88 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
ment of nerves in the segments involved, the matter of topography of 
plexus being secondary to the determination of the segmental position 
of the appendage. 
That there is any direct correlation 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 in phylogeny and in ontogeny, much later than the nervous 
system. 
The view of the relations of nerve and skeletal parts, first hinted at 
by Fürbringer (’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 differen- 
tiated earlier than the nerve, and it is also well known that the myo- 
meres have very close relations to the vertebra, so that in any given 
segment there is an interdependence among the three systems, Hither 
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 
in 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 further that it has been shown that any one of several 
