The kidneys of the cat are compact (i.e., not lobulated) and have the usual kidney or bean form. They lie in the abdominal cavity, one on either side of the vertebral column, against the dorsal body wall, in the region between the third and fifth lumbar vertebræ. The right kidney is one or two centimeters farther craniad than the left, and the long axes of the two converge craniad a little. Each is covered by peritoneum on its ventral surface only (i.e., it is retroperitoneal). At the border of the kidney, where the peritoneum passes from it to the body wall, there is an accumulation of fat, which is most abundant at the cranial end of the kidney. Within the peritoneal investment the kidney is enclosed in a special loose fibrous covering, the =capsule= or =tunica fibrosa=, which is continuous with the fibrous coat of the ureter and pelvis. In the middle of the median border of each kidney is a notch, the =hilus=. It gives exit to the ureter (Fig. 108, _c_) and renal veins (_b_), and entrance to the renal artery (_a_). On the ventral surface of the kidney within the capsule are seen grooves radiating from the hilus. They contain blood-vessels. If the substance of the kidney is sliced away parallel to the ventral surface for some distance (Fig. 109), there is exposed a cavity, the =sinus=, which lies near the medial border and the opening of which is the hilus. It contains the pelvis (_d_) (the expanded beginning of the duct of the kidney), and also renal vessels (_e_ and _f_) with their branches. These structures are enclosed in fat, which fills the remainder of the sinus. Upon opening the pelvis the kidney substance is seen to project into it in the form of a cone, the =papilla= (_c_), the apex of which is directed mediad. On the apex of the papilla are the numerous openings of the uriniferous collecting-tubes, some of them opening at the bottom of an apical depression of the papilla. In a section made parallel to the ventral surface and in the median plane, the substance of the kidney is seen to consist of a peripheral darker and more granular =cortical portion= (Fig. 109, _b_), and of a central, lighter, less granular =medullary portion= (_a_). Both portions are marked by lines which converge to the apex of the papilla (_c_).