Digestive System organs for changing food

mechanically and chemically for absorption by body tissues. This process, called digestion, varies among vertebrates; it is unique in ruminants, which use intestinal symbiotic organisms to prepare such foodstuffs as cellulose for use.

Digestion includes both mechanical and chemical processes. The mechanical processes include chewing to reduce food to small particles, the churning action of the stomach, and intestinal peristaltic action. These forces move the food through the digestive tract and mix it with various secretions.

The small intestine is the largest component of the digestive tract and the major site of digestion and absorption. In addition to receiving chyme from the stomach, the initial segment of the small intestine, the duodenum, receives bile from the gall bladder and digestive enzymes from the pancreas. The pancreatic enzymes are produced in an inactive form and only become active in the lumen of the duodenum. The small intestine is divided into three parts, the duodenum (25 cm), the jejunum (2.5 m) and the ileum (3.5 m).

Low Power View of duodenum

Duodenal mucosa and submucosa

The large intestine consists of the colon, cecum, appendix, rectum and anal canal. We will look at slides of the colon, appendix and anorectal junction. The principal functions of the colon are the reabsorption of electrolytes and water and the elimination of undigested food and waste. The cecum is a blind pouch just distal to the ileocecal valve whose histology resembles that of the colon. The appendix is a thin, finger-like extension of the cecum. The rectum is the dilated distal portion of the alimentary canal. Its histology resembles that of the colon, but it is distinguished by transverse rectal folds in its submucosa and the absence of tenia coli in its muscularis externa (see below). The anal canal is the most distal part of the alimentary canal, extending from the anorectal junction to the anus.

Differences in the anatomy of vertebrate digestive tracts is often correlated with the nature & 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)

Mouth & oral cavity. The oral cavity begins at the mouth & ends at the pharynx. Fish have a very short oral cavity, while tetrapods typically have longer oral cavities. The mammalian mouth is specialized to serve as a suckling and masticatory organ (with muscular cheeks).

• Palate = roof of the oral cavity

oprimary palate - internal nares lead into the oral cavity anteriorly

secondary palate - nasal passages are located above the secondary palate and open at the end of the oral cavity

Tongue:

•Gnathostome fish & primitive amphibians - tongue is a simple crescent-shaped elevation in

the floor of the oral cavity caused by the underlying hyoid skeleton & is called the primary tongue

•Most amphibians –

primary tongue (or hypobranchial eminence) + glandular field (or tuberculum impar) ('stuffed' with hypobranchial musculature)

•Reptiles & mammals –

primary tongue + glandular field (or tuberculum impar) + lateral lingual swellings (more hypobranchial muscle)

•Birds –

Lateral lingual swellings are suppressed & intrinsic muscle is usually lacking

Stomach = muscular chamber(s) at end of esophagus

•serves as storage & macerating site for ingested solids & secretes digestive enzymes

•Vertebrate stomachs:

Cyclostomes - weakly developed; similar to esophagus

amphibians, & reptiles - increasing specialization (more differentiated from the esophagus)

Birds - proventriculus (glandular stomach) and ventriculus (muscular stomach, or gizzard)

Mammals - well-developed stomach; ruminants have multichambered stomachs:

INTESTINE

Fishes - relatively straight & short intestine in cartilaginous fishes & in primitive bony fishes (lungfish & sturgeon). However, the intestine of cartilaginous fishes has a spiral valve.

Amphibians - intestines differentiated into coiled small intestine and short, straight large intestine

Reptiles & Birds - coiled small intestines & a relatively short large intestine (that empties into the cloaca)

Mammals - small intestine long & coiled and differentiated into duodenum, jejunum, & ileum. The large intestine is often relatively long (but not as long as the small intestine). A cecum is often present at the junction of the small & large intestines in herbivores.