THE THIRD AND FOURTH PAIR OF NERVES. 487 



corpora albicantia of Willis are situated.* Anterior to this 

 substance they come in contact with each other, and again 

 separate, in such a way, that it is an undecided question whether 

 they decussate each other, or whether each forms an angle, and 

 is in contact with the other at the angle. 



— The innermost fibres of the two nerves are now ascertained 

 to cross each other, those of the right side passing to the left 

 eye, and vice versa. The external portions of the nerve con- 

 tinue straight to the eye of the same side. — 



From this place of contact, each nerve proceeds to its respec- 

 tive foramen opticum, where it receives a coat from the dura 

 mater, which extends with it to the eye, as has been described 

 in the account of that organ. 



The Third Pair of Nerves, 



Are sometimes called Motores Oculorum, in consequence of 

 their distribution to several muscles of the eye. They arise at 

 the inside of the crura cerebri, and make their appearance on 

 the basis of the brain, at the anterior part of the pons Varolii. 



They originate by numerous threads, which soon unite so as 

 to form a cord, which passes through the dura mater, on each side 

 of the posterior clinoid process, and continues through the caver- 

 nous sinus, and the foramen lacerum, to the orbit of the eye. 



Before this nerve enters the orbit it generally divides into two 

 branches which are situated one above the other. The Upper- 

 most Branch is spent principally upon the rectus superior mus- 

 cle of the eye, but sends a twig to the levator palpebrae. The 

 Inferior Branch is distributed to two of the recti muscles, viz. 

 the internus and the inferior, and also to the inferior oblique. 

 It likewise sends a twig to a small ganglion in the orbit, called 

 the Lenticular or Ophthalmic Ganglion,^ from which proceed 

 the fine ciliary nerves that perforate the sclerotic coat. J 



The Fourth Pair of Nerves, 

 Are called the Pathetic, in consequence of the expression of 



* See vol. ii. page 387. 



f This ganglion, which is considered as among the smallest in the body, lies 

 on the outside of the optic nerve, near its entrance into the orbit, and is gene- 

 rally surrounded by soft adipose matter. 



% See vol. ii. page 417. 



