Comparative Digestive Systems Curriculum Content

Special Materials and Equipment:

Diagrams or actual organs of the digestive systems of man, horses, cows, sheep, swine or chicken.

Digestive organs of a simple and compound stomach animal.

Field trip preparation.

Overhead projector and handouts.

Slide sets on digestive systems of pig and cow (see Activity 3024G).

Supplemental Handouts 3024A & 3024B.

Diagrams of the ruminant, nonruminant, and avian digestive systems. Actual digestive organs may be obtained from slaughter plants or local butchers.


Quiz, worksheet, handouts, diagrams, class discussion, participation, and completion of Supplemental Worksheet 3024B.


1. Take three pieces of paper, tape them together and fold them accordion style. Compare to intestines and have student understand the importance of a folded lining in the intestine (See Activity 3024D).

2. Obtain the four stomachs of a ruminant animal from a local processing facility to be dissected in class.

3. Visit a local processing facility to dissect ruminant and a non-ruminant animals, comparing the digestive tracts of each animal.



A. General description: The digestive system is composed of the organs that are involved in the process of digestion of food.

1. That is, the process by which large complex nutrient molecules are broken down into simpler molecules capable of being used by the organism for food.

2. The alimentary canal is a long winding tube with various enlarged sacs, beginning with the mouth and ending at the anus.

3. Food under goes complex mechanical and chemical changes as it moves through the alimentary canal or tube.

a. The man and the pig have a SIMPLE stomach with an extensive intestinal system.

b. Ruminants such as cattle, sheep, and goats have a large compartmented and very COMPLEX stomach with a more simple intestinal system.

c. Horses and rabbits have a SIMPLE stomach with an extensive intestinal system with an enlarged cecum.

d. The cecum acts like a rumen and is involved with microbial digestion (fermentation).

B. The mouth is the initial opening of the alimentary canal.

1. The tongue is a tool of "prehension" that is used to grasp the food or to guide it in the mouth and on to the throat.

2. Salivary glands (exocrine glands) secrete juices in the mouth that are mixed with the food.

a. These juices contain enzymes that START carbohydrate digestion.

b. The moistening also makes the food easier to swallow.

3. "Mastication" is the term used to describe the chewing and crushing of food in preparation for swallowing. Mastication is more complex in herbivores than in carnivores.

C. The pharynx is a short, funnel-shaped, muscular sac between the mouth and esophagus.

1. It is part of the digestive and respiratory tracts.

2. Food passes through the pharynx into the esophagus by muscle action.

D. The esophagus is muscular tube that connects the pharynx to the stomach. Muscular contractions move the food down the esophagus to the stomach.

E. The stomach is located in abdominal cavity between the esophagus and small intestine.

(Note: For a more thorough discussion of the stomach, see Unit 350 - "Nutrition and Feeding," Topic 3045 - "Digestion and Absorption.")

1. The simple stomach.

a. Humans, swine, rabbits and horses have a simple stomach.

b. It is divided into three regions, the cardiac, fundus and pylorus.

c. Digestion in the simple stomach is:

(1) Mechanical (physical), through muscular contractions.

(2) Chemical, through gastric secretions of enzymes which soften food and begin the breakdown of the nutrient molecules.

(3) Enzymes are "catalysts" - they "start" chemical reactions.

d. Protein is the primary food component broken down in the stomach.

e. The three primary sources of enzymes which break down fats, proteins and carbohydrates include:

(1) Gastric - mostly proteins in the stomach.

(2) Liver and pancreatic - mostly fats, in the small intestine (duodenum - first par).

(3) Intestinal - mostly carbohydrates and some proteins, in the small intestine (where digestion is completed).

2. The ruminant stomach.

a. Sheep, cows and goats are all examples of ruminants.

b. The ruminant stomach occupies 3/4 of the abdominal cavity, mainly on the left side.

c. There are four compartments:

(1) The rumen or paunch is a mucous membrane studded with papillae (to increase surface area).

(a) In mature animals, it is 80% of bovine ruminant stomach system.

(b) In young animals it is 30%.

(2) The reticulum (honeycomb) composes about 5% of the bovine stomach.

(a) It has a raised mucosa (1/2" high) for increased absorptive area.

(b) It is located near the heart.

(c) It prevents large undigestible objects from entering the stomach (nicknamed the "hardware stomach").

(3) The omasum (manyplies) is 7-8% of the bovine ruminant stomach and absorbs mostly water.

(4) The abomasum is the true stomach.

(a) In mature animals it composes 7-8% of the area and in young animals about 70%.

(b) The difference is that young animals have not yet developed the rumen and are still living on milk which would spoil in a fully developed rumen; however it is perfect for a simple stomach or its substitute in ruminants, the abomasum.

d. Ruminant digestion (see also Topic 365 "Digestion and Absorption").

(1) Heavy items settle in the reticulum which is open to the rumen.

(2) The digestia (partial digested food) can pass back and forth from the reticulum to the rumen.

e. The lighter food collects in the rumen.

(1) Gas from microbial activities collects above the rumen content and is absorbed.

(2) To function effectively the rumen requires a lot of water (provided by saliva).

f. The omasum absorbs most of the excess water from the digestia as it passes from the rumen.

g. The abomasum is the true stomach with typical enzyme action. The food remains here for a relatively short time compared with the time in the rumen.

3. Rumination is the regurgitation, remastication, resalivation and reswallowing of food.

a. The purpose is to further smash and break up the less digestible parts of food.

b. This breakdown gives more surface area for the bacteria to work on the cellulose (less digestible plant part) before it goes "down the line."

c. Resalivation helps provide the water needed for rumen (and bacterial) action.

B. The small intestine is a long coiled tube connecting the stomach with the large intestine.

1. The food moves through the intestines by muscular contractions called "peristaltic movement."

2. The ingesta is further mixed with digestive enzymes and the rest of the breakdown and absorption of nutrients occurs here.

3. To improve absorption, the surface area is increased with "villi."

C. The large intestine is considered to include the cecum, colon and rectum.

1. It absorbs the water, and the residue (feces) becomes more solid.

2. Other than absorption of some vitamins and minerals, very little nutrient absorption takes place here.

3. The exceptions of course are the "cecal fermenters" the rabbit and horse.

D. Accessory organs:

1. The pancreas secretes enzymes which emulsify fats and hydrolyze starch (helps break them down).

2. The liver is the largest internal organ and secretes bile, which helps digest fats.

a. The bile is stored in the gall bladder.

b. The liver is a very complex organ with many other functions in the body including storing iron, handling fats and carbohydrates in the blood, etc.