76 BULLETIN : MUSEUM OF COMPARATIVE ZOOLOGY. 
branch is distributed to the wall of the pelvic cavity. The strength of 
the whole nerve before branching varies from } to 3 that of nerve xx, 
the average being between 4 and 4; i. e. it is usually weaker than nerve 
XIX, but may (four cases in twenty) equal that nerve in strength. 
To gain evidence on the relation of the plexus to the skeleton, I have 
in each specimen determined the position of the first vertebra which 
bears a heemal arch, for this is often assumed as a fixed point in discuss- 
ing variations in the vertebral column. By reference to the Table on 
page 81, (see also Parker, ’96, p. 712, and Bumpus, ’97, p. 457,) it is seen 
that the position of the first heemal arch is variable; but, as the fol- 
lowing tabulations show, there is no correlation between these variations, 
and those of the “ strength-centre” of the plexus as represented by the 
a and ß types. 
a type, with first heemal arch on vertebra 22 = 4 cases, 
a “ 113 66 6“ 173 93 = 6 113 
ß [74 66 66 “ 66 22=6 «“ 
B 174 113 [13 66 114 23 = 4. 66 
Group B includes the individuals having the sacrum borne on the 
20th vertebra, and of these there were seven specimens. The nerves 
involved in the plexus (Plate 1, Fig. 4) are xıx to xxu inclusive. Nerve 
xvIII in no case entered the plexus. In three cases a delicate branch 
from xIx was contributed to the formation of N. cruralis. Nerve xIx 
occupies the same relative position as did nerve xvi in group A, but 
shows more of a tendency to enter the plexus (three cases in seven, 
as against three cases in twenty in group A) than did the clement 
in the same position in group A. Moreover, it is distinctly (a half) 
stronger than are the ordinary spinal nerves. For these reasons it 
must be considered as belonging to the plexus. 
Nerve xx corresponds in position and distribution to xıx in group 
A. In every case it branches into three parts, the anterior forming 
N. ileohypogastricus, the middle N. cruralis, and the posterior entering 
N. ischiadicus ; i. e. the 8 type of group A only is represented, but with 
the modification that at first sight, the whole plexus is apparently one 
segment posterior to the position in the 8 type specimens of group A, 
The whole of nerve xxr in each case enters into the formation of N. 
ischiadicus, for it sends no important branches cephalad or caudad. Its 
strength averages six times that of an ordinary spinal nerve, i. e. it is a 
fourth weaker than the nerve (xx) occupying the corresponding position 
in group A. Its average strength is about 14 times that of the next 
anterior nerve, while in group A the ratio is 12 to 1. 
