The Alimentary Canal
The alimentary canal is a tube that extends from the mouth to the anus, about thirty feet in length, and lined throughout by mucous membrane. The alimentary canal walls have four basic layers or tunics. These are the mucosa, the submucosa, the muscularis externa, and either a serosa or adventitia. Each of these layers has a predominant tissue type and a specific function in the digestive process. The mucosa is the wet epithelial membrane abutting the alimentary canal lumen. The major functions of the mucosa are secretion, absorption of digested foodstuffs, and protection. The submucosa’s major function is nutrition and protection. The muscularis externa is the major regulator of GI mobility. The serosa functions to reduce friction, anchors and protects the surrounded organs.
The mouth is at the beginning of the alimentary canal. The mastication of food takes place here. The lining of the mouth, a thick stratified squamous epithelium, protects it from abrasion by sharp pieces of food during chewing. From the mouth, swallowed food passes into the oropharynx and then the laryngopharynx, which are lined by a stratified squamous epithelium, which protects them against abrasion. They contract in sequence to squeeze bolus into the esophagus. The pharyngeal constrictors are skeletal muscles, as swallowing is a voluntary action, innervated by the vagus nerve. The esophagus is a muscular tube that propels swallowed food to the stomach. The stomach then is the widest part of the alimentary canal, it’s a temporary storage tank where food is churned and turned into a paste called chime. Also, the stomach starts the breakdown of food proteins by secreting pepsin. The small intestine is the longest part of the alimentary canal and the place of most enzymatic digestion and virtually all absorption of nutrients. The small intestine shuffles the chime back and forth contacting with the nutrient-absorbing mucosa. Peristalsis propels chime through the...
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