KIDNEY 827 The medullary substance is composed principal- ly of tubes passing nearly straight inward to the central receptacle of the secretion. Both these substances are imbedded in interlacing Vertical Section through a portion of the Medullary and Cortical Portions of the Rabbit's Kidney. a. Small arteries of the cortical portion, b. Corpora Mal- pighiana. <l. Capillary blood vessels of the cortical por- tion, e, n, p. External surface of the kidney, ff, ft, i. Blood vessels of the medullary portion, t. Straight uriniferous tubes of the medullary portion, becoming convoluted in the cortical portion. fibres, most abundant in the medullary. In mammals the kidneys are supplied with blood directly from the arterial system, but the renal artery divides very soon after entering the organs into minute twigs which pierce the capsule of the Malpighian tufts ; from the con- volutions of these tufts arise the efferent ves- sels which surround the uriniferous tubes, and from which the renal veins are formed ; thus the urinary secretion is produced from blood which has passed through the Malpighian cap- illaries, the efferent trunks from which have been compared to a portal system within the Malpighian Body from near the base of one of the Medullary Cones, a. Arterial branch, q/. Afferent vessel, m. Malpighi- an tuft. ef. Efferent vessel ; ft, its branch- es, entering the me- dullary cone. (Mag- nified 70 diameters.) kidney. The uriniferous tubes end in from 12 to 18 conical bundles, pointing toward the in- terior, and there embraced by 6 or 12 mem- branous ducts received into the central reser- voir or pelvis of the kidney, from which arises the ureter, the membranous tube which con- ducts the renal secretion to the bladder. With- out entering upon physiological questions which will be more properly treated under URINE, it will be sufficient to state that the kidneys serve to regulate the quantity of wa- ter in the system, a large amount of which may be got rid of through their agency. As the skin and lungs, the other chan- nels through which superflu- ous water is re- moved from the blood, are liable to be greatly af- fected by external circumstances, the kidneys perform a very important office in relation to that fluid. Hence the quantity of the renal secretion will depend on the amount of fluid passed off by the skin, being greatest when the cuta- neous secretion is least, and vice versa; the amount of solid ingredients being dependent on the amount of waste and the excess of nitrogen in the system. The kidneys serve to free the blood from highly nitrogenized compounds formed from the decomposition of the albuminous and gelatinous tissues and from some portions of the food ; they also remove certain excrementitious compounds, of which carbon is a principal ingredient, ab- normally increased when the liver and the lungs do not act freely; by them the super- fluous water and various saline matters in ex- cess, and foreign substances introduced into the blood as medicines or otherwise, which would be injurious if retained, are carried off. The kidneys are subject to many painful and dangerous diseases, which can only be alluded to here ; among these are vascular congestion, inflammation, fatty and waxy degeneration, and diseased states produced by retention of urine, by calculi, external violence, and extension from other organs. Bright's disease is one of their most common and fatal affections, the so-called granular degeneration, consisting in the distention of the tubules, the surrounding tissue, and the Malpighian capsules, with ex- udation matter, and the subsequent atrophy of portions or even the whole of the cortical sub- stance. Invertebrates have special organs for the secretion of urine, opening into the intes- tines or into the branchial cavity. In fishes
