Ca) 
74 The Nerves and Muscles of the Leg 
tral margins of the distal portion of the rectus muscles are brought into 
approximation when the symphysis pubis is formed. By the rotation of 
the hip bone the crest of the ilium is brought up against the dorsal por- 
tion of the distal margin of the abdominal musculature. The ventral 
portion of this margin becomes converted into the inguinal ligament. 
The courses of the abdominal nerves and the hypogastric and inguinal 
nerves are determined by their positions in the abdominal musculature. 
The genital nerve takes a more direct course towards its region of termi- 
nation, although it too is usually bound up for some of the distal part 
of its course with the distal margin of the abdominal wall. 
The peripheral region to which the lumbo-inguinal nerve extends is 
carried in a ventral, medial and posterior direction by the rotation of the 
limb. The main trunk of the lateral cutaneous nerve is caught by the 
rotating hip bone usually in the vicinity of the future anterior superior 
iliac spine and is carried up against the inguinal ligament. Thus by this 
rotation and shifting marked changes in the relative positions of the 
more anterior nerves arising from the lumbar plexus are brought about. 
b. Variation. 
A study of variation in the distribution of the fibre bundles of the 
spinal nerves to the various peripheral areas of the limb reveals the fact 
that any two nerves shown in Plate III, Figs. 1, 2 and 3, may be com- 
bined into a single trunk when they arise ordinarily in succession, but 
not otherwise. Thus the 12th thoracic and the hypogastric, the hypo- 
gastric and the inguinal, the inguinal and the genital, the lumbo-inguinal 
and the lateral cutaneous, the lateral cutaneous and the femoral, may be 
bound together for a greater or less part of their courses from the plexus 
to the limb. On the other hand, two or more nerve trunks may serve 
to convey fibres commonly carried in a single nerve. Separate iliac 
branches, extra lumbo-inguinal and genital nerves belong to this category 
as do also those “ ? 
middle cutaneous ” nerves which arise directly from 
spinal nerves, and the accessory obturator nerve. The frequency of varia- 
tion of this sort in the border nerves I have previously described in this 
journal, o2. In the same paper I have treated of the frequency of 
variation in segmental origin of the various border nerves. This is most 
marked. Thus the hypogastric nerve arose in 2% of instances from the 
19th and 20th spinal nerves; in 32% from the 20th; 34%. from the 20th 
and 21st; and 32% from the 21st. The iliac arose in 2.1% of instances 
from the 19th and 20th spinal nerves; 27.4% from the 20th spinal nerve ; 
