STRUCTURE OF THE CRANIAL NERVES 551 



substance (fig. 4). In Pal-Weigert preparations the bluish-black 

 myelin rings were separated in places by small unstained areas, 

 corresponding closely with the location of the unmj'elinated 

 fibers seen in pyridine-silver preparations. No essential differ- 

 ences were seen in the Xlth nerve of the cat and rabbit. The 

 large myelinated fibers of the spinal root of the accessory of the 

 cat were the largest of all the fibers measured, some of them 

 having a diameter of 18 micra. 



The hypoglossal nerve of the dog showed a pictm-e closely 

 resembling that of the oculomotor nerve, large and small mye- 

 linated fibers in about the ratio of three to one, measuring 11 

 to 15 micra and 3 to 6 micra in diameter. No unmyelinated 

 fibers were seen within the roots of the nerve, but slender bun- 

 dles of sympathetic fibers could be followed as they approached 

 the nerve, entered its sheath, and finally joined the nerve sub- 

 stance (fig. 5). The hypoglossal of the cat and rabbit showed 

 similar pictures. 



Nowhere did the umnyelinated fibers seen in sections of the 

 Xlth and Xllth aj^jiear evenly distiibuted among the mye- 

 linatetl fibers, as in the sj)inal nerves and the vagus; but always 

 grouped in clusters, as hi the \'Ith nerve, and in largest numbers 

 near the periphery. 



SUMMARY 



1. Unmyelinated fibers are present in the Vth, Vltli. IXth, 

 Xlth and Xllth cranial nerves. Those in che Vlth, Xlth and 

 Xllth are probably all derived from the sympathetic system. 

 In these nerves they have an arrangement characteristic of 

 sym]iathetic fibers; they are grouped in clusters, and are most 

 numerous near the periphery of the nerve. 



2. The oculomotor and trochlear nerves are strikingly similar 

 in composition; they are composed of large and small myelinated 

 fibers, without any accessions or unmyelinated fibers from the 

 sympathetic system. 



3. The abducens is similar to the Ilird and R'th nerves as 

 regards its myelinated fibers, but in addition receives a large 

 number of unmyelinated fibers from the sympathetic system, 



