WAITE : PLEXI OF NECTURUS. del 
Nerve xxi has here topographically the same relations as nerve XXI 
in group A, but its strength relative to the rest of the plexus is much 
less, and in two cases (in seven) it fails to send a branch into the plexus, 
The position of the first heemal arch is in every case on the 23d 
vertebra. 
At first sight the whole plexus in group B seems to have moved with 
the girdle one segment caudad as contrasted with the position in group 
A, a condition which might be explained by the interpolation of a pre- 
sacral segment; but for several reasons I do not believe such an expla- 
nation to be sufficient. If a presacral segment had been interpolated, 
thus entailing a change in position to the extent of an entire segment, we 
should expect still to see variations in the position of the first hemal 
arch. In one half the cases in group A (see Table, p. 81) the first heemal 
arch was on vertebra 23, three segments posterior to the main nerve of 
the plexus. Hence in group B, if the interpolation hypothesis be true, 
the hemal arch ought to occur, at least occasionally, in the same rela- 
tive position, i. e. on the 24th vertebra, instead of maintaining as it 
does a constant position on vertebra 23, only two segments posterior to 
the main nerve (xx1) of the plexus. The specimens examined by Parker 
(96, p. 712) show the same result, i. e. a constancy of position of the 
first haomal arch on the 23d vertebra in all specimens bearing the girdle 
on the 20th vertebra. Bumpus (97, p. 473), however, among thirty-five 
specimens having this position of the girdle finds three (9%) in which 
the first heemal arch is on vertebra 24. This shows that such a position, 
though possible, is rare. 
Again, we should not expect the tendency of the last trunk nerve 
(xvi in group A, XIX in group B) to enter the plexus to be so much 
greater in group B (43%) than in group A (15%), nor should we expect 
to find its strength in group B distinctly exceeding that of the ordi- 
nary spinal nerves, as it does, since there is no indication of such an 
excess in group A. 
If interpolation has occurred, nerve xxır should show about the same 
average relative strength as the element (xx1) corresponding to it in 
group A. This is not found to be so, for xxii in group B is relatively 
and absolutely much weaker than XXI in group A, and by completely 
failing in two cases (28%) to enter the plexus shows an inconstancy 
not seen in the corresponding nerve (xx1) in group A. This leads 
to the conclusion that in group B this most posterior nerve (xxii) 
is not an essential element of the plexus. 
Nerves xx and xxı in group B correspond, in position relative to the 
