THE CEREBRAL NERVES. 07 



oblique muscles. The third pair of nerves issues from the 

 crura cerebri, or inferior division of the metencephalon, upon 

 the base of the brain ; the fourth pair, from the fore-part of 

 the upper division of tlie metencephalon, immediately be- 

 hind the optic lobes, upon the superior surface of the brain. 

 This region is known as the Valve of Vieussens in the Main- 

 nialia. 



All the other cerebral nerves originate in the posterior di- 

 vision of the hind-brain — the myelencephalon. Tlie great 

 Jifth pair [trigemini) passes out from the sides of the me'.en- 

 cephalon, and supplies sensory nerves to the integument of 

 the head, and motor nerves to most of the muscles of the 

 jaws, by its three divisions — the ophthalmic, the superior 

 maxillary, and the inferior maxillary, nerves. 



Of these divisions the two latter are, very generally, closely 

 connected together, while the ophthalmic division remains 

 distinct. The ophthalmic division passes to the cleft between 

 the trabecula and the maxillary process (which nearly corre- 

 sponds with the orbit, and might be termed the orbito-nasal 

 cleft), and is distributed to the inner and the outer side of that 

 cleft. Hence its main branches are nasal and lachrymal. The 

 two maxillary nerves, on the other hand, are distributed to the 

 inner and outer sides, or anterior and posterior boundaries, of 

 the buccal oleft. Hence the superior maxillary belongs to the 

 posterior, or outer, side of the maxillary process, while the in- 

 ferior maxillary appertains to the anterior region of the first 

 visceral arch. The superior maxillary commonly unites with 

 the outer, or lachrymal, division of the ophthalmic ; the in- 

 ferior maxillary with the anterior division of the facial. 



In the higher Vertebrata, the trigeminal nerve usually has 

 two very distinct roots, a dorsal sensory, provided with a gan 

 glion (the Casserian ganglion), and a ventral motor, non-gan- 

 glionated. The fibres of the latter pass almost exclusively into 

 the inferior maxillary division. In addition, the ophthalmic 

 division may have a ganglion (ciliary) ; the superior maxillary 

 another [sphenopalatine or Meckelian), and the inferior maxil- 

 lary a third (otic). 



The sixth pair [cibducentes) issues from the inferior surface 

 of the brain, at the junction of the myelencephalon with the 

 metencephalon. It supplies the external straight muscles of 

 the eye ; with the muscles of the nictitating membrane, and 

 the retractor bulbi, or musculus choanoides, when such mus- 

 cles exist. 



The seventh pair (faciales) supplies the superficial facial 



