Small Animal Abdomen: Comprehensive Veterinary Anatomy Guide

Anatomical Boundaries of the Abdominal Cavity

  • General Definition: The abdominal cavity is bounded by muscle and bone, lined by serous membranes, and contains the viscera.
  • Cranial Border: Formed by the diaphragm muscle.
  • Caudal Border: Formed by the pelvic inlet.
  • Dorsal Border: Composed of the vertebrae, sublumbar muscles, and the crura of the diaphragm muscle.
  • Lateral Border:
    • Intrathoracic portion: Formed by the ribs, costal arch, and intercostal muscles.
    • Extrathoracic portion: Formed by the muscles of the abdominal wall.
  • Ventral Border: Formed by the rectus abdominis muscle.

Blood Supply to the Abdominal Wall

The abdominal blood supply is organized by topographic quadrants:

  • Caudodorsal Quadrant: Supplied by the deep circumflex iliac artery, which arises directly from the aorta.
  • Cranioventral Quadrant: Supplied by the cranial epigastric artery and the cranial superficial epigastric artery.
  • Caudoventral Quadrant: Supplied by the caudal epigastric artery and the caudal superficial epigastric artery.
  • Craniodorsal Quadrant: Supplied by the phrenicoabdominal artery, which consists of the caudal phrenic artery and the cranial abdominal artery. This artery arises from the aorta.

Blood Supply to the Mammae

  • Cranial pathway: Internal thoracic artery →\rightarrow Cranial epigastric artery →\rightarrow Cranial superficial epigastric artery.
  • Caudal pathway: External pudendal artery →\rightarrow Caudal superficial epigastric artery.

Somatic Innervation (Nerve Supply)

  • Origins: The walls of the abdominal cavity are innervated by the ventral branches of spinal nerves T13T13 through L4L4. These branches are divided into medial and lateral branches, with lateral cutaneous branches arising from the lateral branches.
  • Specific Lumbar Spinal Nerves (visible on the lateral surface of the transversus abdominis muscle):
    • Costoabdominal nerve: T13T13
    • Cranial iliohypogastric nerve: L1L1
    • Caudal iliohypogastric nerve: L2L2
    • Ilioinguinal nerve: L3L3
    • Lateral cutaneous femoral nerve: L3/L4L3/L4

Branches of the Abdominal Aorta

The abdominal aorta features several paired arterial branches:

  • Lumbar arteries (segmental).
  • Phrenicoabdominal arteries.
  • Renal arteries.
  • Testicular/ovarian arteries.
  • Deep circumflex iliac arteries.
  • External iliac arteries.
  • Internal iliac arteries.
  • Median sacral artery: A single artery located at the terminus of the aorta.

The Peritoneum

  • Definition: A mesothelial layer divided into three regional components:
    • Parietal peritoneum: Lines the body wall.
    • Visceral peritoneum: Covers the organs.
    • Connecting peritoneum: Double sheets extending between components, connecting parietal to visceral parts (includes mesenteries, omenta, and ligaments).

Connecting Peritoneum and Specialized Folds

  • Omentum (Extended Mesogastrium): Attaches the stomach to the body wall or other organs.
    • Greater Omentum: Attaches the greater curvature of the stomach to the dorsal body wall. It extends caudally as the superficial leaf between the jejunum and ventral abdominal wall, turns dorsally near the pelvic inlet, and returns as the deep leaf dorsal to the stomach. The deep leaf contains the left lobe of the pancreas.
    • Lesser Omentum: Extends from the lesser curvature of the stomach to the porta of the liver. It covers the papillary process of the liver and attaches the liver to the duodenum via the hepatoduodenal ligament. It contains the portal vein, hepatic artery, and bile duct.
  • Mesoduodenum: Originates from the dorsal abdominal wall and root of the mesentery. It extends to the duodenum and encloses the right lobe of the pancreas. It is continuous with the mesojejunum.
  • Duodenocolic Fold: A secondarily attached fold connecting the ascending duodenum to the mesocolon of the descending colon.
  • The Mesentery (Mesojejunoileum): Attaches to the abdominal wall opposite the second lumbar (L2L2) vertebra at the root of the mesentery. It is continuous with the mesocolon.
  • Falciform Ligament: A fold containing fat extending from the umbilicus to the diaphragm and liver (between left medial and quadrate lobes). In young animals, the round ligament of the liver (remnant of the umbilical vein) is visible in its free border.
  • Ligaments of the Urinary Bladder:
    • Lateral ligaments: Contain the remnants of the umbilical arteries (round ligaments of the bladder) and distal ureters.
    • Median ligament: Peritoneal fold caudal to the umbilicus; in the fetus, it contains the urachus and umbilical arteries.

