306 The Nerves and Muscles of the Leg 
The rectus is phylogenetically the oldest part of the quadriceps. In the 
urodela, where it is represented by that part of the ilio-tibialis supplied by 
the femoral nerve, there seem to be no muscles which correspond with the 
vasti but these are differentiated in the reptiles and the higher forms. In 
some of the mammals the three vasti of the muscle are more or less fused. 
In man the chief variations found in the quadriceps result from a greater or 
less division of the primitive extensor mass into individualized parts. 
b. Nerve Variation. 
1. Variation in Origin of the Femoral Nerve. 
The femoral nerve in the great majority of instances arises in the main 
from the 22d, 23d, and 24th spinal (2d, 3d, and 4th lumbar) nerves. 
There is, however, considerable variation in the size of the nerve bundles 
derived from the 22d and 24th spinal nerves. The 21st spinal nerve 
usually contributes some fibres, and the 20th and 25th spinal nerves occa- 
sionally do so. In Table XIV there is shown the frequency with which 
certain root origins of the femoral nerve were found and the relation of 
these various modes of origin to various types of plexuses. A study of 
these variations in relation to race, sex, and side of body has revealed no 
marked associations, and therefore the tables embracing these data are 
omitted. 
2. Relations of the Branches Springing from the Femoral Nerve to the 
Nerve Roots. 
In considering the cutaneous nerves we have seen reason to believe 
that the peripheral segmental distribution of the spinal nerves disclosed 
by physiological experiments, is due to a directness of growth which a 
given spinal nerve has toward a given peripheral area so that the nerve 
can send more fibres into this area than its neighbors can and hence serves 
in the main to innervate the area. This is also probably true of muscle 
innervation. In Plate III, Fig. 3, is shown the position .of the femoral 
nerve as it enters the femoral muscle mass in a young embryo. The fol- 
lowing diagram shows a cross section of the adult femoral nerve as it 
passes under the inguinal ligament. The regions occupied by the fibres 
of the chief motor and sensory branches are outlined, while the approxi- 
mate areas occupied by the main bulk of the fibres of each spinal nerve 
are shown by stippling the position occupied by the fibres of the 23d 
spinal (3d lumbar) nerve. While the diagram is schematic it may serve 
to illustrate the relation of peripheral to spinal nerves of the femoral 
