80 Neuroanatomy: Draw It to Know It
Dermatomes
Here, we will draw the dermatomal sensory innervation
of the limbs and trunk. Note that the dermatomal maps
have broader clinical signifi cance and are simpler than
the cutaneous nerve maps of the limbs and trunk.
Draw the anterior and posterior outlines of the body.
Th en begin with the hand; show that C7 covers the
middle fi nger, C8 the medial hand, and C6 the lateral
hand and lateral forearm. Next, show that C5 covers the
upper lateral arm. Th en, show that T1 covers the medial
forearm and T2 the medial upper arm.
Next, to show the important dermatomes of the
thorax, abdomen, and pelvis, indicate that T4 covers
the nipple line, T10 covers the umbilicus, T12 covers
the suprapubic area, L1 covers the inguinal region, S2
covers the proximal external genitalia, and S3 covers the
distal external genitalia.
Now, show the sloping dermatomal coverage of the
anterior lower extremity. First, indicate that L2 begins its
descent from the superolateral anterior lower extremity;
then, indicate that the coverage of L3 and L4 crosses the
knee; then, show that L5 covers the great toe; and then,
that S1 covers the ankle and little toe.
Next, let’s draw the posterior lower extremity and
gluteal coverage. First, show that the coccyx covers the
center of the anus and then show the dermatomal rings
that surround it: the innermost is S5, then going out-
ward is S4, then S3, and then show that S2 encircles S3
but also extends down the posteromedial lower limb.
And lastly, show that S1 extends down the posterolateral
lower limb and covers the Achilles.
Finally, show that C2 covers the back of the head, C3
and C4 cover the posterior neck, and T2–L5 cover the
upper back to the buttocks (note that L2 is sometimes
listed as the lowest lumbar dermatome).
We show the trigeminal nerve sensory innervation to
the face in Drawing 13-1.^1 – 4 , 8