topography of equine anatomy

Stunted external abdominal oblique / heave line

Chronic forced expiration in heaves (RAO/equine asthma) causes hypertrophy of the external abdominal oblique muscle, producing a visible "heave line" along the caudoventral abdomen

Tunica flava abdominis

A well-developed yellow elastic tissue layer deep to the external oblique aponeurosis in the horse (and ox) that helps support abdominal viscera; much less developed in the dog

Prepubic tendon rupture

Rupture of the common tendinous insertion of the abdominal muscles on the pubis, seen especially in heavily pregnant mares, causing loss of ventral support, a dropped abdomen, and ventral edema

Equine stomach

Relatively small (~8-15L) C-shaped stomach with a nonglandular (saccus cecus) region and glandular region separated by the margo plicatus; strong cardiac sphincter and oblique esophageal entry prevent vomiting

Equine gall bladder

Absent in the horse (unlike the dog); bile drains continuously via hepatic ducts directly into the duodenum

Equine spleen

Large, flat, comma/triangular-shaped organ in the left dorsal abdomen, attached by the gastrosplenic ligament (to stomach) and nephrosplenic ligament (to left kidney)

Equine kidney position

Asymmetric: right kidney lies more cranially (partly under the ribcage, contacting the liver), left kidney lies more caudally

Significance of microbial fermentation in herbivores

Allows symbiotic microbes to break down cellulose into volatile fatty acids, providing an energy source the horse's own enzymes cannot digest

Fermentation chambers in the horse

Cecum and ascending (large) colon; both are greatly enlarged compared to the dog, where they are small/vestigial

Cecum arrangement (base to apex)

Base is dorsal on the right side near the right kidney/body wall; body curves ventrally and cranially; apex points cranioventrally toward the xiphoid/sternal region

Course of the ascending colon

Right ventral colon → sternal flexure → left ventral colon → pelvic flexure → left dorsal colon → diaphragmatic flexure → right dorsal colon → transverse colon

Pelvic flexure location

Normally located in the left paralumbar fossa (left flank)

Right flank topography

Upper: base/body of cecum, right dorsal colon; Lower: right ventral colon; deeper structures: liver and descending duodenum

Left flank topography

Upper: left dorsal colon, spleen, left kidney (nephrosplenic space); Lower: left ventral colon; also jejunum, stomach, liver

Clinical significance of flank topography

Guides percussion, auscultation, and rectal palpation used to localize colic lesions (e.g., displacements, impactions) to specific bowel segments

Right side structures to project on live horse/skeleton

Base and body of cecum, right ventral colon, right dorsal colon, liver, descending duodenum

Left side structures to project on live horse/skeleton

Left ventral and dorsal colons, small colon, jejunum, spleen, stomach, liver

Key abdominal structures to identify

Stomach, spleen, liver, pancreas, duodenum (descending/transverse/ascending), jejunum, ileum, gastrosplenic ligament, nephrosplenic ligament, cecum (base/body/apex), ascending/transverse/descending colon, small colon, cecocolic fold, ileocecal fold, kidneys, adrenal glands

Epiploic foramen

Located on the right side, ventral to the liver; clinically important as a site where small intestine can become entrapped (epiploic foramen entrapment), causing colic

Stallion accessory genital glands

Stallions possess multiple accessory genital glands (ampullae, vesicular glands/seminal vesicles, prostate, and bulbourethral glands), in contrast to the dog, which lacks vesicular and bulbourethral glands