The vertebrate gastrointestinal tract, from the inside out, is composed of an epithelial mucosal layer responsible for secretion and absorption, an intermediate muscular muscularis layer responsible for propelling materials through the digestive tract, and an outer serosa layer composed of connective tissue, that anchors the gut in place and through which the gut communicates via blood vessels and nerves. Carnivores have pointed teeth and lack flat grinding surfaces. Herbivores have large, flat teeth to pulverize the cellulose plant wall. The four types of teeth in adult humans are incisors for biting, canines for tearing food, and premolars and molars, both for grinding and crushing. Birds lack teeth.
Salivary amylase initiates digestion of starch.
It is secreted by the salivary glands and starts working on starch immediately, even while still in the buccal cavity. The two types of muscles that propel food down the esophagus are skeletal in the upper third of the esophagus and smooth in the lower two-thirds. The rhythmic contractions of these muscles that propel food down the esophagus are termed peristalsis. The parietal cells secrete hydrochloric acid. Pepsinogen is secreted by the chief cells.