154 bulletin: museum of compaeative zoology. 



the optic lobes and the base of the epencephalon. Turning ventrad and 

 cephalad, it passes through tl\e orbit, running raediad and dorsad of the 

 posterior and dorsal rectus muscles, and ends on the ocular face of 

 the dorsal oblique muscle (Fig 1, mn, db. cZ.). 



Abducent Nerve. Lilie the other eye-muscle nerves, the abducens has 

 its nidulus in the somatic motor column. It lies in the ventral part 

 of tlie metencephalon, and from it the abducent neuraxous run ventrad 

 to emerge from the ventral face of this division of the brain, not far 

 from the median plane. The trunk of the nerve (Fig. 1, n. abd.) pro- 

 ceeds cephalad, and, crossing dorsad of the oculomotor nerve, divides into 

 several terminal ramifications, which are distributed to the portion of 

 the posterior rectus muscle lying laterad of the ophthalmic branch of 

 the trigeminus {rm. opth. trig.), v^hioh passes througli the proximal part 

 of tlie muscle. Shortly before terminating in this way, the abducens 

 sends cephalad a branch (Plate 1, Fig. 2, rm. mu. qd. + pyr.) to the 

 muscles of the nictitating membrane. Tliis branch soon bifurcates, and 

 between its forks the communicating ramus connecting the trigeminal 

 and oculomotor nerves often passes. 



Ophthalmic Branch, of the Trigeminal Nerve. The Gasserian ganglion 

 presents two well-marked divisions, an ophthalmic portion (Fig. 1, gn. 

 Gas.') extending cephalad, and a maxillo-mandibular portion directed 

 ventrad. The first becomes gradually narrowed into the ophthalmic 

 branch of the trigeminal nerve (rm. opth. trig.) which, running ceph- 

 alad, usually penetrates the posterior rectus muscle, and then passes, 

 ventrad of the proximal ends of the dorsal rectus and dorsal oblique 

 muscles, to the anterior boundary of the orbit. Here it divides, one 

 large nasal ramus (rm. na.) entering the nasal chamber, while several 

 small branches (rm. f.) extend dorsally to a more superficial distribution, 

 and, taken together, correspond to a frontal ramus. 



From a point opposite the distal extremity of the ciliary ganglion, 

 tlie ophthalmic branch sends out a communicating ramus (Figs. 1 and 2, 

 rm. comn.), which unites with the oculomotor ciliary nerve about 

 one mm. distad of the ciliary ganglion. However, not all the nenraxons 

 which here leave the ophthalmic branch reach the oculomotor ciliary 

 nerve, since a slender bundle of them emerges from the communicating 

 ramus about midway in its course, and proceeds toward the eyeball as 

 an independent trigeminal ciliary nerve (Fig. 2, n, cil. trig.'). In some 

 cases, another trigeminal ciliary nerve (n, dl. trig.") leaves the com- 

 municating ramus near its union with the oculomotor ciliary nerve, and 

 likewise passes as a separate fibre to the eyeball. 



