54 ELECTRO-PHYSIOLOGY CHAP. 



more or less remote parts of the central nervous system. The 

 ramification of (peripheral) nerve-fibres is, however, most striking 

 in the electrical organs of certain fish (vide Electrical Fishes}, 

 which must in the majority of cases be regarded as transformed 

 muscles. Thus in Malapterurus the whole of the paired organ, 

 consisting of thousands of separate plates, is supplied by a single 

 nerve - fibre, which must accordingly bifurcate an immense 

 number of times in order to subserve each electrical plate, and 

 the same is found in other electrical fishes. These are the 

 cases which throw most light on the functional significance of 

 the bifurcation of peripheral nerve-fibres. It is obvious that this 

 must occur most freely in cases where no isolated activity of 

 the end-organ is required, but where, on the contrary, the indi- 

 vidual elements are affected as far as possible simultaneously, 

 and for the same purpose. This also applies to muscles which 

 subserve movements of a low degree of complexity, e.g. the 

 rigidly-imprisoned muscles of Crustacea and insects, where the 

 nerves display a wealth of ramifications (Fig. 150). 



The same relations exist, according to Stannius, in most 

 motor nerves of fish, and in those to certain muscles of Amphibia 

 (Fig. 159), where they are again explained by the low grade of 

 co-ordinated movement in these animals. The higher the latter, 

 the more a muscle is appointed, bundle by bundle, to engage in 

 co-ordinated movements the more local will be the distribution, 

 and the less numerous must be the ramifications, of its motor 

 nerve-fibres (Fig. 159). 



As regards the mode of division in the non-medullated fibres 

 of invertebrates, there is a great diversity, both in the central 

 organs and in the periphery. Every kind of transition exists, 

 from simple dichotomous branching to the richest arborisation. 

 As two main types we may take the ramifications of the axis- 

 cylinder processes from the central ganglion-cells in the ventral 

 cord of worms and Crustacea, and the muscular nerves of the latter. 

 Both these types occur along the same axis-cylinder, the central 

 and more or less richly branched portion being separated from the 

 peripheral expansion by an undivided or but little branched part. 

 Within the central organs the thick fibres give off numerous and 

 very fine lateral branches, which again arboresce freely, so that 

 the difference in calibre between stem and branches of the axis- 

 cylinder is often considerable, while in the peripheral expan- 



