334 



ZOOLOGY FOR MEDICAL STUDENTS 



CHAP. 



Oculomotor (Eig. 138, III). — ^The Third nerve supplies the four 

 remaining eye-muscles — Superior, Inferior and Internal Rectus and the 

 Inferior Oblique. It arises from the mesencephalon towards its ventral 

 side. 



Trigeminal (Fig. 140, V). — ^The Fifth nerve is the largest of the 

 cranial nerves in the Dogfish. It is given the name Trigeminal in verte- 

 brates from the characteristic fact that it divides into three main branches, 

 known as the Ophthalmic (I), Maxillary (II), and Mandibular (III) 

 divisions of the nerve. 



Of these branches the ophthalmic passes forwards through the cavity 

 of the orbit and is distributed eventually to sensory cells in the skin on 

 the dorsal side of the snout. In Acanthias it divides into two, a ventral 



Wfe 



HI Yz infr 



Fig. 139. 



Dogfish {Acanthias). View of right orbit with the eyeball and its muscles. A, eyeball in 

 position ; B, eyeball removed, e.r, External rectus ; i.o, inferior oblique ; i.r, internal rectus ; 

 inf.r, inferior rectus ; oph.s, superficial opthahnic nerves ; s.o, superior oblique ; s.r, superior rectus '■ 

 I, supporting table for eyeball ; II, optic nerve ; III, oculomotor (branch to inferior oblique) ; 

 IV, pathetic ; V, trigeminal {p, deep ophthalmic branch ; j, mandibular branch ; 3, maxillary 

 branch) ; VII6, facial (buccal branch). 



deep ophthalmic branch (Fig. 140, Yd.o) which passes forwards beneath 

 the superior rectus muscle and a superficial ophthalmic (Fig. 140, Ys.o) 

 lying more dorsally, along the mesial wall of the orbit, and enclosed 

 in a common sheath with the similarly named branch of VII. In 

 ScylUum there is no separate deep ophthalmic branch. 



The maxillary (Fig. 140, Ymx) and the mandibular (Vmn) divisions 

 of the trigeminal, which in ScylUum are fused together into a common 

 trunk for some distance, pass respectively to the upper and to the lower 

 side of the mouth. The maxillary, purely sensory, is distributed to the 

 skin of the roof of the mouth and the ventral side of the snout, while 

 the mandibular carries the motor fibres for the muscles connected with 

 the lower jaw together with sensory fibres for the same region. 



