Vertebrate Anatomy and Digestive System

Mesentery and Divisions of the Vertebrate Alimentary Canal

• Ventral mesentery is absent in adult vertebrates, except in the regions of the liver and gall bladder (falciform ligament)
• Special mesenteries for gonads and their ducts
• In mammals, the mesentery of the stomach develops a special prolongation called the greater omentum
• Accessory organs include the tongue, teeth, oral glands, pancreas, liver, and gall bladder

Anatomy of Vertebrate Digestive Tracts

• Differences in anatomy correlated with the nature and abundance of food
  • Readily absorbed (e.g., hummingbirds) vs. requiring extensive enzymatic activity (e.g., carnivores)
  • Constant food supply (e.g., herbivores) vs. scattered supply (e.g., carnivores)
• Filter-feeding is the oldest craniate method of acquiring food
  • Still employed by lampreys, a few jawed fishes, and baleen whales

Oro-Pharyngeal Cavity

• Oral cavity begins at the mouth and ends at the pharynx
• Fishes have a very short oral cavity, while tetrapods typically have longer oral cavities
• Primary palate - roof of the oral cavity of fishes and amphibians; secondary palate in reptiles and mammals

Nasal Passageways and Oral Glands

• Nasal passageways open into the oropharyngeal cavity in lobe-finned fishes, and into the oral cavity in tetrapods with primary palate
• Multicellular oral glands open onto the roof, walls, and floor of the cavity
• May contain venom, saliva, including several enzymes, anticoagulant (lampreys, bats), nutrients in catfish, and mucus
• Oral glands are scarce in fishes

Mammals

• Mouth specialized to serve as a suckling and masticatory organ (with muscular cheeks)
• Oral vestibule separates the gums or alveolar ridges from the cheek and mouth

Pharynx

• Pharyngeal pouches may give rise to gill slits
• Fishes - gills and gill slits
• Tetrapods - includes glottis (slit leading into the larynx), openings of auditory (Eustachian) tubes, opening into esophagus, and location of tonsils in mammals

Esophagus

• Distensible muscular tube connecting the pharynx and the stomach
• Fishes - closes so the stomach doesn't become filled with respiratory water
• Birds - may have a diverticulum called the crop, which has digestive enzymes and allows hoarding of food
• Pigeon milk is an esophageal secretion in doves for nestlings

Stomach

• Muscular chamber(s) at the end of the esophagus that serves as storage and macerating site for ingested solids and secretes digestive enzymes
• Vertebrate stomachs:
  • Cyclostomes - weakly developed; similar to esophagus
  • Fish, amphibians, and reptiles - increasing specialization (more differentiated from the esophagus)

Birds

• Store large quantities of food temporarily in the crop, releasing it for digestion as needed
• Stomach is divided into:
  • Proventriculus (glandular stomach)
  • Ventriculus (muscular stomach, or gizzard)
• Lacking teeth, they swallow small stones which lodge in the muscular gizzard
• As the gizzard churns, the stones grind against the food like numerous tiny millstones
• Birds must constantly replace their gizzard stones by swallowing new gravel

Ruminant Stomach

• Serves mainly as a storage sac for large quantities of vegetable matter
• In the absence of cellulase, little digestion takes place in the small intestine, and food is diverted into a long dead-end side branch, the caecum
• The caecum houses a huge population of bacteria, some of which produce enzymes that convert cellulose to sugars, while others manufacture amino acids and other nutrients
• Micromolecules are absorbed directly through the epithelium of the caecum, and the waste material is released into the colon for disposal

Intestines

• Located between the stomach and the cloaca or anus
• Vertebrate intestines are differentiated to varying degrees into small and large intestines
• Cartilaginous fishes - have a short, thick, tapering intestine with a spiral valve where food passes slowly as it moves toward the colon

Herbivores

• The tract includes an expanded colon, cecum, or forestomach
• An enlarged colon is the principal site for microbial fermentation
• Haustrations extend over the cecum and the entire length of the colon of most species