Variation* in Spinal Nerves of Hyla mwea. 287 



We have yet to describe in connection with the sacral plexus, 

 the further relations which a decreasing thickness of XI. bears 

 to the advancement of the nerves in this region. 



According to Table L, nerve XI. being in a state of decrease 

 in thickness, we see that form i. is the most primitive, and form 

 iii. the most advanced, form ii. being an intermediate stage. 



Fig. 4 will show the bearing of this on the results of previous 

 tables. In form i. we see the closest connection of XL with the 

 sciatic nerve, the whole of nerve XL entering into the composi- 

 tion of the sciatic. In ii., we have nerve XL entering the sciatic, 

 or joining X. by two or more trunks, while in form iii., XL has 

 no share in the sciatic at all, merely joining branches of that 

 nerve which supply only the viscera at the posterior end of the 

 body with some of their muscles. 



On comparing these results of observations on the sacral plexus 

 of Hyla a urea with those made by Adolphi on Bufo variabilis, 

 and Pelobates fuscus, it will be seen that there is a marked 

 similarity with those results, viz., that the centre of gravity of 

 the sacral plexus is gradually moving towards the head, or in 

 other words, nerves VIII. and IX. increase in thickness as X. 

 and XL decrease in thickness, and pari passu with this change 

 in size and consequent advancement, nerves VIII. and IX. 

 are gradually losing their original relations to the body wall and 

 are assuming a greater importance in the innervation of the leg. 

 It would further appear from the tables relating to the most 

 posterior spinal nerves of Hyla aurea, especially XL, that there 

 is at the same time a change whereby these nerves are losing 

 their connection with the leg, and are, and will supply, in its 

 place, the viscera of the hinder part of the trunk, perhaps 

 preparatory to dying out altogether, as XII. is now doing. 



It remains to be seen what relations the thickness of the 

 anterior nerves of the spinal column bears to their form. 



Since we found in Tables B, 20 and 23, that IV. is increasing 

 in thickness, then the most primitive form of that nerve will occur 

 most frequently with the smaller thicknesses of the nerve. In 

 Table M, therefore, form i. is the most primitive, and form iv. 

 the most advanced, forms ii. and iii. being intermediate stages. 



