comparative account of digestive system
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The avian digestive system shows adaptations for a high metabolic rate and flight. Enlargements of the esophagus, collectively called the crop, permit the temporary storage of food prior to digestion. The stomach is typically divided into a glandular proventriculus and a muscular gizzard, the latter lying near The cardiac (heart) muscles and smooth muscles of the viscera of birds resemble those of reptiles and mammals. The smooth muscles in the skin include a series of minute feather muscles, usually a pair running from a feather follicle to each of the four surrounding follicles. Some of these muscles act to raise the feathers, others to depress them. The striated (striped) muscles that move the limbs are concentrated on the girdles and the proximal parts of the limbs. Two pairs of large muscles move the wings in flight: the pectoralis, which lowers the wing, and the supracoracoideus, which raises it. The latter lies in the angle between the keel and the plate of the sternum and along the coracoid. It achieves a pulleylike action by means of a tendon that passes through the canal at the junction of the coracoid, furcula, and scapula and attaches to the dorsal side of the head of the humerus. The pectoralis lies over the supracoracoideus and attaches directly to the head of the humerus. In most birds the supracoracoideus is much smaller than the pectoralis, weighing as little as one-twentieth as much; in the few groups that use a powered upstroke of the wings (penguins, auks, swifts, hummingbirds, and a few others), the supracoracoideus is relatively large.
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