STRUCTURE OF THE CRANIAL NERVES 545 



Microscopic sections of the trochlear nerves of the dog closely 

 resembled those of the oculomotor nerve; large and small mye- 

 linated fibers, 11 to 15 and 2 to 6 micra- in diameter, in about 

 the ratio of three to one, with a few fibers of intermediate size. 

 No communicating branches from the cavernous plexus or the 

 ophthalmic nerve were seen. 



The abducent nerve of the dog near the brain stem consisted 

 of a single group of large and small myelinated fibers. 11 to 15 

 and 3 to 6 micra in diameter, in about the ratio of three to one. 

 Within the cavernous sinus the ner\-e was joined by a large 

 bundle of sympathetic fibers, the majority of which formed an 

 intimate union with three of the six or seven fasicles which made 

 up the nerve at this ])()int (fig. 2). More distally the main grou]) 

 of sympathetic fibci-s left the ner\(' to ]mrsue an inde])(Mident 

 course. 



The striking cliaracteristic of these llnce ii('i\-es is their simi- 

 larity in structure, botli as to the size of the fibers and the ])ro])or- 

 tion of large and small fibers. (Jaskell and Carjienter noted the 

 presence of small fibci-s in tlie oculomotor nerve, and considered 

 the ciliary ganglion ;is their destination, though they did not 

 say that all the small lilxM's entered the ganglion, (laskell 

 further noted tlie similarity in structure between the Ilird and 

 IVth ncrNcs, but said the destination of the small fibers of the 

 latter was as yet unkjiown. Tli(> Xltli nerve he described as 

 ('om])()sed of large myelinated fibers '"with a few smaller ones; 

 with no .sign of any distinct group of small fibers as in the Ilird 

 and I\'th nerves." In our preparations all three nerves showed 

 strikingly similai- cliaracteristics as to \\\v siz(> of the fibers and 

 the ratio of tlie small and large fibers. The \Tth nerve could 

 not be distinguished from the other two by a diminished number 

 of small myelinated fibers, but was characterized by the pres- 

 ence of unmyelinatetl fibers from the sym])athetic system which 

 came into inthnate contact with tlie myelinated fibers and 

 travelled distalward with them as far as the nerve was followed. 

 The only clue to an explanation of this fact is Jegorow's sugges- 

 tion that a distribution of fibers of the Vlth nerve to the eyeball 

 in the birds might be accounted for by the presence of syinpa- 



