The Ot bit. 441 



tive position from within outwards, is the fourth, the 

 frontal and the lachrymal ; while along the outer wall, 

 from above downwards, are the superior division of the 

 third nerve, next the nasal, then the inferior division of 

 the third, and, lastly, the sixth. The ophthalmic vein lies 

 below, at the angle of junction of the inner and outer 

 walls. In the orbit, near its apex, the relative situation of 

 the nerves is, the fourth, the frontal and the lachrymal, 

 and these lie close to the roof of the orbit and in the order 

 named from within outwards ; then, the superior division 

 of the third beneath the superior rectus muscle ; next, the 

 nasal crossing above the optic nerve; on a lower level, is 

 the inferior division of the third lying below the optic 

 nerve, and, lastly, the sixth nerve, about on the same level 

 as the third and lying near the external rectus muscle. 



Effects of Injury to Orbital Nerves. Optic 

 nerve. In tracing, say, the right optic nerve, back- 

 wards from the retina, we find that, as it approaches 

 the chiasma, it divides into two portions ; one, the smaller 

 division, continues the course backwards and assists in the 

 formation of the right optic tract, which goes to the right 

 side of the cerebrum ; the other portion decussates with a 

 similar division of the left nerve, so that, the right tract 

 contains fibres from the right side of each retina, while the 

 left contains fibres from the left portion of each retina. 

 An injury to the optic nerve of one eye will cause loss of 

 vision of that eye; an injury to the chiasma or point of de- 

 cussation, involving the crossed fibres, will cause loss of 

 vision in the nasal halves of the retinae. If the lesion be 

 more extensive and affect the direct or non-decussated 

 fibres as well, then, in addition to the above effects, there 

 will be added loss of vision in the outer half of the eye to 

 which the non-decussated fibres belong. Should the dis- 

 ease be still more extensive and involve all the fibres of 



