Inguinal Canal Anatomy and Anatomy of the Abdomen

I. ANATOMY OF THE ABDOMEN

Layers of the Anterior Abdominal Wall

1. Skin
2. Superficial fascia - has a superficial fatty layer (Camper's fascia) and a deep membranous layer (Scarpa's fascia)
3. External oblique muscle / aponeurosis - fibers run downward and medially (like hands in pockets)
4. Internal oblique muscle - fibers run upward and medially (perpendicular to external oblique)
5. Transversus abdominis muscle - fibers run horizontally
6. Transversalis fascia - thin but important fascial layer lining the inner surface
7. Extraperitoneal fat
8. Peritoneum (parietal)

Rectus Sheath

• Two rectus abdominis muscles run vertically on either side of the midline, separated by the linea alba
• The rectus sheath is formed by the aponeuroses of the three flat muscles:
  • Above the arcuate line: anterior sheath = external oblique + anterior lamina of internal oblique; posterior sheath = posterior lamina of internal oblique + transversus abdominis
  • Below the arcuate line (Douglas's line): all three aponeuroses pass anterior to rectus abdominis, leaving only transversalis fascia behind it

Blood Supply

• Superior epigastric artery (from internal thoracic artery) - runs in the rectus sheath superiorly
• Inferior epigastric artery (from external iliac artery) - enters the rectus sheath below the arcuate line, lying just medial to the deep inguinal ring
• Lateral wall: deep circumflex iliac and lower intercostal/subcostal vessels

Internal Surface of the Anterior Abdominal Wall

1. Supravesical fossa - between median and medial umbilical folds
2. Medial inguinal fossa (= Hesselbach's triangle) - between medial and lateral folds
3. Lateral inguinal fossa - lateral to the lateral fold; site of the deep inguinal ring

• THIEME Atlas of Anatomy - General Anatomy and Musculoskeletal System

II. THE INGUINAL CANAL

Overview

• Males: spermatic cord
• Females: round ligament of the uterus
• Both sexes: ilioinguinal nerve and lymphatics

The Two Openings (Rings)

Deep (Internal) Inguinal Ring

• An opening in the transversalis fascia
• Located midway between the anterior superior iliac spine (ASIS) and the pubic tubercle, approximately 2-3 cm above the femoral artery pulse
• Lies in the lateral inguinal fossa
• The inferior epigastric vessels lie just medial to it
• Bounded by: the interfoveolar ligament, inguinal ligament, and lateral umbilical fold
• The transversalis fascia is invaginated here into the inguinal canal to form the internal spermatic fascia around the cord

Superficial (External) Inguinal Ring

• An inverted V-shaped (slit-like) defect in the external oblique aponeurosis
• Located lateral to the pubic tubercle
• Bounded by:
  • Medial crus (superomedially)
  • Lateral crus (inferolaterally)
  • Intercrural fibers (superiorly, connecting the crura)
  • Reflected inguinal ligament (inferiorly, completing it internally)

Walls of the Inguinal Canal

• Bailey and Love's Short Practice of Surgery, 28th Ed., p. 1087
• THIEME Atlas of Anatomy - General Anatomy and Musculoskeletal System

The Conjoint Tendon

Contents of the Inguinal Canal

• Vas deferens (ductus deferens)
• Testicular artery
• Pampiniform plexus of veins (become the testicular vein)
• Lymphatics from the testis
• Cremaster muscle (from internal oblique)
• Genital branch of the genitofemoral nerve
• Ilioinguinal nerve (runs on the cord's anterior surface)

• Round ligament of the uterus (ends in the labia majora)
• Ilioinguinal nerve
• Lymphatics

Key Nerves in the Region

Hesselbach's Triangle (Medial Inguinal Fossa)

• Lateral: Inferior epigastric vessels
• Medial: Lateral edge of rectus abdominis
• Inferior: Inguinal ligament (iliopubic tract)

Clinical Anatomy: Hernia Types

Processus Vaginalis and Its Clinical Significance

• Complete patency → indirect inguinal hernia
• Partial obliteration → hydrocele of the cord
• Obliteration at the testis only → communicating hydrocele

Summary Memory Aid

"My External Oblique And Transversalis Keeps Contents Right"

• Anterior = External oblique aponeurosis
• Posterior = Transversalis fascia
• Roof = Conjoint tendon (Int. oblique + Transversus)
• Floor = Inguinal ligament (= Poupart's ligament)

Deep ring = in transversalis fascia, lateral to inferior epigastrics Superficial ring = in external oblique aponeurosis, lateral to pubic tubercle

Skin Anatomy and Sebaceous Cysts

I. SKIN ANATOMY

Functions of the Skin

Thick vs. Thin Skin

Layer 1: EPIDERMIS

The 5 Layers of the Epidermis (deep to superficial)

• Single layer of cuboidal to low columnar cells resting on the basal lamina
• Contains stem cells (keratinocytes) that divide mitotically - the regenerative layer
• Also contains melanocytes interspersed between basal cells
• Cells connected to basal lamina by hemidesmosomes and to each other by desmosomes
• Basophilic cytoplasm due to abundant ribosomes

• Several cells thick; cells are larger and more polygonal
• Named for the numerous short "spines" (cytoplasmic processes connected by desmosomes) visible on light microscopy
• Cells contain tonofilaments (keratin intermediate filaments) bundled into tonofibrils
• Upper spinosum begins producing keratohyalin granules and lamellar bodies

