Fat is found in all the tissues and fluids of the body, usually forming distinct masses or globules, which do not combine with the other elements of the body, but remain free, either suspended in fluids, or lodged between fibres, or deposited in cells.

Fat is essential to the growth and nutrition of the tissues, a larger proportion of fat being met with wherever cell growth is going on rapidly, that in tissues which are fully developed. Fat by its combustion in the economy furnishes a quantity of force to maintain the energy, and heat to supply the temperature of the body; indeed fat may be regarded as the storehouse of carbon; and one apparent advantage of its freedom from combination with other elements is that it is always ready for immediate service, whenever the requirements of the system. demand it.

The chief source of the fat of the tissues is of course from the oleaginous constituents of the food: but fat is also formed by the decomposition of the saccharine and albuminous principles, which yield. fatty acids, and which, probably combining with glycerin, are converted into fat before their ultimate reduction to carbonic acid and water.

Fat is found in only extremely minute quantities in the healthy human excretions, since in the body it is always decomposed into carbonic acid and water, and in this form passes out of the economy. In certain diseases however fat appears in the excretions. In cases of occlusion, for instance, of the pancreatic and biliary ducts the fats introduced with the food

into the intestinal canal are not emulsionized and saponified, and consequently are not absorbed, but pass unaltered out of the system with the fæces. Again, in chyluria, fat globules are met with in the urine.

Whenever the process of oxidation is impeded or imperfectly performed, we find that fat accumulates in the organs and tissues; for example, we find it in the fatty degeneration of the liver and voluntary muscular fibre met with in all cases of phthisis, or pulmonary diseases which have run a chronic course; and in obesity, the penalty of sedentary or self-indulgent habits. Whenever the supply of blood is cut off from a part, or its flow diminished, the oxidation of that part is of course arrested, and, as a consequence, fatty degeneration occurs; for instance, a thrombus blocks up a cerebral artery, and acute softening of the cerebral substance supplied by that artery is the result. And even if the supply be only diminished instead of entirely arrested, the result is the same only not so rapid. If an organ or member is long disused, or its functional activity impaired, it undergoes fatty degeneration, since the physiological stimulus being no longer supplied, the same quantity of blood does not circulate through the part as when it was in full activity and vigour; the fatty degeneration of the muscular fibres of the uterus after delivery, and the tendency to accumulate fat after the active work of life is over, illustrate this point.

Certain poisons, as the salts of the bile acids,

phosphorus, &c., when introduced into the system produce rapid fatty degeneration of the organs and tissues, by causing the destruction of the blood corpuscles and the consequent diminution of the oxidizing power of the blood.

Chemical and physical properties

Test 1. They are insoluble in cold water.-Shake up a little olive oil with water in a test-tube. The oil becomes broken up into small globules, which readily reunite and float on the surface of the water.

Test 2. Readily soluble in ether; benzol, fluid oils, chloroform, and hot alcohol.-Place a drop of olive oil on a glass slide, add a drop or two of ether, the oil is dissolved; on evaporating the ether with the breath the oil is left as a greasy residue on the glass slide.

Test 3. They are highly inflammable.-Burn a small portion of fat on a piece of platinum foil, it flares away with great intensity, leaving little or no residue.

Test 4. They form an emulsion with a solution of albumin.—Shake up a small quantity of olive oil with solution of albumin. A milky fluid is formed, this under the microscope will be seen to consist of minute globules. On standing, these globules slowly unite, and form larger ones, and finally, a considerable portion of the oil will separate itself and float free on the surface of the solution of albumin.

Test 5. Heated with alkalies they are saponified.Shake up some olive oil with an equal quantity of liquor potassæ and heat, a soapy fluid, which lathers on agitation, results. (Saponification consists in the decomposition of the fat or oil into fatty acid and glycerine. The fatty acid unites with the alkali to form a soap whilst the glycerine remains in solution.)

II. The non-saponifiable fatty matters are distinguished from the preceding by not being decomposed or saponified when treated with alkaline solutions. Consequently they can be separated from the other

fats by adding a solution of caustic potash to the etherial solution, which causes the saponifiable fats to dissolve out, leaving the non-saponifiable in solution.

(10) CHOLESTERIN, C26H44O. Is the only member of the group which requires particular notice. This substance is generally regarded as an excretory product formed in the substance of the brain and nervous tissue, whence it is absorbed by the blood and carried to the liver; here it is separated from the blood and discharged with the bile into the intestines where it undergoes decomposition.

Cholesterin can be obtained from nearly all the tissues and fluids of the body; it also occurs in several morbid products, as gall-stones, the fluid of hydatid, ovarian cysts, &c.

Preparation. The fluid or tissue from which cholesterin is to be extracted, must be thoroughly exhausted with ether. The etherial solution is then filtered, agitated with a solution of caustic potash to remove the saponifiable fats. After standing a few minutes the etherial solution containing the cholesterin will separate from the solution of caustic potash. The etherial solution is then drawn off by means of a pipette, and placed in a glass flask and evaporated over boiling water. (N.B.-Care must be taken not to bring the flask near the gas-burner, or the vapour of the ether will take fire. It is advisable to bring the temperature of the water to boiling-point, then withdraw the flame, and plunge the flask up to its neck in the water. When the temperature of the water has fallen to 40° C., remove the flask and replace the burner, and raise the temperature of the water to boiling-point again, then remove the burner and replace the flask, and so on till the whole is evaporated.) Add to the residue left at the bottom of the flask just so much boiling alcohol as will dissolve it.

The alcoholic solution on cooling deposits cholestearin in glistening crystalline plates.

Chemical and physical characters.-Cholestearin, as obtained by the above process, is a white crystalline substance, somewhat resembling spermaceti.

Test 1. It is lighter than water, and floats on the surface; and it is not dissolved by it.

FIG. 1.-Cholestearin.

Test 2. Very soluble in ether.— Place a minute fragment of cholestearin on a glass slide, and touch it with ether, it is at once dissolved. Evaporate the etherial solution with the breath, and the cholestearin is deposited.

Test 3. Red coloration given when evaporated with nitric acid, and the residue touched with ammonia.-Place a minute fragment on a white porcelain dish, add a drop of nitric acid, evaporate to dryness, touch the dried residue with a drop of ammonia, a deep red-brown coloration is developed.

Test 4. Violet coloration with hydrochloric acid and ferric chloride.—Place a minute fragment of cholestearin on a white porcelain dish, and add two or three drops of strong hydrochloric acid and one or two drops of dilute solution of ferric chloride, evaporate gently, the residue acquires a beautiful violet colour.

Requisites for Demonstration II.

MATERIALS.-Mutton suet.

Olive oil. Gall stones or etherial extract of brain-pulp (made by digesting ox-brains in ether). Solution of Albumin.

REAGENTS.-Distilled water. Alcohol. Ether. Nitric acid. Hydrochloric acid. Solutions of Ammonia. Potassium hydrate. Ferric chloride.

APPARATUS.-Thermometer. Water bath Pipette. Small glass flask. Glass slide. Stirring-rod. Test-tubes. Platinum foil.

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