_Definition._ The term egagropile, literally goat-hair, has been given to the felted balls of wool or hair found in the digestive organs of animals. The term has been applied very widely, however, to designate all sorts of concretions of extraneous matters which are found in the intestinal canal. In cattle the hair licked from their skin and that of their fellows rolled into a ball by the action of the stomach and matted firmly together with mucus and at times traces of phosphates, are the forms commonly met with. In sheep two forms are seen, one consisting of wool matted as above and one made up of the fine hairs from the clover leaf similarly matted and rolled into a ball. In pigs the felted mass is usually composed of bristles, (exceptionally of paper or other vegetable fibre), and in horses felted balls of the fine hairs from the surface of the oat, mingled with more or less mucus and phosphate of lime make up the concretion. These are found in the stomach, and intestines. In predatory birds the feathers and in insectivorous birds chitinous masses are formed in the gizzard and rejected by vomiting. _Causes._ Suckling animals obtain the hair from the surface of the mammary glands hence an abundance of hair or wool on these parts favors their production. The vicious habit of calves of sucking the scrotum and navel of others is another cause. In the young and adult alike the habit of licking themselves and others especially at the period of moulting is a common factor. _Composition._ Hair, wool, and the fine hairs of clover are the common predominant constituents, but these are matted together more or less firmly by mucus and phosphates, the ammonia-magnesian phosphate uniting with the mucus and other matters in forming a smooth external crust in the old standing balls of adult animals. The centre of such balls is made up of the most densely felted hair. In balls of more recent formation the external crust is lacking and the mass is manifestly hairy on the surface, and the density uniform throughout. These have a somewhat aromatic odor, contain very little moisture, and have a specific gravity approximating .716 (sheep) to .725 (ox). Ellagic, and lithofellic acids, derivatives of tannin, are usually present, and are abundant in the egagropiles of antilopes. In the balls of recent formation, as seen especially in sucking calves, the hair is only loosely matted together, and often intermixed with straw and hay, and is saturated with liquid and heavier than the old masses. These are usually the seat of active putrefactive fermentation, and being occasionally lodged in the third or even the fourth stomach, the septic products act as local irritants, and general poisons. They are therefore far more injurious than the consolidated hairballs of the adult animal, and often lay the foundation of septic diarrhœas and gastro-enteritis. The balls may be spherical, elliptical, ovoid, or, when flattened by mutual compression, discoid.