cakpenter: development of the oculomotor neeve. 213 



rise to a variable number of very small branches, which accompany it to 

 the eyeball. 



III. Abducent Nerve. 



1. Tbe sixth nerve arises at the beginning of the fourth day of incu- 

 bation as a slender bundle of processes of neuroblasts, which emerge 

 from tbe ventral face of the hind-brain. The abducent nidulus is situ- 

 ated in the ventral wall of the hind-brain near tbe median plane. The 

 nerve exhibits a number of attenuated roots arranged in a longitudinal 

 series. 



2. Indifferent medullary cells migrate out into the roots of the abdu- 

 cent nerve. These distribute themselves along the nerve trunk, assume 

 an elongated form, and become recognizable as the cells which later 

 develop the sheaths of Schwann. None of these indifferent cells give 

 rise to ganglion cells. 



3. The fundament of the posterior rectus muscle becomes differentiated 

 during the third day before the appearance of the abducent nerve. The 

 embryonic abducent neuraxons, upon emerging from the neural tube, 

 grow rapidly cephalad to connect with the muscle fundament. 



4. The abducent nerve of the adult is composed of both large and 

 email meduilated neuraxons. 



1. Cells resembling the indifferent cells of the neural tube are present 

 in the embryonic Gasserian ganglion. Certain of these cells may, by 

 differentiation, give rise to ganglion cells. Others appear to migrate 

 from the Gasserian gauglion into the ophthalmic branch of the trigeminal 

 nerve, and there assume the characters of Schwann's-sheath cells. Simi- 

 lar cells remaining within the Gasserian ganglion possibly develop later 

 into the nucleated capsules of the ganglion cells. 



2. In the adult the communicating ramus from the ophthalmic branch 

 of the trigeminus does not pass directly to the ciliary ganglion, as in the 

 embryo, but to the oculomotor ciliary nerve, with which it connects 

 about one mm. from the distal extremity of the ganglion. Certain of 

 the neuraxons of the communicating ramus, however, turn centrally and 

 enter the sympathetic region of the ciliary ganglion ; the remaining neu- 

 raxons accompany those of the oculomotor ciliary nerve to the eyeball. 



3. A trigeminal ciliary nerve is given off by the communicating ramus 

 about midway in its course. A second trigeminal ciliary nerve is occa- 

 sionally to be seen arising from the communicating ramus near the 

 termination of the latter in the oculomotor ciliary nerve. 



