The principal organized structures in urinary sediments are: tube-casts; epithelial cells; pus-corpuscles; red blood-corpuscles; spermatozoa; bacteria; and animal parasites. They are much more important than the unorganized sediments just considered. 1. Tube-casts.--These interesting structures are albuminous molds of the uriniferous tubules. Their presence in the urine probably always indicates some pathologic change in the kidney, although this change may be very slight or transitory. Large numbers may be present in temporary irritations and congestions. _They do not in themselves, therefore, imply organic disease of the kidney._ They probably occur only in urine which contains, or has recently contained, albumin. While it is not possible to draw a sharp dividing-line between the different varieties, casts may be classified as: (1) Hyaline casts; (2) waxy casts; (3) granular casts; (4) fatty casts; (5) casts containing organized structures--(_a_) epithelial casts; (_b_) blood-casts; (_c_) pus-casts; (_d_) bacterial casts. As will be seen, practically all varieties are modifications of the hyaline. The significance of the different varieties is more readily understood if one considers their mode of formation. Albuminous material, the source and nature of which are not definitely known, probably enters the lumen of a uriniferous tubule in a fluid or plastic state. It there hardens into a mold, which, when washed out by the {114} urine, retains the shape of the tubule, and contains within its substance whatever structures and débris were lying free within the tubule or were loosely attached to its wall. If the tubule be small and have its usual lining of epithelium, the cast will be narrow; if it be large or entirely denuded of epithelium, the cast will be broad. A cast, therefore, indicates the condition of the tubule in which it is formed. The search for casts must be carefully made. The urine must be fresh, since hyaline casts soon dissolve when it becomes alkaline. It should be thoroughly centrifugalized. When the sediment is abundant, casts, being light structures, will be found near the top. In cystitis, where casts may be entirely hidden by the pus, the bladder should be irrigated to remove as much of the pus as possible and the next urine examined. In order to prevent solution of the casts the urine, if alkaline, must be rendered acid by previous administration of boric acid or other drugs.