THE THIRD, FOURTH, AND SIXTH CRANIAL NERVES. 223 



blended with it at level 50. Two other small twigs, b, c, join the 

 main trunk at level 40, run by its side in intimate connection 

 with it in the cavernous sinus to levels 52 to 54, when they 

 diverge again, without having fused with the 4th nerve or lost 

 their identity. The branches of distribution become embedded 

 in muscle at level 80. 



No granular amorphous material, embedded in the nerve, is shown 

 in the figure. 



The transverse sections were 25/x. in diameter, were stained in 

 osmic acid, and are magnified 14 diameters. The diagram of the 

 main trunk and the millimetre scale accompanying it are of the 

 natural size. 



Fig. 5 exhibits the 6th cranial nerve in transverse section, 

 together with a diagram of the nerve reconstructed from a series 

 of sections made at intervals of a quarter of a millimetre, as in figs. 

 1, 2, and 4. 



In the first three sections the bundles of origin are seen lying 

 loosely side by side, not yet closely bound together by the dura 

 mater, which is first seen investing the nerve in section (20). With 

 the exception of a few very fine outlying twigs seen in sections (25) 

 to (40), no branches are seen until the nerve is near its termination, 

 sections (50) to (65). In section (65) only a few of the branches of 

 distribution are still to be seen. 



The sketch of this nerve, like that of the 4th cranial nerve in 

 fig. 4, shows its simplicity of structure, corresponding to its distri- 

 bution to a single muscle. The arachnoid membrane joins the nerve 

 at about ten millimetres from its origin, and the dura mater en- 

 sheathes it about seven millimetres farther on. Several very small 

 nerve-bundles, composed chiefly of non-medullated fibres, come into 

 relation with the main trunk at from twenty-five to twenty-seven 

 niillimetres from its origin, leaving again about ten millimetres lower 

 down. With these exceptions the only branches are those of distri- 

 bution to the external rectus. At about fifty-five millimetres from 

 its origin the nerve-branches enter this muscle. 



The transverse sections were 25ya thick, were stained in osmic acid, 

 and are magnified 14 diameters. The diagram of the nerve and the 

 millimetre scale are of natural size. 



VOL. XXXV. (N.S. vol. XV.) — JAN. 190]. 



