88 bulletin: museum of compaeatiye zoology. 



ment of nerves iu the segments involved, the matter of topogi-aphy of 

 plexus being secondary to the determination of the segmental position 

 of the appendage. 



That there is any direct con-elation between skeletal and nervous tis- 

 sues seems improbable, for there is no obvious reason why nerves should 

 directly influence skeletal parts, nor is it probable that skeletal parts 

 directly influence the position of nerves, since the vertebrate skeleton 

 arises, both iu phylogeny and in ontogeny, much later than the nervous 

 system. 



The view of the relations of nerve and skeletal parts, first hinted at 

 by Fiirbringer ('79), and more clearly formulated by Eisler ('92), seems 

 much more reasonable than the idea of direct correlation. According to 

 Eisler's conception, skeletal and nervous structures exhibit variations 

 independently, but in parallel directions. The control of such paral- 

 lelism rests with the musculature. The latter has very definite re- 

 lations to the nerves on the one hand, and to the skeleton on the 

 other. Bolk ('94) has shown that in Homo the position of the nerves 

 depends upon the position of the muscle segment, which is diflferen- 

 tiated earlier than the nerve, and it is also well known that the myo- 

 meres have very close relations to the vertebrae, so that iu any given 

 segment there is an interdependence among the three systems. Either 

 nerve or vertebra may vary within certain limits, without necessarily 

 affecting the other, but extensive variation of either would presumably 

 influence the other through intermediation of the musculature. 



From the nature of the case skeletal variations within narrow limits 

 are less easily observable than nervous ; but the examination of almost 

 any series of vertebrae shows variation not only serially, but also on the 

 right and left sides of the same vertebra. Such variations have been 

 recorded by Paterson ('92, pp. 523, 524) from dissections of a large 

 series (265) of human adults. 



The conception of parallel variations of skeleton and nerve under 

 control of the musculature is able to explain the fluctuations in posi- 

 tion and strength of nerves belonging to any one segment involved 

 iu the plexus. It also offers an explanation of the inverse correlation 

 of nerves of adjacent segments, seen in the weakening of the nerve 

 of one segment, accompanied by a strengthening of that of the next 

 segment ; for since many muscles are innervated by two or more 

 nerves from different segments, if one of these becomes weaker, the 

 other becomes stronger to make up the deficiency. It should be 

 remembered fui'ther that it has been shown that any one of several 



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