Variations in Spinal Nerves of Hyla aurea. 283 



Table D and Fig. 2 show a steady progression in the manner 

 in which the cruralis originates. Since nerve VIII. is in a 

 condition of increasing thickness, we learn from Table D that the 

 most advanced form is viii., this being the only form occurring 

 with the highest thickness, 8, and also since its greatest 

 frequency, even excluding that with thickness 8, is at least with 

 as high a thickness (7) as that of any other form. Further, we 

 learn from the same source, that form i. is the most primitive, its 

 frequency reaching a maximum with the lowest thickness 3, and 

 gradually decreasing up to thickness 5. Referring to Fig. 2, we 

 see that these two extremes point to the same kind of advance- 

 ment as took place in the ileo-hypogastric, in other words, we 

 find that in form i., the most primitive, the cruralis consists of 

 fibres from nerve IX. only, while in form viii., the most advanced, 

 the fibres come from nerve VIII. only, a transference of 

 function from IX. to VIII., continuing that of the ileo-hypo- 

 gastric, which, being nearer the exit of the nerve from the spinal 

 cord, seems to lead the way in this respect. Of the intermediate 

 stages, it would seem from the table that ii. is the most primitive, 

 and iii. the next, iv., v., vi. and vii. following in succession. 

 Fig. 2 shows us the following characters for each of these several 

 forms. In form ii., we see that the cruralis is formed of chiefly 

 IX. fibres, with a small branch from VIII. In form iii., similar 

 relations exist between the fibres from IX., and from VIII., with 

 the addition of a certain number of fibres from X. The next 

 advance is seen in the greater proportion in the cruralis of fibres 

 from VIII., which equals that of the IX. fibres in both iv. and 

 v., the cruralis in iv. showing fibres from X., which are absent in 

 v. Up to this, forms i., ii., iii., iv., and v. have occurred from 

 the lowest thickness of VIII. upwards. Form vi., however, 

 does not occur with the lowest thickness 3, showing the next step 

 upwards, while form vii. in the table shows another similar step 

 in advance. In Fig. 2 these two forms vi. and vii. exhibit the 

 same relationships to one another as were seen in the two 

 previous pairs. Thus, in both, the cruralis consists chiefly of 

 VIII., with only a small branch from IX., fibres from X. 

 entering into its composition in form vi., but not in form vii. 

 Bearing in mind the general transferrence in the origin of the 

 cruralis from IX., and probably previously from X., though of 



