Dunn, Noi'c Fibers of the Frog. 299 



tion of the nerve fiber in its course is found in the constantly- 

 decreasing average area for the nerve fibers at successive levels 

 in a region where no branches arise. 



7. In this series of observations it has always been found 

 that in a cross section of a nerve the area of the axis cylinder 

 was approximately equal to the area of the medullary sheath — 

 therefore all the observations made upon the area of the fiber 

 as a whole can be expressed in terms of the more functionally- 

 active portion — the axis cylinder. 



Introduction. 



The innervation of the thigh in the frog, Rana virescens 

 brachycephala, Cope, has been quite fully discussed in a paper, 

 Dunn, 1900, which embodied the results of a study carried on 

 in this laboratory. The present study is an attempt to verify 

 the results then obtained by a repetition of the investigation 

 for the thigh branches and a further determination of the num- 

 ber and diameter of the fibers innervating the shank and the 

 foot 



Tliis present investigation contains data based upon a 

 study of the right and left hind extremities of a single frog. 

 The results obtained from the two sides, while acting as mutual 

 controls, are of further value in illustrating the similarity of 

 innervation for the right and left sides of the same individual. 



Section I. Gross Anatomy of the Muscular and Cutaneous 

 Nerves Supplying the Thigh and the Shank. 



The Vllth, Vlllth and IXth^ spinal nerves unite to form 

 the plexus lumbo sacralis from which the skin and muscles of 

 the hind extremity of Rana virescens receive their innervation. 

 From this plexus the fibers pass to their destination by way of 

 two main nerve trunks, the crural and the sciatic nerves. The 

 crural nerve, the smaller of these trunks, sends branches only 

 to the thigh. The sciatic nerve, the larger of the trunks, con- 



1 Gaupp designates these nerves as the Vllllh, IXth and Xth, numbering 

 ;the spinal nerves from II to XI, inclusive. See Gaupp's edition, Ecker's und 

 WiEDERSHElM's Anatomic des Frosches, Part )I, p. 156. 



