The tissues of the body are constantly wearing away and being renovated from the nutritive substances contained in the blood, and the blood is at the same time constantly taking into itself the worn-out materials or waste resulting from the wear and tear of the body, some of the more important of which are discharged through the kidneys in the form of urine. Upon the efficiency, therefore, of the latter organs to free the blood from these impurities will largely depend the health of the animal.

The normal urine consists of water carrying in solution certain organic and mineral salts.

In the horse it is a yellowish fluid, having a peculiar odour and an alkaline reaction. Its specific gravity varies more or less according to the nature of the food consumed, work, etc, the average being about 1.042. It is nearly always more or less turbid in appearance, sometimes actually muddy. If healthy urine be allowed to stand, a copious sediment of earthy-looking matter falls to the bottom of the vessel. A small quantity of this placed under the microscope is found to consist of minute crystals of carbonate of lime (fig. 134).

A few drops of nitric acid added to horses' urine decomposes the carbonate of lime and causes the fluid to effervesce, just as when tartaric acid is added to carbonate of soda. As a result of this, the sediment disappears and the urine becomes bright and clear.

In disease the urine is liable to undergo very striking alterations in its physical characters, as well as in its chemical composition, and in these connections it sometimes affords valuable assistance, not only in locating a disorder, but likewise in determining its nature.