520 



THE CRANIAL NERVES. 



a manner as to form a connection at the same time between the two 

 opposite sides, and between the eye and the quadrigeminal tubercle on 

 the same side. Here also the apparent majority of the fibres cross each 

 other completely from side to side, though intermingled at the chiasma 

 in such slender bundles that they are distinguishable only by microscopic 

 examination. But at each side of the chiasma there is also a layer of 

 fibres, which, according to Henle, hardly exceeds one-twentieth of a 



Fig. 172. 



COURSE OF THE OPTIC NERVES IN MAN. 1,2. Right and left eyeballs. 3. Decussation 

 of the optic nerves. 4,4. Tubercula quadrigemina. 



millimetre in thickness, passing continuously along its outer border and 

 thence running forward with the optic nerve to the eye of the same side. 

 These fibres, like those of the white substance in general, do not keep the 

 same horizontal level, but follow a more or less spiral course, winding suc- 

 cessively outward, downward, and inward, from the upper surface of the 

 chiasma to reach the under surface of the optic nerve in front- It is 

 not known in what special parts of the retina the two sets of fibres on 

 each side, namely, the decussating and the direct, find their termination. 

 At the anterior angle of the chiasma there are also fibres which pass 

 from side to side in a curved direction, running symmetrically across 

 from one optic nerve to the other, and forming a transverse commissural 

 baud between the retinae of the two eyes ; and finally at the posterior 

 angle of the chiasma there are similar transverse commissural fibres, 

 which pass forward on each side with the corresponding optic tract, 

 cross the median line at the chiasma, and return backward with the optic 

 tract of the opposite side. Thus there is effected a fourfold connection 



