Femoral sheath

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Here is a complete account of the femoral sheath, drawn from Gray's Anatomy for Students and Mulholland & Greenfield's Surgery.

Femoral Sheath

Definition and Shape

The femoral sheath is a funnel-shaped (cone-shaped) sleeve of fascia that surrounds the femoral vessels and associated lymphatics as they pass beneath the inguinal ligament from the abdomen into the thigh. It tapers inferiorly and merges with the connective tissue of the vessels approximately 3-4 cm below the inguinal ligament.

Formation (Wall Contributions)

The sheath is formed by downward extensions of two fascial layers of the abdomen:

Wall	Fascia
Anterior	Transversalis fascia
Posterior	Iliopsoas fascia (part of the transversalis/endoabdominal fascia)

The iliopectineal arch also contributes to the proximal sheath - it joins the iliopubic tract in forming the femoral sheath. The arch itself separates two important compartments at the inguinal ligament level:

Lacuna vasorum (vascular compartment) - contains the femoral vessels, covered by the sheath
Lacuna musculorum (muscular compartment) - contains iliopsoas muscle, femoral nerve, and lateral femoral cutaneous nerve (outside the sheath)

Contents and Internal Compartments

The sheath is divided by two vertical septa into three compartments (lateral to medial):

Compartment	Contents	Notes
Lateral	Femoral artery	Most lateral structure
Middle	Femoral vein
Medial	Femoral canal	Lymphatics + areolar tissue

Important: The femoral nerve is NOT within the sheath - it lies lateral to the sheath, separated from it by the iliopectineal arch.

Femoral sheath, ring, and canal - lateral view

Schematic view of the femoral sheath, ring, and canal. The transversalis fascia forms the anterior portion of the sheath and the iliopsoas fascia forms the posterior portion. Septae separate the vessels from each other, and the vein from the femoral canal. - Mulholland & Greenfield's Surgery

The Femoral Canal

The femoral canal is the medial-most, cone-shaped compartment within the sheath:

Contents: Lymphatic vessels, areolar tissue, and a lymph node (Cloquet's node / node of Rosenmuller) at its upper (proximal) end
Normal state: Contains only wisps of connective tissue and small lymphatic nodes
Length: Short (~1.25 cm)
Shape: Wider superiorly, narrowing inferiorly

Femoral Ring

The femoral ring is the superior (abdominal/extraperitoneal) opening of the femoral canal. Its boundaries are:

Boundary	Structure
Anterior	Inguinal ligament
Posterior	Iliopubic tract + pectineal (Cooper's) ligament
Lateral	Femoral vein (septum of the sheath)
Medial	Curved edge of transversus abdominis aponeurosis (NOT the lacunar ligament, which inserts more medially)

The femoral ring is normally closed by the femoral septum - a plug of extraperitoneal fat and areolar tissue. It is covered superiorly by peritoneum.

Clinical Significance

Femoral Hernia

The femoral canal represents a potential weak point in the lower abdominal wall. When abdominal contents (usually a knuckle of small bowel or omentum) protrude through the femoral ring into the canal and potentially beyond, a femoral hernia results.

Key facts:

More common in women (wider pelvis, broader femoral ring)
Passes below and lateral to the pubic tubercle (differentiating it from inguinal hernia, which is above and medial)
The rigid walls of the femoral ring make femoral hernias prone to strangulation
The sharp medial boundary (lacunar ligament) makes reduction difficult

Vascular Access

The femoral artery and vein - contained within the sheath - are commonly used for:

Cardiac catheterization (coronary angiography, angioplasty)
Peripheral vascular procedures
Central venous access (right heart catheterization, access to renal/gonadal veins, pulmonary artery)
Contralateral/ipsilateral limb angiography

The femoral artery can be palpated just below the inguinal ligament at the midpoint of the inguinal ligament (midway between ASIS and pubic symphysis) as it passes over the femoral head.

Summary Diagram

Femoral triangle contents - compartments of the femoral sheath

Contents of the femoral triangle showing the three compartments within the femoral sheath and the relationship of the femoral nerve lying outside it. - Gray's Anatomy for Students

Mnemonic for sheath contents (lateral to medial): NAVY

N - Nerve (femoral nerve - lateral but OUTSIDE the sheath)
A - Artery (femoral artery)
V - Vein (femoral vein)
Y - Y-fronts (femoral canal with lymphatics - the "empty" space)
Gray's Anatomy for Students, p. 676
Mulholland & Greenfield's Surgery (7e), pp. 3650-3652

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