216 



in 23 out of 124 plexuses (18,1 7o)- Of these 10 occurred in the 19 

 plexuses in which the XXV spinal nerve contributed to the anterior 

 nerves of the Umb, 12 in the 105 "normal" plexuses and one in the 

 three plexuses in which the third lumbar nerve contributed to the 

 sacral plexus. Paterson (op. cit.) formed a natural separation of the 

 sciatic nerve in three out of 23 plexuses (13 "/o). 



As a rule the peroneal nerve passes through the piriformis muscle 

 in these cases. In but one instance did it pass wholly above the 

 pyriformis muscle. The occurrence of these plexuses with divided 

 sciatic nerve among the various types of the lumbo -sacral plexus 

 offered a very good control for the artificial separation of undivided 

 sciatic nerves. The conditions of relationship of peroneal to spinal 

 nerves as revealed by natural and artificial separation were in all 

 instances essentially the same. Neither sex, race, side of body nor 

 skeletal conditions seemed to have intimate association with i':he natural 

 separation of the sciatic nerve. 



The Relation of the Position of the Plexus to the Side 



of the Body. 



In table 9 is shown the relation of the various types of plexus 

 to the side of the body. In tables 2 — 8 under "b" is given the fre- 

 quency with which the various types of plexus were found on each 

 side of the body in 246 instances. A glance at the latter tables will 

 show that the various types of plexus occurred with about equal 

 frequency on each side of the body except that the extremely anterior 

 type "B" occurred with greater frequency on the right side, the 

 extremely posterior type "G" with greater frequency on the left side. 

 Table 9 also shows that where there is a difierence in the type of 

 plexus on each side of the body that on the left usually occupies the 

 more distal position. In a considerable number of instances, however, 

 the plexus on the left side is situated the more anteriorly. 



The Relation of Sex to the Position of the Lumbo-sacral 



Plexus. 



This may be seen in tables 2—8 under "b" (see also text-table 

 below p. 217). The "normal" type D is more frequent in females. 

 Distally or proximally placed plexuses are more frequent in males. 

 This difference, especially as regards the proximally placed plexuses, 

 is not well marked. 