Omental Bursa and Epiploic Foramen

  • Omental Bursa: The space enclosed between the superficial and deep leaves of the greater omentum.
  • Epiploic Foramen: The opening between the bursa and the main peritoneal cavity.
  • Borders of the Epiploic Foramen:
    • Ventrally: Portal vein.
    • Dorsally: Caudal Vena Cava.
    • Caudally: Hepatic artery.
    • Cranially: Liver.

The Digestive System: Gastrointestinal Tract

  • Stomach: The largest dilation of the alimentary canal. It is a musculoglandular organ that stores and mixes food with enzymes, mucus, and HClHCl. It lies transversely, primarily on the left side.
    • Regions: Cardia, Fundus, Body, Pyloric region (Pyloric Antrum, Pyloric Canal, and Pylorus/Sphincter).
    • Curvatures: Greater and Lesser.
    • Gastric Folds (Rugae): Internal folds of the stomach lining.
    • Topography: When empty, it does not contact the abdominal wall and is within the rib cage. When fully distended, it can extend to the umbilicus, pushing the liver right and intestinal mass caudally.
  • Small Intestine:
    • Duodenum: Initial portion, attached to the abdominal roof by mesoduodenum. Parts include the cranial part, cranial flexure, descending duodenum, caudal duodenal flexure, ascending duodenum, and duodenojejunal flexure.
    • Jejunum and Ileum: Compromise the jejunal mass on the abdominal floor. The ileum opens into the large intestine via the ileocolic orifice. Note: The dog has no ileocecal orifice.
  • Large Intestine: Main function is dehydration.
    • Cecum: A blind-ended pouch opening into the ascending colon via the cecocolic orifice.
    • Colon: Divided into Ascending (right side), Transverse (travels right to left), and Descending (longest, follows left flank, terminates at pelvic inlet as the rectum).

Accessory Organs of Digestion

  • Liver: Largest gland, located in the cranial abdomen. Functions include exocrine bile production and endocrine metabolism.
    • Lobes (6 total): Left lateral, Left medial, Quadrate, Right medial, Right lateral, and Caudate (Caudate process and Papillary process).
    • Ligaments: Right triangular (to right crus of diaphragm) and Left triangular (to left crus of diaphragm).
    • Porta (Vena) of Liver: Entrance for vessels/nerves between caudate and papillary processes.
  • Gallbladder: Stores and concentrates bile. Located between the right medial and quadrate lobes.
    • Duct System: Cystic duct and Hepatic ducts join to form the Bile duct, which opens at the major duodenal papilla.
  • Pancreas: Functions as exocrine (digestive enzymes) and endocrine (insulin, glucagon, gastrin from islet cells).
    • Structure: Right lobe (in mesoduodenum), Body, and Left lobe (in greater omentum).
    • Ducts: Open at the major duodenal papilla (pancreatic duct) and minor duodenal papilla (accessory pancreatic duct).

