No. I.] THE CRANIAL NERVES OE AMPHIBIA. 127 



nents can be traced with certainty ; that of the second one is 

 not so easily made out, as will be seen below. This is partly 

 owing to the fact that the fibres of W\b are remarkable for 

 their uniform coarseness and heavy medullary sheath, and those 

 of W\aa for their fineness and very thin sheath. On the 

 other hand, those of W\ab, though tolerably coarse and well 

 sheathed, do not possess such well-marked characteristics. 



As the R. hyomandibularis proceeds ectad, flattened out 

 under the quadrate, W\aa comes to occupy the inner side of 

 the nerve, next to VII ab, while outermost is VII b, which also 

 overlies W\ab. This is about their relation when joined by 

 the communicating branch from the IX + X (690), and is 

 shown in Fig. 27. This latter branch occupies, as is seen in 

 the figure, the outer third of the joint nerve. It is composed 

 of mixed fibres, similar to those of the Trigeminus. 



As the R. hyomandibularis emerges from beneath the car- 

 tilage it acquires a round outline, as seen in transverse section, 

 but the relations of the components are much the same, except 

 that the portion from the IX + X takes up a somewhat more 

 dorsal position, pushing mesad ^ VII b. In Fig. 29 a 

 small twig is seen leaving the ventral side of the nerve and 

 deriving its fibres from W\ab (483). This twig passes ectad 

 close under the nerve, and after proceeding cephalad a short 

 distance enters a vertical muscle just outside the nerve, which 

 is identified as the M. orbitohyoideus described by F. E. Schulze 

 (54). A twisting and flattening of the nerve now takes place, 

 so that the positions seen in Fig. 30 are assumed. W\aa is 

 now uppermost, y^NWb and NWab next, while the compo- 

 nent from the IX + X is ventral. Along in this part of the 

 nerve it is at times difficult, for the reasons mentioned above, 

 to trace VII ^<^. There is little doubt, however, but that its 

 course is as given here. 



As indicated in Fig. 30, the nerve divides into two parts. 

 The ventral of these two divisions, the R. hyoidcns, is com- 

 posed largely of fibres from the IX + X, with the addition of 

 a bundle from VII ^^. It proceeds ventrad (433) and caudad 

 through the M. orbitohyoideus, and then turns mesad. It then 

 divides (508), as indicated in the chart, one part supplying the 



