The Digestive System
The digestive system of a chicken has some similarities and some distinct differences to the human digestive system. In this article, we will explore this marvelous system, how it is put together, and how it works.
The alimentary canal, or food tube, travels through the whole digestive system. Through this tube, we will follow a kernel of corn as it travels from the beak to the vent. The changes that occur are the magic of the digestive system.
Did you ever hear that old saying, “Scarcer than hen’s teeth?” Something that is so scarce as to be non-existent? Well, that is where we start our journey through the digestive system of our feathered friend, the chicken. The mouth of our bird is called a beak. It has no teeth, at least, it hasn’t for the past 80 million years. When Henrietta the hen picks up her kernel of corn, it is moistened in the mouth with saliva from glands to make it easier to swallow, not unlike what happens in our own mouth. Amylase, an enzyme that is in the saliva, begins the digestion process. This enzyme starts the breakdown of complex starches into more simple sugars. Again, the same process occurs for us in our mouth. Try this experiment for yourself. Place a plain cracker on your tongue. Let it stay for several seconds. Note how the initial taste is a little blah (that’s why we use dip). Now note as you start to chew and swallow your cracker has become sweeter. The amylase in your saliva has broken down that complex starch into a sweeter simple sugar.
With a push of the tongue, we swallow and so does Henrietta. The corn has entered the esophagus, a flexible tube sometimes called the gullet. No digestion occurs in this organ. The esophagus acts as transportation by muscular action to the crop. Our own esophagus takes our chewed food directly to our stomach. Henrietta’s crop is located just outside the body cavity at the base of the neck. It evolved as storage for birds. Birds have to eat quickly and hide fast. By the end of the day, the crop will look full and feel hard from the day’s hard seeds and corn. If you have ever processed a bird, you know not to rupture this sack before you remove it. It can be messy.
The kernel of corn hasn’t changed much yet. When the corn leaves the crop it goes to the proventriculus or “true stomach.” It is only a little wetter and a bit softer from its recent storage and exposure to amylase. The proventriculus is similar to our own stomach in that primary digestion begins with this organ. Here we start secretions of HCI (hydrochloric acid), which works on proteins and weakens the hard coating on the corn. Pepsin and other enzymes start to work at this stage for both humans and poultry. Realize, however, that Henrietta has done little or no mechanical digestion (chewing) to this point. Before Henrietta can start to assimilate (absorb) basic nutrients, she must crush this corn into small enough particles to take a ride in the transport system (blood). Following the proventriculus, as the word implies, is the ventriculus, more commonly referred to as the gizzard.
The ventriculus (gizzard) is a very muscular organ. It is also found in reptiles, earthworms, and fish. In ancient times it was dried and used as a remedy for various ailments. Today it can be found as a tasty ingredient in our Thanksgiving stuffing. Our kernel of corn has been weakened by the previous chemical processes in the stomach but has not been acted on by mechanical digestion. By this point, humans would have already chewed their food some 30 times before swallowing for good digestion. At least that was what was told to me many years ago at the dinner table. Remember Henrietta’s scarcity of teeth? Her chewing is replaced by the mechanical action of the gizzard. Through muscular contraction as the force and grit (small particles of stone) as grinding wheels, this organ will grind, mix and mash her corn into particles small enough to be absorbed. I have found various items in cleaning gizzards over many years of processing poultry. One that comes to mind is a 22-caliber shell casing carelessly discarded in the yard. Chickens pick up all sorts of items and store them in their crops. As poultry keepers, it is our responsibility to keep their areas clear of undesirable debris.
The corn arrives at the small intestine from the gizzard as a fine soup. The small intestine is vital to the entire digestive process. This is where the final chemical digestion and most absorption of nutrients occur. The names of the large and small intestines refer to their diameter, not their length. In Henrietta the small intestine is about four feet long. The duodenum refers to the first section of the small intestine. This is where the corn finishes breaking down. In this initial area of the small intestine (duodenum) the liver and the pancreas do their part in the process. The liver produces bile that is stored in the gall bladder. This bile travels by small tubes (ducts) to the duodenum to assist in the breakdown of fats. The pancreas, by similar means, injects enzymes that complete the breakdown process of proteins. The rest of the wrinkled-lined tube is surrounded by vessels of the transport system for the assimilation of nutrients into the chicken’s cells.
Intersecting where the small and large intestine join is the ceca. The ceca are a pair of pouches. Their purpose is to advance the digestion of the materials continuing into the large intestine, although at present time the ceca is believed to have little if any effect on the health of the chicken.
From the intersection with the ceca begins the large intestine (colon). It is only about four inches in length, but its diameter is twice that of the small intestine. The primary function of the large intestine is to reabsorb water. The action is similar to the human colon. Henrietta’s large intestine also acts as the rectum or holding area for later waste release.
Before Henrietta can eliminate her waste there is one last interaction, the cloacae. The cloacae are identified as the place where the digestive, urinary, and reproductive systems meet. Chickens do not urinate. Therefore, with no bladder, uric acid, metabolic waste from the kidneys is mixed and dried with solid waste from the digestive system. Uric acid is identified by the white portion of the feces (poop). Do not be alarmed when considering your breakfast egg must pass through this area. During the egg-laying process, the opening to the reproductive tract covers the excretory openings.
We have reached the end of the alimentary canal, known as the vent. The vent is a multipurpose external opening to the outer environment. It is through this vent that eggs are released and the elimination of waste happens.
I hope I have helped you better understand the biology of Henrietta and your own feathered friends. This trip through the alimentary canal, also known as the food tube, should help you better understand your birds’ needs.