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Most birds have approximately 175 different muscles, mainly controlling the wings, skin, and legs. Overall, the muscle mass of birds is concentrated ventrally. The largest muscles in the bird are the pectorals, or the pectoralis major, which control the wings and make up about 15–25% of a flighted bird's body weight.
Birds swallow food and store it in their crop if necessary. Then the food passes into their glandular stomach, also called the proventriculus, which is also sometimes referred to as the true stomach. This is the secretory part of the stomach. Then the food passes into the gizzard (also known as the muscular stomach or ventriculus).
The proventriculus is a standard part of avian anatomy, and is a rod shaped organ, located between the esophagus and the gizzard of most birds. [2] It is generally a glandular part of the stomach that may store and/or commence digestion of food before it progresses to the gizzard. [3]
In diving birds, the air sacs can aid in helping birds with respiration. [17] Movement of the muscles involved in diving can cause a pressure differential between the air sacs which would cause more air to move through the parabronchi. [17] This would then increase the uptake of oxygen stored in the respiratory system. [17]
The pelvic limb muscles of emus contribute a similar proportion of the total body mass as do the flight muscles of flying birds. [35] When walking, the emu takes strides of about 100 cm (3.3 ft), but at full gallop, a stride can be as long as 275 cm (9 ft). [36] Its legs are devoid of feathers and underneath its feet are thick, cushioned pads. [36]
The ribs are flattened and the sternum is keeled for the attachment of flight muscles except in the flightless bird orders. The forelimbs are modified into wings. [ 80 ] The wings are more or less developed depending on the species; the only known groups that lost their wings are the extinct moa and elephant birds .
Muscle disorders, such as scleroderma or lupus, which affect the muscles involved in swallowing. “Stress, anxiety or depression can sometimes lead to difficulty swallowing,” Dr. Nocerino says.
In these reptiles, gastralia provide support for the abdomen and attachment sites for abdominal muscles. The possession of gastralia may be ancestral for Tetrapoda and were possibly derived from the ventral scales found in animals like rhipidistians, labyrinthodonts, and Acanthostega, and may be related to ventral elements of turtle plastrons.