leaves the body, passes into the urine in the manner just mentioned. Probably the greater quantity of the taurine formed in the intestine is reabsorbed as such. Small quantities of taurine have been separated from liver, muscle, urine (of ox), and spleen (of certain fishes).

(i) Cystine CH,NSO, is amido-lactic acid in which one H is replaced by HS.

Lactic acid-C1HO.

Amido-lactic acid :- C3H5(NH)03.

Cystine :-CH1(HS)(NH2)O3=C3H,NSO3.

If heated on a silver surface it gives a black spot of silver sulphide. It crystallises in the hexagonal system either as colourless hexagonal plates or rhombohedra. The crystals are insoluble in water, alcohol, and ether, but soluble in alkalis, mineral acids, and oxalic acid.

FIG. 37. Cystin crystals.

It is found as a constituent of a rare form of urinary calculus. Its origin is unknown, but cystinuria appears to be hereditary. On exposure to the air these calculi turn green; they form only in acid urine owing to their solubility in alkaline fluids.

According to recent observations by Goldmann and Baumann,1 cystine is a normal constituent of urine, but is present in very minute quantities.2

7

(j) Aspartic (or asparaginic) acid C1H,NO, is amido-succinic acid. It is obtained from the substance asparagine (C1H ̧N2O3) in plants; and is a product of the decomposition of proteids.

8 2

(k) Glutamic (or glutaminic) acid C5H,NO is amido-glutaric acid, i.e. an amido-compound of the same series to which succinic acid belongs (the oxalic acid series, see p. 67). This is also a product of the decomposition of proteids.

(1) Carbamic acid CH,NO2 is amido-formic acid. It is not known in the free state; its ammonium salt is a product of the decomposition of proteids.

The Bile Acids. The bile contains the sodium salts of complex amido-acids called the bile acids. The two acids found in human bile are glycocholic acid and taurocholic acid.

Glycocholic acid C26H43NO6 is especially abundant in the bile of herbivora, and in man its amount is increased by a vegetable diet. By

1 Zeit. physiol. Chem. xii. 254.

2 Delépine (Proc. Roy. Soc. vol. xlvii. 1890, p. 198) states that the formation on cystine may result from the activity of a torula-like organism in the urine.

the action of dilute acids or alkalis and also in the intestine it takes up water and splits into glycocine and cholalic acid.

C26H43NO6+H2O=C¿H ̧NO2+C24H4005

It forms brilliant colourless needles soluble in water and alcohol, but not in ether. Its taste is first sweet and afterwards bitter. The alcoholic solution is dextrorotatory (a)»=+29°.

The glycocholate of soda C26H42NaNO is the compound that occurs in the bile. It crystallises in stellate needles, soluble in water

FIG. 38. Sodium Glycocholate.

FIG. 39.-Cholalic Acid.

and alcohol, but not in ether. (a)D=+257°. Glycocholate of potash occurs in the bile of certain fishes.

Taurocholic acid C26H45NO,S is especially abundant in the bile of carnivora. By the action of hydrating reagents and in the intestine it is decomposed into taurine and cholalic acid.

C26H45NO,S+H2O=C24H40O5+C2H-NO3S.

It forms silky needles, soluble in water and alcohol, and intensely bitter. (a)+24.5°. Sodium taurocholate C26H4NaNO,S is the compound that occurs in the bile, except in certain fishes where the potassium salt is found.

Cholalic (or cholic) acid C24H4005 derived from the bile acids, forms large, shining, deliquescent crystals, slightly soluble in water, and readily soluble in alcohol and ether. (a)=+35°. Boiled with acids or heated to 200°, it loses either one molecule of water to form choloidic acid, or two to form dyslysin (C24H36O3). Latschinoff has recently assigned to cholalic acid the formula C25H4205, but Mylius' has shown that the correct formula is without doubt that originally assigned to it by Strecker C24H4005.

Choleic acid is an acid which has been separated from ox-bile by

1 Ber. d. deutsch. chem. Gesell. xix. 369, 2000; xx. 1968.

42

Latschinoff. It occurs in two forms: anhydrous C25H4204, and hydrated C25H2O,+1H,O. The latter is called deoxy-cholic acid by Mylius. 2 Fellic acid. The cholalic acid from human bile differs in some of its reactions and solubilities from that obtained from ox-bile (Hammarsten).3 Bäyer calls it anthropo-cholalic acid and assigns to it the formula C18H28O4. Schotten suggested that the difference was due to an admixture with choleic acid. He, however, subsequently found this was not the case, but that another acid is present with formula C23H40043 to which the name fellic acid is given. It is due to admixture with fellic acid that cholalic acid from human bile appears to be different from that obtained from other sources.

Hyo-glycocholic and hyo-taurocholic acids combined with soda form the bile salts of pig's bile. In these acids cholalic acid is replaced by a nearly related acid called hyo-cholalic acid (C2H4003). The hyoglycocholate is more abundant than the hyo-taurocholate, and has been separated by Jolin into two varieties a and 3, the former of which is precipitable by sodium sulphate, the latter not. In the bile of the goose cholalic acid is replaced by cheno-cholalic acid (C27H4404).

Pettenkofer's Reaction.-If a thin film of bile be spread on a porcelain dish, a drop of solution of cane sugar, and a drop of strong sulphuric acid be added, a beautiful purple colour is developed, especially on the application of heat. This test is given by all the acids and salts found in the bile. The reaction is due to the formation of furfuraldehyde from the sugar and sulphuric acid; the furfuraldehyde forms a coloured compound with cholalic acid. It is by no means distinctive of bile acids, however, as Mylius and Udranszky have shown. Of the numerous organic substances which give the colour or a very similar one, one only, a-naphthol, gives it more readily than the bile acids. The spectroscopic appearances differ, however, in many instances. In the case of the colour produced by bile there is a band between D and E and another at F.

