68 Neuroanatomy: Draw It to Know It
The Thigh (Cont.)
Next, let’s consider some of the fi ner details regarding the
primary muscles of the thigh. First, note that in regards to
the iliopsoas muscle, the psoas muscles (psoas major and
minor) are actually innervated by direct branches from
ventral lumbar rami from L1 to L3 (and not the femoral
nerve). However, because the psoas muscles cannot be
isolated and tested clinically, they are lumped in with the
iliacus muscle, and all three are considered collectively as
the femoral nerve-innervated iliopsoas muscle.
Now, show the sciatic nerve’s innervation to the
adductor magnus muscle, and indicate that the adductor
magnus has both an adductor portion, supplied by
L2–L4, and a hamstrings portion, supplied by L4, L5.
Finally, note that the peroneal division of the sciatic
nerve innervates the short head of the biceps femoris and
that the tibial division innervates the other hamstrings
muscles: the long head of biceps femoris, semitendino-
sus, and semimembranosus. Th e peroneal innervation of
the short head of the biceps femoris is especially impor-
tant in localization because a peroneal neuropathy at the
fi bular head (the most common peroneal nerve entrap-
ment site) will spare the short head of the biceps femoris,
but a peroneal neuropathy proximal to the fi bular head
will aff ect the short head of the biceps femoris.
Now, let’s include the lesser clinical muscles of the
thigh. First, show that the femoral nerve innervates the
sartorius muscle, which is the longest muscle in the body.
It is a superfi cial muscle that crosses the thigh: it spans
from the anterior superior iliac spine to the medial knee.
Th ink of someone checking the bottom of his or her
shoe to imagine the sartorius’ action; it provides knee
fl exion in combination with hip abduction and lateral
rotation. Functionally, it serves to decelerate the lower
extremity during climbing movements.
Next, show that the obturator nerve innervates the
gracilis muscle, supplied by L2, L3. It also lies superfi -
cially and spans the medial line of the thigh, and it pro-
vides hip adduction and also knee fl exion and medial
rotation.
Th en, show that the femoral nerve innervates the
pectineus muscle, which lies in the femoral triangle;
occasionally, the obturator nerve helps innervate the
pectineus muscle, as well. Th e pectineus assists in both
hip fl exion and adduction. It cannot be isolated and
tested clinically.
Finally, show that the obturator nerve innervates the
obturator externus muscle, which is one of the short rota-
tors of the hip and which provides external rotation of the
hip in hip extension and hip abduction in hip fl exion.
Now, let’s include the sensory branches of the thigh;
we map their sensory coverage in Drawing 5-5. First,
draw the lateral femoral cutaneous nerve (aka the lateral
cutaneous nerve of the thigh), supplied by L2, L3. It pro-
vides sensory coverage to the lateral aspect of the thigh.
Th en, from the femoral nerve, draw the medial and
intermediate cutaneous nerves of the thigh, which cover
the anterior thigh and which are collectively known as
the anterior femoral cutaneous nerve. Th en, draw the
saphenous nerve, which extends down the medial leg to
the instep of the foot and provides sensory coverage to
that same area. Lastly, show that the saphenous nerve
produces the small but clinically important sensory
branch called the infrapatellar branch, which innervates
the anterior knee and which can be injured in knee
arthroscopy.
Now, draw the posterior cutaneous nerve of the thigh,
which S1–S3 supply; it covers the posterior thigh, and
also some of the pelvic and proximal leg regions. Finally,
note that the obturator nerve, itself, covers a small cuta-
neous area on the medial thigh.
Before we conclude, let’s consider the anatomic rela-
tionships of the neurovascular structures of the femoral
triangle because they are critical to know when perform-
ing femoral venous cannulation. Within the femoral tri-
angle, the nerve lies most laterally; medial to it is the
artery; and medial to it is the vein. Th e mnemonic NAVY
is helpful because it incorporates the position of the
midline genitalia, the “Y,” into the acronym.^1 – 4