WAITE : PLEXI OF NECTUEUS. 75 



111 the a type (Plate 1, Fig. 2) X. cruralis (cru.) is formed from the 

 union of the posterior branch of xix with a small anterior branch of xx, 

 while X. ischiadicus (isch.) is formed by union of the remaining part 

 (posterior branch) of xx with the anterior branch of xxi. 



In the yS type (Plate 1, Fig, 3) X. cruralis (cru.) is the middle branch 

 of XIX, which may (three cases out of ten) receive a delicate branch 

 from XVIII ; while X, 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 /? 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 X. cruralis ; on the other hand, the yS type (Fig. 3) shows a back- 

 ward tendency, since nerve xix sends a branch into the next posterior 

 segment to enter X. ischiadicus, and also nerve xviii occasionally enters 

 into X. cruralis. 



The successive spinal nerves in the region of the plexus may now be 

 considered individually. 



Xerve xviii makes only a small and inconstant contribution to the 

 plexus, since in three cases only, — all in the jS type, — it gives a deli- 

 cate branch to X. 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. 



Xerve xix 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 §, 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 (3, Fig. 3) branches. In the /3 type the middle 

 (cruralis) and posterior branches show considerable variation in strength 

 in relation to each other. 



Xerve 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 X. cruralis. This anterior branch is always 

 much weaker than the remaining (posterior) one, which forms the major 

 part of X. ischiadicus. In the p type the entire nerve becomes a part 

 of N. ischiadicus. 



Xerve xxi has a uniform relation to the plexus in the two types. Its 

 stronger anterior branch enters X. ischiadicus, while the weaker posterior 



