THE ELASMOBRANCH FTSHER 223 



CRANIAL NERVES 



The first cranial or olfactory nerve has its end organs in the mucous membrane 

 of the olfactory organ. The nerve passes backward in two short bundles, one 

 median and the other lateral in position (7, fig. 200a) to the olfactory bulbs 

 (ol.h.) The bulbs are joined to the olfactory lobes (ol.l.) of the brain by means 

 of long olfactory tracts {ol.t.) . 



Running parallel with this tract is the terminal nerve {tn.), which in Hep- 

 tanchus arises near the recessus neuroporicus and passes along the tract as a 

 slender strand. Near the olfactory bull) it runs laterad and enters the fissure 

 dorsally between the lateral and median divisions of the olfactory nerve. To 

 the median nerve it gives an exceedingly slender strand and continues with 

 the lateral division of the nerve, finally reaching the mucous membrane of 

 the cup. 



The second or optic nerve (77, figs. 200a and 200b) has its origin from the 

 retina of the eye. As a thick stem it passes inward, crosses in the optic chiasma, 

 and enters the diencephalon. 



The third cranial nerve or oculomotor (777) springs from the ventral side 

 of the midbrain and passes to muscles of the eye. In the orbit it divides so as to 

 supply branches to the inferior, the superior, and the anterior rectus, and 

 to the inferior oblique muscles of the eye. 



The trochlearis or fourth nerve (7F) also arises from the mesencephalon, 

 but the fibers perforate the roof of the brain instead of the floor. The fibers 

 then as a small band pass under the overhanging cerebellum and down over the 

 mesencephalon forward and outward to the superior oblique muscle of the eye. 



The trigeminal or fifth cranial nerve in Heptanchus maculatus arises from 

 the brain in common with, but slightly anterior and ventral to, the seventh 

 nerve. A short distance from its origin it separates into its main branches. 

 The first of these nerves, the ophthalmicus profundus (op. V, fig. 200a), runs 

 to the median side of the eyeball. Before reaching the latter, however, it 

 gives off the ciliary branch (cl.) to the eye; it then perforates the cartilagi- 

 nous capsule surrounding the eyeball and continues forward under the supe- 

 rior rectus, next emerging from the cartilage to pass forward under the ante- 

 rior rectus muscles and between the superior and inferior obliques. It leaves 

 the orbit by a separate foramen, and finally breaks up in the skin over the 

 nasal capsule. The ophthalmicus superficialis of the fifth (os.V) is a small 

 branch or branches running with the ophthalmicus superficialis of the seventh 

 nerve. The maxillary branch (inx.V, fig. 200a) of the trigeminal breaks up 

 into three or four main divisions which go to supply the region of the upper 

 jaw; while the mandibular division of the fifth (md.V) passes backward and 

 downward around the angle of the mouth to the mandibular region. 



The abducens or sixth nerve {VI, fig. 200b), after leaving the mid ventral 

 line of the myelencephalon back of the fifth-seventh complex, passes under the 



