the buccal branch of the facial nerve) to the upper jaw. Make 

 out the mandibuiar branch of the trigeminal nerve, arising behind 

 the maxillary and passing through the cartilage to the lower jaw. 

 (viii) Slice away the cartilage alongside the spiracle so as to expose the 

 semicircular canals of the ear (otocyst), and make out the foUowhig 

 nerves arising together with the trigeminal (V) from the side, of the 

 medulla, anteriorly : — 



(a) The hyomandibular branch of the facial nerve (VII), passing 

 between the spiracle and the hyomandibular cartilage, and 

 dividing into an external mandibular nerve to the lower jaw 

 and a hyoidean nerve to the hyoid arch. 



