Lecture Notes on Gastrointestinal System: Salivary Glands, Esophagus, and Peritoneal Cavity

Salivary Glands

  • Function of Salivary Glands:
    • Small glands located within the mouth produce mucus.
    • Larger glands, located outside the mouth, produce a more serous (watery) fluid and contain digestive enzymes to initiate digestion.
  • Types of Salivary Glands:
    • Parotid Gland:
    • Produces mostly serous saliva.
    • Located ventral to the ear.
    • Larger in herbivores and highly productive.
    • Opens into the caudolateral side of the mouth.
    • Zygomatic Gland:
    • Located ventral to the zygomatic arch and orbit.
    • Known as the "Dorsal Buccal Gland" in herbivores.
    • Mixed secretions; opens near the first molar on the dorsolateral side.
    • Mandibular Gland:
    • Produces mixed mucous and serous saliva.
    • Found close to the angle of the jaw and is smaller than the parotid gland.
    • Larger in herbivores and opens into the floor of the mouth.
    • Sublingual Gland:
    • Produces mixed mucous and serous secretions.
    • Located rostral to the mandibular gland, caudoventral to the root of the tongue.
    • May have several parts; opens on the floor of the mouth under the tongue.

Soft Palate

  • Function:
    • Separates the dorsal nasopharynx from the ventral oropharynx.
    • Composed solely of soft tissue (no bone); continuation of the hard palate.

Pharynx

  • Structure and Function:
    • Comprised of oropharynx and nasopharynx.
    • The narrowness of the oropharynx limits the size of objects that can be swallowed.
    • Serves as the space where the gastrointestinal (GI) tract and respiratory tract intersect; located caudal to oral/nasal cavities and rostral to the trachea/esophagus.
    • Lacks clear boundaries.

Esophagus

  • Structure:
    • Transports food from the pharynx to the stomach.
    • Located dorsally, slightly left of the trachea, and runs within the mediastinum of the thorax.
    • Passes through the diaphragm via the esophageal hiatus.
    • Dorsally traverses the liver en route to the stomach.
  • Layers of Esophagus:
    • Outermost layer: serosal layer (connective tissue).
    • Middle layer: muscular layer (comprised of two layers running in different directions).
    • Innermost layer: mucosal layer (includes the middle muscular layer and outer submucosa).
  • Muscle Types:
    • Contains striated muscle cranially, transitioning to smooth muscle within the thorax.
    • Collapses when not in use, unlike the trachea which remains open.

Deglutition (Swallowing)

  • Process of Swallowing:
    • Initially voluntary while food is in the mouth; becomes involuntary as the swallowing reflex kicks in.
    • The tongue presses against the hard palate, isolating the food bolus caudally.
    • The tongue pushes the bolus towards the pharynx; touching the pharyngeal mucosa triggers a reflex action.
    • During this, the soft palate raises and the epiglottis tilts back to cover the glottis.
    • Peristalsis (rhythmic contraction of esophageal muscles) carries the food bolus to the stomach.

Abdominal Cavity

  • Description and Location:
    • Lies caudal to the diaphragm and cranial to the pelvic cavity.
    • Largest body cavity, covered cranially by the caudal ribs and surrounded caudally by abdominal muscles.

Peritoneal Cavity

  • Characteristics:
    • Encased by the serous membrane called the peritoneum.
    • Most abdominal organs reside within the peritoneal folds, making them intraperitoneal.
    • Organs adhered directly to the body wall are retroperitoneal (e.g., kidneys).
  • Peritoneum:
    • Comprises a single layer of cells connecting to the viscera via fibroelastic tissue.
    • Stores fat under the peritoneum near the organs.
    • Contains minimal lubricating fluid between itself and the organs.
    • Visceral Peritoneum: Adhered to intraperitoneal organs.
    • Parietal Peritoneum: Associated with the body wall.

Mesentery

  • Function and Structure:
    • Comprises double folds of peritoneum that support viscera, house fat, and supply blood.
    • Specific Types:
    • Mesentery: Supports small intestines.
    • Mesocolon: Supports colon.
    • Mesometrium: Supports uterus.

Omentum

  • Function:
    • Comprises double folds of peritoneum supporting the stomach.
    • The Greater Omentum is attached to the greater curvature of the stomach; involves significant fat deposition.
    • Less critical for blood supply but contains blood vessels.

General Visceral Topography

  • Overview:
    • Variations may be seen between species, or by age (adult vs. juvenile), recent ingestion (full vs. empty), or pregnancy status.
    • Terms:
    • Proximal: Closer to the mouth.
    • Distal: Further from the mouth.