270 Proceedings of the Royal Society of Victoria. 



I have found present in 3 - 2 per cent, cases in Hyla aurea, 

 in each of which it either joins XI. directly or joins the network 

 formed by the branches of X. and XI. This plexus supplies 

 branches to the Coccygeus and Levator ani muscles, and to the 

 bladder, cloaca, and oviduct ; while, in addition, the last three 

 seem sometimes to have nerve fibres from XT. only. 



The thickness of a nerve is an essential element in deciding 

 which is its more primitive, and which its more advanced, 

 condition. I have therefore drawn up a series of tables com- 

 parable to those made by Adolphi for Bnfo variabilis and Pelo- 

 bates fusais. 



As a basis for the valuation, I have taken (1) the most 

 frequently occurring thickness of nerve VI., the central nerve of 

 those three which do not send any fibres to the limbs, as thickness 

 4. As in Bufo variabilis, nerves V., VI., and VII. have generally 

 very nearly the same thickness, and as a rule in animals of 

 approximately equal size, these three nerves have also about the 

 same thicknesses. VJI. is generally the same thickness as V. and 



VI. sometimes slightly thicker or thinner. In the one case in 

 which I found any connection between it anil the sacral plexus, 



VII. had one of the smaller thicknesses noted for that nerve. 

 (2) The most frequently occurring thickness of nerve IX., in the 

 aacral plexus, is represented as thickness 9. The figures to 19 

 show, of course, only the relative thickness of the nerves, though 

 in such a manner that for each increase in the absolute thickness 

 of -1 millimetre, there is an increase of 1 in the relative thickness ; 

 e.g., thickness 4 corresponds to - 3 millimetres, thickness 5 to "4 

 millimetres and so on. 



So that this may be clearer I have drawn a curve showing 

 the exact value of the separate thicknesses to 19. Along the 

 horizontal line are placed the numbers of the relative thicknesses 

 consecutively, while the vertical represents the actual value of 

 those separate thicknesses, increased some twenty times. 