The Uric Acid Group.- (a) Uric acid C ̧H,N403.-There is much diversity of opinion with regard to the chemical nature of this substance; the different views held on this point, and also its relationship to urea, and to other substances of a similar nature (allantoin, alloxan, xanthine, &c.) will be more appropriately discussed in connection with the physiological uses of these bodies (see Urine).

1 Ber. d. deutsch, chem. Gesell, xviii. 3039.

3 Maly's Jahresb. 1878, p. 263.

5 Ibid. xi. 268.

7 Ibid. xi. 492.

2 Ibid. xix. 369.

4 Zeit. physiol. Chem. ii. 358; iii. 292. 6 Ibid. xi. 417.

8 Ibid. xii. 355. Udranszky here gives a list of 76 organic substances that give the furfuraldehyde reaction.

Pure uric acid crystallises in colourless rhombic plates or prisms. When obtained from urine it is more or less tinged with pigment, and it assumes many crystalline forms (dumb-bells, whetstones, &c.). It is without taste or smell. It is insoluble in alcohol and in ether; it requires for its solution 15,000 parts of cold and 1,900 of hot water. Its solutions give only a feeble acid reaction.

It is not found in the free condition in the urine, except in cases of disease (gravel, calculus); but it is combined with bases to form urates.' The acid is dibasic.

The amount excreted per diem by an adult averages 0.5 to 1 gramme, but its amount is raised by much animal food, and by want of exercise. Urates occur also in the blood, and as chalky deposits in and around the cartilages of gouty persons. The solid urine of birds and reptiles consists almost entirely of urates. Traces of uric acid have been separated from various tissues, kidneys, spleen, lungs, brain and muscle.

Urates of sodium.-The neutral salt C,H,NO,Na, forms nodular masses, and the acid salt C,H,NO,Na is usually amorphous in urine; they form the deposit in urine commonly called lithates. The chalky deposits in gout are chiefly composed of the acid salt, which is then crystalline.

Crates of potassium corresponding to these seldom occur.

Acid ammonium urate CHÂN,O,(NH) (neutral salt unknown) forms globular collections of crystals; it is found in the deposit in alkaline urine, and is the chief component of the excrement of reptiles and birds.

3- 4

Acid calcium urate (CH3N,O3) Ca occurs in the form of fine needles in urinary sediments, calculi, and in gouty deposits.

Acid lithium urate CH3N403Li is the most soluble salt of uric acid, hence the use of lithia as a drug in cases of gouty diathesis.

Murexide test.-Evaporate to dryness with nitric acid; the residue is reddish yellow, and becomes reddish purple on the addition of ammonia, and bluish violet with soda or potash.

(b) Xanthine C,H,N,O, differs from uric acid by containing one atom of oxygen less. It is a pale yellow, amorphous powder, insoluble in alcohol or ether, soluble in cold water. When evaporated to dryness with nitric acid, a yellowish residue remains which turns red with caustic potash, and reddish violet on being heated.

Xanthine occurs normally in minute quantities in the urine, and has been obtained from many organs such as pancreas, spleen, liver, brain, and thymus.

1 The question of quadurates will be discussed under Urine.'

Urinary calculi, consisting of xanthine, varying in size from a pea to a pigeon's egg, occasionally form.

(c) Hypoxanthine CH,NO differs from xanthine by containing one atom of oxygen less. It generally occurs with xanthine. It has been described in the spleen, pancreas, muscles, liver, marrow, blood, and urine. In leucocythæmia its quantity in the blood and urine is increased. In acute yellow atrophy, the amount in the liver rises.

5 5

(d) Adenine CH,N, can be obtained from the nuclei of cells. On heating it with sulphuric acid NH is replaced by O, and hypoxanthine thus formed C¿H¿N ̧+H2O=C¿H ̧Ñ1O+NH ̧. Both substances

[adenine]

5

4 4

[hypoxanthine]

contain a radicle C,H,N, called adenyl (Kossel 1). Adenine is a crystalline substance; the crystals contain three molecules of water of crystallisation.

(e) Guanine CH,NO has been found in the liver and pancreas, in guano, in the excrement of spiders, and in the skin of many reptiles and fishes. It bears the same relation to xanthine that adenine does to hypoxanthine (C ̧H ̧Ñ ̧O+H2O=C2H1N1O2+NH3). It is amorphous,

insoluble in water, alcohol, and ether, but readily soluble in acids and alkalis. The crystals of chlorate of guanine are characteristic.

(f) Allantoin CH,NO, is found in the amniotic and allantoic fluids and in the urine of newborn animals. It crystallises in colourless prisms which are soluble in water. By the action of dilute nitric acid it takes up water and splits into allanturic acid (C-H10N6O6) and urea.

FIG. 40.-Allantoin crystals.

(g) Carmine C,H,N40, which has been separated from muscle, is a crystalline substance, the crystals containing one molecule of water of crystallisation. It

is soluble in warm water, insoluble in alcohol and ether. It may be considered as a compound of hypoxanthine and acetic acid:

Other Nitrogenous Acids.— (a) Inosinic or Inosic acid C10H14N4011•

1 Zeit. physiol. Chem. xiii. 292. Hypoxanthine adenyl oxide; adenine is adenyl imide. Compounds of the radicle NH are called imides.