290 Proceedings of the Royal Society of Victoria. 



is less marked, here the coraco-clavicular is composed chiefly of 

 nerve II. having only a tine branch from III. A modification 

 in the proportions of the fibres from the two nerves would 

 readily give rise to form i\\, in which the coraco-clavicular arises 

 as a branch from III. with a few fibres from II. passing either 

 to III. itself or to the coraco-clavicular. In form v., nerve II. 

 has no connection whatever with the brachial plexus. This is 

 what might have been predicted from the fact that IV. is taking 

 an increasing share in the brachial plexus. But when we come 

 to compare with this the Table (P) showing the thicknesses of 

 nerve II. with which these forms are found we meet with a 

 strange discrepancy. 



If we regard nerve II. as being in a state of decrease as 

 deduced from Tables 13, 18, 25, etc., we find that the order of 

 the forms in Fig. 6, according to their point of development, 

 beginning with the most primitive, should be v., iv., ii., iii. — 

 excluding i. From this we obtain the remarkable result that as 

 nerve II. decreases in size, it assumes fresh duties in addition to 

 those which it had previously. Especially is this unexpected 

 here, as since II., in a diminishing condition, takes upon itself to 

 "ive rise to the coraco-clavicular, it is difficult to conceive from 

 whence the region supplied by this branch is to derive its nervous 

 supply, as II. diminishes to a thickness of 0. Further, if in the 

 face of Tables B, 13, 18, 25 and 27, we accept the evidence of 

 Table B, 28, viz., that II. increasing in thickness, we merely have 

 the difficulty reversed, for an increase in thickness of a nerve is 

 unnecessary for a more and more restricted area of action. I 

 have considered that probably this irregularity is due to the 

 change in habit of Hyla a/irea, originally an arboreal form, to 

 a terrestrial life. In this connection, it would be interesting to 

 know whether the true Tree frogs of Europe show any deviation 

 from the general combinations of Bufo variabilis and Pelobates 

 fuscus. In the absence of a more extended knowledge on this 

 point, we may accept for the present, at least, the evidence of 

 Table O, since it seems most in accordance with all the facts 

 relating to nerves IV., III. and II. 



The increasing thickness on the part of nerves IV. and III. 

 and the apparent decreasing thickness of nerve II. together with 

 the above tables, indicate the fact that in the brachial plexus of 



