74 BULLETIN: MUSEUM OF COMPARATIVE ZOOLOGY. 
the “strength-centre ” 4 
in displacement anteriorly ; but this is not correlated with the variations 
of the plexus, such variation usually resulting 
in the position of the pelvic girdle, — indeed most such cases were found 
in specimens in which the pelvic girdle was normally placed, — nor have 
I found any correlation between the displacement of the strength-centre 
in the brachial and lumbo-sacral plexi respectively. This is not in accord 
with the conclusions of Adolphi (96, p. 118), who says for Anura “. . . 
die beiden Extremitätenplexus, der Plexus sacralis und der Plexus brachi- 
alis, ihren Schwerpunkt in der gleichen Richtung verlegen . . .” Such 
a condition indicates that the variations in the brachial plexus are inde- 
pendent of variation in the position of the pelvic girdle ; and since there 
is no posterior displacement of the brachial plexus, nor any change of 
topography in cases where the pelvic girdle is placed on the 20th ver- 
tebra, we have evidence that there has been no interpolation of vertebre 
in the part of the column anterior to the posterior limit of the brachial 
plexus. 
THE LUMBO-SACRAL PLEXUS. 
The variations in the lumbo-sacral plexus are most conveniently 
grouped under three heads, viz. : A, those in which the girdle is attached 
to the 19th vertebra; B, those with the girdle carried by the 20th ver- 
tebra; and OC, those in which the constituents of the girdle have an 
unsymmetrical position (see the Table, page 81). 
Group A is represented by twenty specimens. The nerves here form- 
ing the plexus (Plate 1, Figs. 2, 3) are xvi to xxı inclusive, neither 
XVIL nor XXII in any case entering it. The plexus is distributed by three 
trunks. The anterior is N. ileohypogastricus (¢L/’ga.), which is the 
anterior branch of nerve xıx, and extends cephalad on the inner body- 
wall; its strength and relation to other nervous parts seem uniform. 
The middle trunk (cru.) is N. cruralis (obturator), which passes out 
between the ilium and pubis, to be distributed to the anterior part of the 
thigh. The posterior is N. ischiadicus (%sch.), which is the main trunk, 
and is distributed to the posterior part of the thigh and to the leg. 
We may distinguish in the topography of the plexus two types, based 
on differences in the source of the branches which go to form N. cruralis 
and N. ischiadicus; these I shall designate as a type and ß type. 
1 The term “ strength-centre ” has been adopted to designate that point in the 
plexus which is central with regard to the combined “strengths” of the component 
nerves, We may thus conceive the whole “strength” of the plexus as concen- 
trated at this point, the “strength-centre.” 
