320 MANUAL OF HISTOLOGY. 



of this repeated splitting-up of the primitive fibres further, a considerable 

 number of terminal filaments could be formed from a few of the latter (2). 

 So far the difficulty of following up the nerves is but small, if we take, 

 for instance, the platysma of the frog. 



But experience teaches that this repeated division of motor nerve fibres 

 is peculiar to the lower orders of vertebrate animals. In fishes also it. may 

 give rise to the formation of more than one hundred terminal filaments 

 from one such ; and primitive fibres are by no means rare, which supply 

 upwards of fifty muscular fibres. 



Among the higher orders of vertebrata, on the contrary, this splitting 

 up becomes less and less frequent, so that its occurrence is only excep- 

 tional among the mammalia. The number of muscle and nerve fibres 

 becomes almost the same, a fact of great physiological importance. 



If we examine one of the thin transparent muscles of a frog, we find 

 without any difficulty the small trunks of the nerves, which have entered 

 the substance of the latter, lying sometimes obliquely, sometimes parallel 

 to its fibres, and giving off numerous branches and anastomotic twigs. 

 In human and mammalian muscle likewise a plexiform interchange of 

 fibres between adjacent twigs may be observed. 



At the points of division of the latter, and especially when they have 

 attained a considerable degree of fineness, and contain but few primitive 

 fibres, we not un frequently perceive the manner in which a nerve-fibre 

 suddenly breaks up into two or more branches, with a contraction generally 

 at the point where the latter part from one another. These branches 

 possess the same medullated appearance as the parent stem, and pass 

 onwards, according to the character of the whole nerve, more or less 

 divergent. Deceptive appearances, however, are matters of possibility here. 

 At those points, however, where, in the further course of their ramifica- 

 tions, the nerve fibres come to lie either singly or in extremely small 

 number together, traversing thus the muscle in a usually oblique direction 

 (309, a), their further subdivision may be most distinctly observed. 



This most frequently occurs by division into two threads, more rarely 

 into three or four. The latter may either correspond to one another in 

 breadth, or are unlike in that respect (a below and in the centre). Con- 

 tractions at the points of division may not be present, or if so, only 

 slightly marked ; or again, they may be very strongly pronounced. But 

 complete solution of continuity, to such an extent that the empty primitive 

 sheath alone remains, is always an artificial production. The axis 

 cylinder, on the other hand, divested entirely of medullary sheath, appears 

 frequently as a natural formation. 



The division of this axis cylinder is for the rest hardly anything more, 

 than the separation of the original primitive fibrillse" into two new 

 fasciculi of smaller diameter. 



In consequence of this repeated division, the nerve fibres (which 

 possessed at the outset a medium diameter of 0-0142-0-.0113 mm. and a 

 double contour) become gradually reduced in calibre down to 0*0056 mm., 

 and lose their double outline (6). 



Finally, however, we observe the terminal twigs of 0'0045-0'0038 mm. 

 in breadth approaching the single muscle fibres in the form of free axis 

 cylinders having lost their dark medullated appearance, and apparently 

 ending here in two short branches. 



It was formerly believed that we had here in these pale fibres the trco 

 termination of the nerves, while it still remained a matter of uncertainty, 



