Two acids exist in the bile united with soda to form soluble soap-like salts. They may be recognized by the purple-violet color produced by cane sugar and sulphuric acid at a temperature of about 700 C. (Pettenkofer's reaction).

Taurocholic Acid, C26H15NS07, is most plentiful in the bile of carnivora, where it occurs combined with soda. It is decomposed by prolonged boiling with water into taurin and cholic acid, thus: -

Taurocholic Acid. Taurin. Cholic Acid

C26H15NS07 + H20 = C2H7NS03 + C24H10O5.

Glycocholic Acid, C26H15N06, found in the bile of herbivora and man. It crystallizes in fine white, glistening needles. It exists as the glycocholate of soda in the bile. By boiling with weak acid, it yields glycin and cholic acid.

Glycocholic Acid. Glycin. Cholic Acid

C26H15N06 + H20 = C2H5N02 + C24H10O5.

In the bile certain matters also exist to which the color is due, the principal being bilirubin in man and carnivora, and biliver-din in herbivora. They are probably derived from the coloring matter of the blood. They can be recognized by treating the solution with nitric acid which is colored with red fumes, when a play of colors is seen passing through stages of green, blue, violet, red and yellow.

Lecithin, C44 H90 NPO9, is a complex nitrogenous fat found in most tissues and fluids of the body, particularly in the nerve tissues and yelk of egg. It is an interesting product of decomposition of the constituents of the brain, and is related in constitution to the neutral fats; it may be regarded as an acid glycerine ether. It is easily decomposed when heated with baryta water, splitting into glycerin-phosphoric acid, neurin, and barium stearate.

Another body called Cerebrin, not containing any phosphorus and of doubtful composition, can be obtained from brain substance, and is also found in nerve fibres and pus corpuscles. It is a light colorless powder which swells in water.

Protagon, C160H308N5PO35, is by some supposed to be the chief constituent of brain substance, and by others a mixture of the last two bodies.

Neurin (Choliri), C5H15N02, is an oily liquid only found in the body as a product of the decomposition of lecithin, but it has been obtained synthetically.

Cholesterin, C26H140 + H20, exists throughout the body where active tissue change is going on, particularly the nervous centres. It is a monatomic alcohol, and is the only one existing free in the body. It may be obtained from gall stones, some of which consist entirely of cholesterin. It may occasionally be found in a crystallized form in many of the fluids of the body but never in the tears or urine, and only seems to be an effete product, nearly all that produced in the body being discharged with the effete portions of the bile. It may be recognized by the shape of the crystals formed from a solution in alcohol, which are rhombic plates, in which one corner is generally deficient.