WAITE: PLEXI OF NECTURUS. 79 
the movement of the entire plexus is but the summation of the move- 
ments of its elements, the individual nerves. (2) Nerve xxi is so incon- 
stant and contributes so feebly to N. ischiadicus, that, in compensation 
for this deficiency, all of xx1 is taken up to form N. ischiadicus, and thus 
it can contribute nothing to N. cruralis. Again, I have shown that, in 
group B, xxı is relatively weaker, and xx relatively stronger than the 
corresponding elements of the plexus in group A. Hence a contribution 
from xx to N. ischiadicus (in group B) serves to make up the deficiency 
arising from the inconstancy and stronger posterior tendency of xxi 
(cf. Fig. 4). (3) It is to be noticed that in group A (Figs. 2 and 3), 
xxı always entered N. ischiadicus and that nerve only, while xx was 
(see Table) in many cases (a type) divided between N. ischiadicus and 
N. cruralis. This gives to xx the definite relation of a component part 
to N. cruralis in group A, a relation which might be expected to be 
retained after migration, i. e. in group B. But in group A XxI in no 
case had any relation to N. cruralis. It is therefore less probable that 
in the new condition xxı should form a new connection entering N. oru- 
ralis, than that it should give its entire strength to N. ischiadicus, as it 
does, with which it had the definite relation of a component part in the 
old condition. The general backward movement of the parts of the 
plexus, and the former (a type of group A) branched condition of xx, 
may account for the fact that this nerve sends a branch to N. ischi- 
adicus in this post-migration condition, 
These considerations lead me to believe that the ß type is the only 
condition to be expected in group B, if the plexus has reached its posi- 
tion there by means of migration of the locus of its strength-centre. If 
however, interpolation of a presacral segment has occurred, we should 
expect to find about the same variations in type of topography in group 
3 as in group A. These do not occur, therefore the persistence of the 
B type of topography in group B; the inconstancy of nerve XXI, and 
the added activity of xıx in their contributions to the plexus; together 
with the slight variation in the position of the first hemal arch shown 
in this group ;—all combine as evidence against the theory that the 
position of the pelvic girdle in group B is the result of interpolation of a 
presacral segment. 
Group C includes three individuals (10% of the total number ex- 
amined), in which the attachment of the girdle is unsymmetrical. 
Of these one (Plate 2, Fig. 5) bore the right sacral rib on the 19th 
vertebra, the left on the 20th, while the first hemal arch was on the 
