Hippuric Acid, , is a compound of great interest both to the chemist and to the physiologist. It derives its name from its having been first discovered in the urine of the horse, and that fluid, or the renal secretion of the cow, affords us the best and readiest means of obtaining it. The crystals of hippuric acid are moderately large, colourless, but subsequently becoming milk-white, four-sided prisms, which are devoid of odour, but have a faintly bitter taste. They dissolve readily in boiling water and in spirit, but are only sparingly soluble in cold water and in ether. It is an abundant normal constituent of the urine of the horse, cow, sheep, goat, hare, elephant, &c., and most probably is to be found in the urine of all vegetable feeders. In the human urine of healthy persons living on an ordinary mixed diet it occurs in very small quantity, but it is increased by an exclusively vegetable diet, and in the well-known disease diabetes.
The hippuric acid occurring in the animal organism exists in combination with bases, and chiefly as hippurate of soda and hippurate of lime. The last-named salt can be obtained by the mere evaporation of the urine of the horse. The chief interest of the substance is that it was one of the first to be discovered of a long series of complex bodies, which we now know are formed synthetically in the animal body. Hippuric acid readily splits into benzoic acid and glycocoll. If benzoic acid is administered it is excreted as hippuric acid, combining with glycocoll in the body. In herbivorous animals the benzoic acid is largely derived from the food; in animal feeders even in starvation it occurs in small amount in the urine, and we must therefore conclude that its forerunners may be derived from the metabolism of the tissues. That certain bodies closely allied to benzoic acid may be so formed has now been experimentally demonstrated, while glycocoll can also be proved to be so produced. At one time the belief was entertained that these bodies were combined in the liver; but more recent research has shown that the synthesis chiefly takes place in the kidneys.