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soft and waxy in consistency. On exposure to the air they become hard, as an examination of museum specimens will show. The principal constituent of human biliary calculi is cholesterin. It may be easily extracted from them by means of ether, in which it is readily soluble, while the coloringmatter of the calculi is insoluble in that medium. It is a compound which is not only found in biliary calculi, but is widely distributed in small quantity throughout the animal body. It is found in fairly large amount in the tissues of the nervous system. It is also found in semen, pus, etc. It forms the larger part of the fat obtained from the wool of the sheep (lanolin). As existing in sheeps' wool, it has associated with it a substance closely allied to it, isocholesterin, which is distinguished from it by not giving the characteristic reaction with sulfuric acid. It is a monatomic alcohol of the formula C2H4OH. It is insoluble in water, soluble in alcohol and ether, and crystallizes from alcohol in fine plates which are characteristic. It has a fatty feel when rubbed between the fingers.

I. Take a small fragment of a biliary calculus in a test-tube. Add about 0.5 c.c. of alcohol. Heat gently, and decant the alcohol into a watch-glass. Allow the alcohol to evaporate, and examine with the microscope the crystalline residue left.

2. Add to the residue in the watch-glass, placed over a sheet of white paper, a drop of sulfuric acid. Note the change in color to blood red, and the transition of the red color on standing to violet. 3. Dissolve a fragment of cholesterin in I c.c. of

chloroform. Add an equal volume of sulfuric acid. Note the change of the chloroform solution to blood red. Observe the green fluorescence imparted to the sulfuric acid.

4. Heat a small fragment of a calculus with sodium carbonate solution in a test-tube. The solution is filtered. It contains the biliary pigments

FIG. 11.-a, Cholesterin crystals; b, cystin crystals (Salinger and Kalteyer),

in the form of sodium salts. Divide the solution into three portions in small test-tubes.

a. To one add a few drops of a solution of calcium chlorid. Calcium carbonate is precipitated, and carries down with it the biliary pigment. Filter off on a small filter, and add to the calcium carbonate a drop of concentrated nitric acid containing nitrous acid. Note the formation of a green color. This changes rapidly to violet, and finally becomes yellow or almost colorless.

b. Allow the contents of the second test-tube to

stand in the air.

The bilirubin is oxidized to biliverdin. The solution takes on a green color.

c. Nearly neutralize the contents of the third test-tube with hydrochloric acid. Add to the testtube an equal volume of concentrated hydrochloric acid containing th of its volume of nitric acid. Note the formation of a greenish-blue color.

MILK.

Chemically, milk consists of a solution of albumins and globulins with milk-sugar, and inorganic salts, and containing fats in an emulsified condition. Fresh milk has a reaction which is known as amphoteric, and has the property of changing either red or blue litmus-paper to the opposite color. It is an excellent culture-medium for microorganisms, and its composition rapidly changes with the growth of these bodies in it. On heating to boiling, the proteids are not coagulated, but a scum forms on the surface, consisting of a combination of one of the principal constituents of milk, casein with salts of calcium. If this be removed and the milk again boiled, the formation of this substance is again renewed.

The average composition of human and cows' milk is as follows:

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As will be seen from the above analyses, the albumin and globulin are almost twice as great in human milk as in cows' milk, while the casein in human milk is about one-third that in the milk of the cow. The lactose is also greater in human milk, and it is for this reason that lactose is added to cows' milk when that fluid is modified for infantfeeding. For the same reason also the milk is diluted in order to reduce the percentage of casein to that found in the human subject.

As the amounts of the various substances found in milk vary with different individuals of the same species, analyses are made to determine in what proportion a particular milk should be diluted, and lactose and fat added. Even with careful methods, it is a question with practitioners to what extent the so-called modified milk can completely replace that of the human subject.

The most important constituents of the milk are the casein, milk-fat, and lactose. The latter compound has been considered in the section of carbohydrates.

Casein is a proteid containing both phosphorus and sulfur. It is difficultly soluble in water or solutions of salts. It behaves like a fairly strong acid, and therefore forms compounds with metals, such as potassium, calcium, mercury, and silver. The silver and mercury compounds have lately come into prominence as antiseptics. Casein does not exist in true solution in milk, but is probably in a state midway between solution and suspension. Its alkaline salts are easily soluble in water, and

hence opalescent solutions of casein become clear on the addition of an alkali. Casein is also precipitated from its solution by many metallic salts, such as copper sulfate, zinc sulfate, and alum. In the presence of a sufficient quantity of calcium salts. which are normally found in milk, it undergoes a peculiar change when treated with the enzymes found in rennet. As occurring in milk, this phenomenon is known as curdling, and is comparable

FIG. 12. Colostrum.

in many respects with the coagulation of blood. The casein on coagulation is transformed into a substance, paracasein, which makes up the principal part of cheese. Lactalbumin is formed at the

same time.

The fats found in milk exist in the form of a finely divided emulsion of fat-particles, of which there are between 5,000,000 and 6,000,000 in a cubic millimeter. It has been assumed that the

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