In a healthy condition the urine presents a bright yellowish hue. When, as is sometimes the case, carbonate of lime is very abundant, the fluid wears a distinctly muddy appearance.

Change of colour as the result of disease may be brought about by various morbid conditions of the urinary and other organs.

In polyuria, or profuse staling, it loses its yellow appearance and becomes pale and watery in appearance, and the same change is observed in debility and in some forms of disease of the kidney.

In all febrile diseases the colour of the urine becomes heightened, with a tendency to assume a reddish-yellow appearance. This is most noticeable after standing for a short time, when the carbonate of lime held in suspension has subsided to the bottom of the vessel and the turbidity has cleared off.

Bile imparts to urine a deep brownish-yellow hue, which denotes the existence of liver derangement in one or other of its various forms.

Blood gives to this fluid a red or smoky-red appearance, and denotes disease associated with rupture of blood-vessels either in the kidneys, the bladder, the ureters, or the urethra. Its presence is recognized by microscopical examination, which reveals the existence of small biconcave discs or red blood corpuscles (fig. 140). (See "Blood".)

Fig. 140. - Blood Corpuscles and Leucocytes.

A A, Red corpuscles. B; Side view of a red corpuscle. C, Red corpuscle seen edgeways. D, E, F, Leucocytes.

Haemoglobin (fig. 141), the substance contained in the red cells of the blood, is found in the urine in that sudden and fatal disease known as Hemoglobinuria, and imparts to it the colour of porter. When allowed to stand, the urine in this disease throws down a blackish-brown granular flocculent deposit.

If a few drops of nitric acid be added to a small quantity of the fluid, a dense precipitate of coagulated albumen will be formed, and the same result will follow if the urine be heated in a test-tube over the flame of a spirit-lamp.

Pus (matter) is sometimes found in the urine, and when existing in considerable amount imparts to it an opaque, milky-looking appearance.

Fig. 141. - Haemoglobin Crystals from the Blood.

Fig. 142. - Pus.

A, Pus cells. B, Pus cells treated with acetic acid, showing nuclei. c, Pus cells "budding".

If a small quantity of the urine be set aside in a glass, and allowed to stand, the pus settles down to the bottom as a creamy-looking sediment, When such a deposit is examined with a microscope it is found to consist of small, pale, nucleated or granular-looking cells (fig. 142).

A few pus corpuscles may be frequently found in the urine of old horses and mares, which in other respects exhibit no signs of disease. They are mostly the result of a slight irritability of the bladder or prostate, uterus or vagina.

When pus exists in quantities sufficient to impart an opaque appearance to the urine, it may be presumed that there exists some active disease in some part of the mucous membrane of the genito-urinary organs, or in the prostate gland, or the kidney. It is mostly derived from the bladder, as the result of irritation excited by a calculus, or from a chronic catarrhal state of the mucous membrane. In abscess of the kidney a large amount of pus is found in the urine.