How does an ostrich manage to grind rough food so that it is digested in the stomach?

The ostrich is a huge bird, having lost the ability to fly, it had to adapt to survive on land. The stomach of a bird has very developed muscles, and its walls are also very compacted. The goiter in an ostrich is presented in the form of a pregastric chamber, it contains food that the bird has previously eaten. Farmers who breed these birds leave some sand and gravel near their drinkers. This bird specifically eats gravel and sand, which contribute to crushing, grinding and assimilation of various foods that are included in the ostrich’s diet.