WAITE: PLEXI OF NECTURUS. 75 
In the a type (Plate 1, Fig. 2) N. cruralis (eru.) is formed from the 
union of the posterior branch of xıx with a small anterior branch of xx, 
while N. ischiadicus (vsch.) is formed by union of the remaining part 
(posterior branch) of xx with the anterior branch of xxi. 
In the ß type (Plate 1, Fig, 3) N. cruralis (eru.) is the middle branch 
of xıx, which may (three cases out of ten) receive a delicate branch 
from xvm ; while N. ischiadicus (isch.) is formed by union of the small 
posterior branch of xIx, all of xx, and the anterior branch of XXI, 
It will be noticed that the a type (Fig. 2) presents, as compared with 
the ß type, a tendency to a forward migration of the plexus as a whole, 
indicated by part of nerve xx trending forward into the next segment to 
enter N. cruralis ; on the other hand, the £ type (Fig. 3) shows a back- 
ward tendency, since nerve xIx sends a branch into the next posterior 
segment to enter N. ischiadicus, and also nerve xviir occasionally enters 
into N. cruralis. 
The successive spinal nerves in the region of the plexus may now be 
considered individually. 
Nerve xvıır makes only a small and inconstant contribution to the 
plexus, since in three cases only, — all in the 8 type, — it gives a deli- 
cate branch to N. cruralis. Its strength is nearly constant, and is about 
the same as that of the ordinary spinal nerves anterior to it. This 
nerve may therefore be considered as a nearly typical spinal nerve. 
Nerve xıx presents much variation in its strength relative to the 
other nerves of the plexus, It may equal xx, —six cases in twenty, — 
or it may have only + the strength of that nerve. Its average strength 
as compared with nerve xx (the main nerve of the plexus) is between 
} and 2, and this is the relation usually found. . Besides the ileo- 
hypogastric branch, which is constant, this nerve possesses one (type a, 
Fig. 2) or two (type ß, Fig. 3) branches. In the 8 type the middle 
(cruralis) and posterior branches show considerable variation in strength 
in relation to each other. 
Nerve xx is the chief nerve of the plexus. Its strength is usually 
about eight times that of an ordinary spinal nerve. In the a type, it 
gives off a branch anteriorly, which joins the posterior branch of nerve 
XIX, these together forming N. cruralis. This anterior branch is always 
much weaker than the remaining (posterior) one, which forms the major 
part of N. ischiadicus, In the £ type the entire nerve becomes a part 
of N. ischiadicus. 
Nerve xxr has a uniform relation to the plexus in the two, types. Its 
stronger anterior branch enters N. ischiadicus, while the weaker posterior 
