In looking, therefore, for the immediate causes of urinary calculi we must accord a high place to all those conditions which determine the presence of excess of mucus, albumin, pus, blood, kidney casts, blood-coloring matter, etc., in the urine. A catarrhal inflammation of the pelvis of the kidney, of the ureter, or of the bladder, generating excess of mucus or pus; inflammation of the kidneys, causing the discharge of blood, albumin, or hyaline casts into the urinary passages; inflammation of the liver, lungs, or other distant organ, resulting in the escape of albumin in the urine; disorders of the liver or of the blood-forming functions, resulting in hematuria or hemoglobinuria; sprains or other injuries to the back, or disease of the spinal marrow, which cause the escape of blood with the urine; the presence in the bladder of a bacterial ferment, which determines the decomposition of the mucus and urea, the evolution of ammonia and the consequent destruction of the protecting cellular (epithelia) lining of the bladder, or the irritation caused by the presence of an already formed calculus, may produce the colloid or uncrystallizable body that proves so effective in the precipitation of stone or gravel. It has long been known that calculi almost invariably form around any foreign body introduced into the kidney or bladder, and I have seen a large, calculous mass surrounding a splinter of an arrow that had penetrated and broken off in the body of a deer. The explanation is now satisfactory--the foreign body carries in with it bacteria, which act as ferments upon the urine and mucus in addition to the mechanical injury caused by its presence. If such a body has been introduced through the solid tissues, there is, in addition, the presence of the blood and lymph derived from the wounded structures. CLASSIFICATION OF URINARY CALCULI. Urinary calculi are most conveniently divided according to the locality in which they are found. Thus we find first renal calculi, formed in the kidney (Pl. XI, fig. 1), and which for cattle must be again divided into calculi of the uriniferous tubes and calculi of the pelvis. The second class are named ureteral calculi because they are found in the duct leading from the kidney to the bladder (ureter). The third class are the vesical calculi, from the bladder or vesicle in which they are found. The fourth class are the urethral calculi, and are found in the duct leading outward from the bladder through the penis (urethra). The fifth and last class are the preputial calculi, since they are found within the sheath of the penis (prepuce).