Why have birds a gizzard?
Because it may answer the purposes of teeth to masticate their food; the gizzard being composed of thick and tough muscular substance, small in size, but more powerful in its action than the strongest jaw-bone. It consists of four distinct muscles – a large hemispherical pair at the sides, and a small pair at the two ends of the cavity. By their alternate action these muscles produce two effects; the one, a constant friction on the contents of the cavity, the other, a pressure upon them. These muscles are lined with a cuticle which is extremely callous, and which often becomes cartilaginous, and even horny. Reaumur and Spallanzani compelled geese and other birds to swallow needles, lancets, and other hard substances; in a few hours after which, the birds were killed and examined; – the needles and lancets were uniformly found broken off and blunted, without the slightest injury having been sustained by the stomach. Swallowing pebbles also aids the action of the gizzard upon the food, t
