SPINAL NERVES OF THE FROG. 115 



In Astacus fluviatilis, on the other hand, although 

 anatomically very similar, variations in these apertures are 

 comparatively rare, Bateson* only finding 23 variations in 

 586 females and one variation in 714 males. In the Frosr, 

 variations in the RR. ventrales longi of those nerves 

 contributing to the brachial and sciatic plexuses are so 

 common and often so considerable that one sometimes 

 positively hesitates before committing oneself as to what 

 the exact normal is. Thus the brachial plexus is normally 

 constituted by the second " spinal "+ portions of the first 

 and third. But how often is the first entirely 

 independent, and who would care to state definitely, 

 without an elaborate statistical investigation, how often 

 the third should contribute to the plexus — partially and 

 obviously, wholly and invisibly, or not at all ? Again, 

 what extensive variations are there to be found in the 

 ensemble of the sympathetic system, where, for example, 

 the coccygeal ganglia may, according to Wiedersheim, 

 vary in number from one to twelve ! 



The present paper contains a description of three cases 

 of variation in the spinal nerves of the Frog, all of which 

 are, in my experience, of a very uncommon character, 

 and do not fall between the extremes of average varia- 

 tion. In two of these cases I am indebted to my friend 

 Dr. John Beard for the material, and he has, with 

 characteristic generosity, allowed me to dissect and 

 describe it. It must, however, be emphasized that any 

 macroscopic investigation of a subject such as this can 

 have only a very limited scientific value, as the path of a 

 nerve fibre has an interest altogether subordinate to its 

 exact central origin and peripheral distribution. Hence 

 no investigation of the peripheral nervous system can be 

 considered complete, or even trustworthy, unless checked 

 * " Variation," pp. 153 and 154. 