Other Abdominal Organs

  • Spleen: Located in the left cranial abdomen. Attached to the stomach by the gastrosplenic ligament. Functions: erythropoiesis storage, filtering blood (bilirubin production), and lymphocyte production.
  • Adrenal Glands: Paired endocrine organs against the abdominal roof, interacting with the sympathetic system.
  • Urinary System:
    • Kidneys: Filter plasma; located in the lumbar region. They are retroperitoneal (covered by peritoneum only on the ventral surface). In cats and some large dogs, the left kidney is pendulous/floating.
    • Internal Kidney Structure: Cortex, Medulla, Hilus (medial indentation), Renal Sinus, and Renal Pelvis.
    • Ureters and Urinary Bladder.
  • Reproductive Organs: Ovaries, Uterine horns, Uterus, and Cervix. (Ovaries are held by the suspensory ligament and proper ligament).

Visceral Vascularization

Unpaired Arteries from the Aorta
  1. Celiac Artery: Branches into the Hepatic, Splenic, and Left Gastric arteries.
    • Hepatic a.: Supplies the liver and branches into right gastric and gastroduodenal (yielding cranial pancreaticoduodenal).
    • Splenic a.: Supplies the spleen (short gastric branches) and the left lobe of the pancreas.
    • Left Gastric a.: Supplies the stomach.
  2. Cranial Mesenteric Artery: Branches into the Common trunk (Ileocolic, right colic, middle colic), Caudal pancreaticoduodenal, Jejunal, and Ileal arteries.
  3. Caudal Mesenteric Artery: Branches into the Left colic and Cranial rectal arteries.
Specific Organ Supply
  • Stomach: All three celiac branches.
  • Duodenum: Cranial and caudal pancreaticoduodenal arteries.
  • Jejunum: Jejunal arteries.
  • Ileum: Mesenteric ileal (from ileocolic), ileal (from cranial mesenteric), and antimesenteric ileal (from cecal) arteries.
  • Large Intestine:
    • Cecum: Cecal arteries.
    • Ascending/Transverse Colon: Colic branch of ileocolic, right colic, and middle colic.
    • Descending Colon: Left colic and middle colic.
  • Kidneys: Paired renal arteries.
Venous Drainage
  • Caudal Vena Cava: Receives drainage from veins satellite to paired arteries (e.g., renal veins).
  • Hepatic Portal Vein: Formed by the gastroduodenal, splenic, cranial mesenteric, and caudal mesenteric veins. It drains veins satellite to unpaired arteries into the liver.
  • Gonadal Drainage Asymmetry:
    • Right ovarian/testicular vein →\rightarrow Caudal vena cava.
    • Left ovarian/testicular vein →\rightarrow Left renal vein.

Lymphatics and Innervation

  • Lymph Nodes: Many exist (hepatic, splenic, gastric, etc.), but the jejunal lymph nodes are most prominent. Lymph drains into the cisterna chyli, then the thoracic duct.
  • Sympathetic Nervous System (SNS): Drives "fight or flight."
    • Pathway: Sympathetic chain →\rightarrow Splanchnic nerves (major, minor, lumbar) →\rightarrow Pre-vertebral ganglia (Celiacomesenteric and Caudal Mesenteric) →\rightarrow Post-ganglionic fibers to viscera.
    • Celiacomesenteric ganglion: Fusion of celiac and cranial mesenteric ganglia near their respective arteries.
    • Hypogastric nerves: Carry post-ganglionic sympathetics from the caudal mesenteric ganglion to the pelvic plexus.
  • Parasympathetic Nervous System (PSNS): Maintains homeostasis.
    • Vagal trunks: Ventral trunk (liver, stomach, pylorus); Dorsal trunk (stomach, pylorus, and celiac branch to celiacomesenteric plexus). Supply goes as far as the left colic flexure.
    • Pelvic nerves: From sacral spinal nerves; supply the gut distal to the left colic flexure (descending colon, rectum) and urogenital organs.

Medical Imaging Notes

  • Cats: Characterized by a "string-of-pearls" appearance in the descending duodenum due to segmental peristalsis.
  • Urograms: Visualize kidneys, ureters, and bladder.
  • Barium Suspensions: Used for gastrointestinal contrast (stomach, jejunum, ileocolic junction, etc.).