The physical characters of the bile are as follows :—In colour it is always a deep brown, but when seen in thin layers it has a brownish yellow tint. It is very fluid, being viscid only in new-born infants. The specific gravity varies from 1.032 to 1.040. On examining with the microscope bile from the gall-bladder, with which of course a cer tain amount of mucus is mixed, there are observed :-1, Tranaparent or grayish round vesicles, about the 700th of a lino in diameter ; they disappear on the addition of alcohol or ether, and are removed by filtration. 2, Conical yellow bodies, about the 140th of a lino in length, and about the 300th or 400th of a line in breadth, apparently devoid of nuclei ; these are epithelial cells from the gall-bladder. 3, Here and there irregular dark granules, which disappear on the addition of a solution of potash, apparently pigment cells. 4, Occa sionally minute crystals of cholesterin, occurring as colourless rhombic tablets.
Chemically the bile is composed of several elements which have a tendency to arrange themselves during chemical analysis in very various forms. Not only are the four organic elements, carbon, hydrogen, oxygen, and nitrogen present, but also sulphur, phosphorus, sodium, potassium, calcium, and iron. The union of the organic elements in different proportions will account for the various sub stances such its picromel, bilis, choleic acid, colic acid, taurine, which chemists have described in their analyses of biles. According to Dr. Kemp the organic portion of ox-bile may be represented by the formula 48 carbon, 42 hydrogen, 13 oxygen, and 1 nitrogen. The following analyses of ox-bile and human bile, by Therms! and Ikrzelius, will serve to 'show the nature of bile as well as the pro gress of chemical inquiry on this subject when contrasted with more recent analyses. According to Therms' tho composition of bile is an follows :— It will be seen from these analyses that the chief part of the organic elements was found in the form of picromcL It was in the year 1838 that Demarcay announced that bile consisted essentially of an organic acid combined with soda. He termed the acid choleic, and obtained it in the following manner : bile, from which the mucus had been precipitated by alcohol, was evaporated on the water-bath, and 10 parts of the dried residue were dissolved in 100 parts of water, to which 10 parts of hydrochloric acid had been added. Allowing evaporation at a moderate temperature to proceed, it was observed that a dark green oil collected on the surface, while at the nine time the fluid became turbid. On removing the oil and allowing the fluid to rest for some
time, it gradually became clear, with the precipitation of a green deposit. This dark green bitter precipitate is Dernarcay's choleic acid, and is regarded by him as constituting nine-tenths of the solid consti tuents of the bile. It is still mixed with margaric acid, cholesterin, pigment, &c. After their removal it forms a yellow spongy matter, which rapidly absorbs oxygen from the atmosphere, is very bitter, slightly soluble in ether, soluble in water, and very soluble in alcohol. The choleate of soda, obtained by adding an alcoholic solution of soda to an alcoholic solution of choleic acid, and then passkag a current of carbonic acid through it to remove the excess of soda, possesses all the characters of bile ; it yields on evaporation a brown resinous mass, and is soluble in.water and in alcohoL When choleic acid is boiled with hydrochloric acid it yields ammonia, taurine, and choloidic acid ; the latter being insoluble is deposited. The formuke usually assigned to choleic acid, taurine, and choloidic acid differ only slightly from the formula given above for the organic portion of ox-bile.
But it has been recently shown by Redtenbacher that taurine contains as much as 25 per cent. of sulphur.
As an instance of modern chemical analysis we give the two follow ing analyses. The bile in these cases was obtained from healthy men, killed by severe accidents: Platner succeeded in obtaining cholele acid and choleate of soda in a crystallised form. Sugar has also been recently demonstrated to exist in the bile. Gmelin and Strecker have also obtained from dried bile an acid which they call choleic, and other substances have been procured from the bile of lower animals.
One of the uses which the bile serves in the economy is to produce a specific change upon the aliment in a certain stage of the digestive process. The first change which the food undergoes after it has been swallowed is the reduction of it by the stomach into a fluid mass, the appearance of which varies considerably according to the nature of the food. This fluid mass is termed chyme, which when accumu lated in a certain quantity is sent from the stomach into the duodenum.
In the duodenum the food undergoes a further change, and is con verted from chyme into the substance called chyle. These two fluids are distinguished from each other by specific characters. [DIGESTION.] That the bile is the main agent in producing the change by which chyme is converted into chyle is proved by a decisive experiment performed by Sir B. Brodie.