Acrid diuretics, by mouth or skin, microbian infection, retention of urine, urethral calculus, parasites, spasm, enforced suspension of micturition, unclean catheter, adjacent infection, chill. Lesions: hyperæmia of mucosa, thickening, vascular distention, clouding of epithelium, muco-purulent secretion, alkaline fermentation, ammonia, liquefaction of cells, erosion. Symptoms: Slight fever, stiff, straddling gait, urine scanty, cloudy, alkaline, penis or clitoris semi-erect, smearing of tail or prepuce. Crystals of triple phosphate. Treatment: Antiseptics, boric or salicylic acid, gum arabic, astringent antiseptics, laxatives, flax seed, slippery elm, anodynes, diluents, piperazine, drainage, rest, restricted laxative diet, warmth, avoid stimulants. _Causes._ Cystitis is caused in all animals by irritant diuretics like cantharides, copaiba, or oil of turpentine given by the mouth or applied to an extensive cutaneous surface. It is an error, however, to conclude with Williams that this is the sole cause. The very existence of calculi virtually implies bacterial infection, and fermentation. The presence of free ammonia in the urine usually implies fermentation, and fermentation must be looked upon as practically synonymous with microbian invasion. That bacteria may be present without serious injury is undoubted. The protective power of the healthy mucosa is very great. But when the mucosa is weakened, microbes that would otherwise be harmless, find a ready infection atrium, and triumph over the weakened tissues. Hence retention of urine and overdistension of the bladder as in urethral calculus, blocking of the urethra by a parasite, spasm of the sphincter vesicæ, compulsory retention as in the mare in harness, the dog kept indoors, or in railway car on a long journey, or in mares so travelling, may become the occasion of cystitis. Even in cases in which no microbe is present at first, this reaches the bladder by the introduction of an unclean catheter, or by extension from an uretheritis, vaginitis or metritis, or even from a peritonitis, or infected urachus. Or the infection may descend from a suppurating kidney. Another occasion of microbian invasion is the congestion which attends on exposure to cold.