• 3-5 layers of flattened cells containing prominent keratohyalin granules (dark purple on H&E)
• Cells discharge lamellar bodies into the intercellular space - these create the water permeability barrier
• Cells exhibit apoptotic nuclear morphology; nucleus begins to degrade
• pH here is approximately neutral (pH 7.17)

• Thin, glassy, translucent zone between the granulosum and corneum
• Only present in palm and sole skin

• 15-20 layers (up to hundreds in thick skin) of dead, anucleate, keratinized cells (corneocytes)
• Cells filled with soft keratin bundled in a protein matrix
• pH is acidic (4.5-6.0) - allows kallikrein (KLK) serine peptidases to digest desmosomes, causing desquamation
• Provides the main barrier against desiccation and infection

Keratinocyte lifecycle: Stem cell divides in stratum basale → migrates upward → synthesizes keratin → forms lamellar bodies in spinosum → discharges water barrier lipids in granulosum → nucleus and organelles degrade → becomes dead corneocyte → desquamates at surface. Total transit time: ~2-4 weeks. - Histology: A Text and Atlas, p. 1285-1293

Non-Keratinocyte Cells of the Epidermis

Layer 2: DERMIS

Two Sublayers

Dermal Contents

• Hair follicles with attached arrector pili muscles
• Sweat glands (eccrine and apocrine)
• Sebaceous glands
• Blood vessels arranged in superficial and deep plexuses
• Sensory receptors: Meissner's corpuscles (papillary dermis, fine touch), Pacinian corpuscles (reticular/hypodermis, deep pressure/vibration), Ruffini endings, free nerve endings
• Lymphatics
• Fibroblasts, mast cells, macrophages

Layer 3: HYPODERMIS (Subcutaneous Tissue)

• Lies deep to the dermis; equivalent to the subcutaneous fascia of gross anatomy
• Contains variable amounts of adipose tissue in lobules separated by connective tissue septa
• Also contains deeper portions of hair follicles, sensory receptors (Pacinian corpuscles), neurovascular bundles
• Functions: thermal insulation, energy storage, cushioning, tethering skin to underlying fascia/muscle
• Blood vessels in the dermis and hypodermis form plexuses between each layer, all originating from source vessels via fine perforating vessels - Bailey and Love's, p. 1087 (plastic surgery section)

Skin Appendages (Epidermal Derivatives)

• Holocrine glands (entire cell disintegrates to release secretion = sebum)
• Almost always associated with hair follicles - their duct opens into the upper portion of the hair follicle (pilosebaceous unit)
• Absent from palms and soles
• Sebum lubricates hair and skin, has mild antimicrobial properties
• Most numerous on face and scalp

• Simple coiled tubular glands opening directly on skin surface
• Distributed almost everywhere; most numerous on palms and soles
• Produce watery sweat for thermoregulation

• Open into hair follicles (not directly to skin surface)
• Located in axillae, groin, areola, perianal region
• Produce viscous secretion; odorless until acted upon by bacteria

• Invaginations of epidermis into the dermis
• Produce hair via a matrix of actively dividing cells
• Have a bulge region (lower permanent portion) containing stem cells
• Attached to arrector pili (smooth muscle) - contraction causes "goosebumps" (cutis anserina)

• Modified stratum corneum, produced by the nail matrix

II. SEBACEOUS CYST (Epidermoid Cyst)

Terminology

Definition and Pathogenesis

• Benign subepidermal nodules filled with keratin (not sebum)
• Formed by plugging of the follicular orifice (infundibulum) → the follicular epithelium invaginates and proliferates, forming a cyst lined by stratified squamous epithelium
• The cyst wall is the keratinizing epithelium itself - this is the "capsule"
• Contents: white, cheese-like, foul-smelling keratin material (often called "sebaceous material" colloquially)

Location in the Skin

• Its wall made of stratified squamous epithelium (continuous with the epidermis at the punctum)
• Contents occupying the space within the dermis
• A punctum (central pore) = the blocked follicular opening, visible as a small black dot on the surface

Clinical Features

• Roberts and Hedges' Clinical Procedures in Emergency Medicine, p. 887
• Fischer's Mastery of Surgery, p. 6205

Complications

1. Infection - increasingly caused by MRSA; cyst presents as tender, fluctuant, erythematous mass
2. Rupture - releases keratin into dermis → intense foreign body inflammatory reaction ("rupture abscess")
3. Recurrence - if capsule not fully removed
4. Rare: malignant transformation to squamous cell carcinoma (extremely rare)

Treatment

• Complete surgical excision (elliptical incision centered on the punctum, dissecting out the entire capsule intact)
• An elliptical incision is made centered on the punctum, approximately a third of the cyst diameter in width
• The line of cleavage is sought and the cyst shelled out intact
• If rupture occurs during dissection, all fragments of epithelium must be removed or recurrence is certain
• Dead space closed with absorbable sutures, skin closed - Pye's Surgical Handicraft, 22nd Ed.

• Step 1: Incision and drainage (I&D) - express the thick sebaceous material manually (too thick to drain spontaneously)
• Step 2 (delayed): Excision of the pearly white capsule after inflammation has settled, to prevent recurrence
• Alternatively: primary excision at the initial sitting (evidence shows faster healing, fewer days of pain, less scarring - Kitamura et al. RCT, n=71) - Roberts and Hedges' Clinical Procedures in Emergency Medicine

Key surgical principle: Loop drainage is not applicable to sebaceous cysts because the capsule must be removed - this distinguishes it from a simple abscess.

Summary: Sebaceous Cyst vs. Dermoid Cyst