Herbivore Comparative Anatomy

What are the 4 chambers of the ruminant stomach?

rumen, reticulum, omasum, abomasum

The first 3 chambers (rumen, reticulum, and omasum) are known as what?

the forestomach (proventriculus)

Function of rumen and reticulum

together ferments and breaks down complex and rough food to prepare it for further digestion, and absorbs some nutrients

Rumen location

laterally compressed and extends from cardia to the pelvic inlet, from abdominal roof to floor, and from left body wall across midline, may reach the lower right flank

Smaller reticulum lies

cranial to rumen under cover of sixth to eighth ribs and mainly to left of median plane

What is the integrated structure and function of the rumen and reticulum described as?

ruminoreticular compartment

Where do the rumen and reticulum communicate?

over U-shape ruminoreticular fold

The reticular mucosa has what type of appearance?

honeycomb

Lamina propria

submucosa includes bands of smooth muscle within distal parts of reticular ridges

The upper keratinized layer of the epithelium in the reticulum mucosa protects

against abrasion by rough and fibrous diet

Ruminal epithelium characteristics

thick subepithelial lamina propria forms core of papilla through collagen, elastic, and reticular fibers and also contains a dense capillary network

no muscularis mucosae

looser submucosa located directly against lamina propria → contains a vascular network

Rumen mucosa

largest and most densely strewn within blind sacs

Omasum location

lies within intrathoracic part of abdomen to right of midline (between rumen and reticulum to left and liver/body wall to right

lower pole has an attachment to fundic region of abomasum around omasoabomasal orifice

much of right surface is covered by and partly connected to lesser omentum

Omasum interior

large, oval omasoabomasal opening at other extremity is partly occluded by prolapse of abomasal folds

floor of omasal groove smooth except for a few low ridges that run along its length and a scattering of claw-like projections that guard upper opening

keratinized stratified squamous epithelium over laminae raised to cover numerous papillae

mucosa characterized by a lamina propria → dense subepithelial capillary network and encloses a thick muscularis mucosae → thin outer longitudinal layer and a thicker inner circular layer

Abomasum location

larger of 2 limbs forms a piriform sac → reaches forward to left to make contact with body wall between reticulum and atrium and ventral sac of the rumen

passes transversely toward right body wall and ascends to end at pylorus, caudal to lower part of omasum

Abomasum inner structure

at omasoabomasal junction → epithelium changes abruptly to a simple columnar epithelium with occasional goblet cells

area increased about sixfold by presence of dozen large folds that arise around entrance → course over walls of fundus and body before subsiding at flexure

proximal ends of folds forms a mucosal valve or “plug” that discourages reflux of ingesta into omasum

mucosa of pyloric part most remarkable for large swelling or torus that projects from lesser curvature to narrow pyloric passage

Other layers of abomasum

dark mucosa of body and fundus contains true peptic glands

glands of lighter pyloric part secrete mucus alone

muscle coat consists of longitudinal and circular strata

Omasum and abomasum supplied by

left gastric and left gastroepiploic arteries

Sympathetic nerves reach stomach through

periarterial plexuses

Duodenum location

part rises almost vertically toward visceral surface of liver, followed by a run toward pelvis as descending duodenum → turns when almost level with coxal tuber

Jejunum forms many short coils within

free margin of mesentery

Cecum continues into colon without

obvious change in diameter; junction marked only by entrance of ileum

Accessory pancreatic duct opens far down

descending duodenal limb

In small ruminants, greater pancreatic duct is usually

present

1st part of duodenum is supplied from

celiac artery

Descending colon is supplied from

caudal mesenteric artery

Intestinal veins combine to form

cranial mesenteric radicle of portal

Nerves that reach the gut along cranial mesenteric artery consist of both

sympathetic and vagal fibers

Parasympathetic nerves to the last part of the colon are derived from

sacral outflow

Horses are _____ fermenters

hindgut

Left limb of horse stomach comprises

fundus and body

Right limb or pyloric part of a horse stomach is much ______ and extends across _____ to join duodenum

narrower

midline

Haustra

small pouches caused by sacculation, which give the colon its segmented appearance

Microbial fermentation within cecum produces

gas normally discharged at intervals into right ventral colon

Parallel limbs and flexures consist of

right ventral colon, ventral diaphragmatic flexure, left ventral colon, pelvic flexure, left dorsal colon, dorsal diaphragmatic flexure, and right dorsal colon

Venous drainage parallels arterial supply

portal vein formed by union of caudal mesenteric, cranial mesenteric, and splenic tributaries