Seat, colon, cæcum; hair of oat seed, clover leaf, vine tendrils, hair of horse, nucleus, calcic admixture, straw, in horses on dry food, with depraved appetite, or with skin disease. Symptoms: none, or torpid bowels, colics, recurring, fermentations, tympany, obstruction, rupture, peritonitis, rectal exploration. Lesions: impacted ball, with excess of liquid and gas in front, rupture, ragged bloody edges. Treatment: extraction, enemata, eserine, barium chloride. Hair balls, received the name of egagropiles because of their discovery in the alimentary canal of the wild goat, but they are found in various forms in all the domestic animals. In _horses_ they occupy the cæcum and colon and are most frequently composed of the fine vegetable hairs that surround the grain of the oat, or the leaf of clover, of the woody tendrils of vines, and of the hairs of themselves and their fellows taken in at the period of moulting. They sometimes contain a nucleus of leather or other foreign body which has been swallowed but in many cases no such object can be found, the hair having become rolled and felted by the vermicular movements of the stomach and intestines. An admixture of mucus assists materially in the felting, and calcareous and magnesian salts may make up the greater part of the mass, rendering it virtually a _calculus_. They may further have a large admixture of straw and vegetable fibres of larger size than oat or clover hairs. They are most frequent in horses kept on dry food, (sweepings of oatmeal mills) and at hard work, and which show depraved appetite and lick each other. Omnibus horses suffer more than army horses. Skin diseases, by encouraging licking, contribute to their production. _Symptoms._ In the great majority of cases hair balls do not seriously incommode the horse. They do not attain a large size, and being light do not drag injuriously on the intestine and mesentery. They do, however, retard the movement of the ingesta, and when grown to a considerable size they may block the intestine, more particularly the pelvic flexure, the floating colon or rectum. Under such conditions they produce colics which may be slight, transient, and recurrent, or severe and even fatal, having all the characteristics of complete obstruction from other causes. Fermentations, tympanies, and straining without defecation are common features. When the obstruction takes place in the pelvic flexure, the floating colon or rectum, it may often be detected by rectal exploration. When complete obstruction occurs all the violent symptoms of that condition are present, and these may pass into those of rupture (Peuch, Leblanc, Neyraud), and shock or peritonitis. If the animal has passed hair balls even months before, the colics may with considerable confidence be attributed to other balls of the same kind.