The high percentage of mucin in the incompletely developed connective tissue of young children is due to the greater quantity of ground substance present there.

In cases in which fluids are to be analysed for mucin, the precipitate produced by the addition of acetic acid may be collected and weighed, or alcohol may be added to the fluid; this precipitates both mucin and proteids; the former may then be dissolved out from this precipitate by means of baryta-water or lime-water, and precipitated therefrom by acetic acid.

Læbisch states that mucin has an acid reaction, and that the amount present may be measured by the decrease of alkalinity of an alkaline solution employed to dissolve it.

Properties and reactions of connective-tissue mucin.—Mucin is a slimy, glutinous substance insoluble in water and in alcohol. It is

soluble in weak alkaline solutions, such as lime-water, from which it is easily precipitable by acetic acid, and is not soluble in excess of that acid. It may also be precipitated by means of the mineral acids, but is soluble in excess of those reagents. It is insoluble in solutions of hydrochloric acid containing less than 1 per cent. of the acid. In 2 per cent. hydrochloric acid it dissolves slowly, but more quickly on the application of heat. In 5 per cent. hydrochloric acid it dissolves readily. It, however, loses its characteristic properties when treated with acids of this strength, being split into its proteid and carbohydrate constituents, the former being converted into acid-albumin.

Mucin is precipitated from its solutions completely by saturating them with sodium chloride, magnesium sulphate, or ammonium sulphate.

Mucin which has been precipitated by acids is almost insoluble in solutions of common salt (5-10 per cent.), but mucin as it exists in the vitreous humour is more soluble in such solutions, but still not freely soluble.

The various mucins differ from one another as to whether or not they are precipitated by various metallic salts; connective-tissue mucin is precipitated by lead acetate, by ferric chloride, and by copper sulphate. It is not precipitated by potassium ferrocyanide and acetic acid; it is not precipitated by tannic acid.

With copper sulphate and caustic potash it forms a violet solution, which is not reduced on boiling. With Millon's reagent it behaves like a proteid. It also gives the xanthoproteic reaction (orange colour with nitric acid and ammonia).

The prolonged action (six to eight weeks) of absolute alcohol renders mucin insoluble in dilute alkalis. It appears to be converted into a substance like coagulated proteid.

Composition of connective-tissue mucin.-From its percentage composition (C, 483; H, 6·44; N, 11-75; S, 0-81; 0, 32-7) Labisch has calculated the following empirical formula: C160H256N32SO 80. It thus contains the same elements as a proteid, and there is no doubt that mucin is an albuminoid which is very closely allied to the proteids, and that it is the product of the differentiation of the protoplasm of animal cells. Landwehr indeed considers it as only an intimate mixture of a proteid and a carbohydrate; but it is more generally regarded as a chemical unit, a compound of these two substances. The action of dilute mineral acid is first to split up the mucin into proteid and carbohydrate. The proteid is converted into acid-albumin ; the carbohydrate (Landwehr's animal gum) has the empirical formula C6H1005. It does not reduce copper salts; by the further action of the mineral acid, the

gum is converted into a reducing sugar or gummose, which has the formula C6H12O6

Decomposition products of mucin.-1. The proteid constituent of mucin.— This appears to be of the nature of a globulin; it is precipitated by salts like a globulin, and is convertible, like globulins, with extreme case into acid-albumin. Mucin in suspension in water becomes very insoluble in weak alkalis after its temperature has been raised to 70° C. This is presumably because the heatcoagulation temperature of the proteid contained in the mucin has been reached.

2. The carbohydrate constituent of mucin.—Animal gum.'—This substance may be obtained from mucin in the following way: Mucin is dissolved in weak hydrochloric acid by the aid of heat; on neutralising with soda a white flocculent precipitate of proteid is obtained, which is increased in amount by the addition of sodium sulphate crystals and boiling. This is filtered off, and the filtrate contains animal gum; this may be precipitated by means of copper sulphate, and the copper subsequently separated from it Landwehr has obtained this carbohydrate from the mucin of tendon, of the saliva, of synovia, from colloid cysts, from metalbumin and paralbumin, from chondrin, and in small quantities from all parts that contain connective tissue, such as the brain, kidney, spleen, &c.; he has also separated it in small quantities from the urine."

Animal gum forms an opalescent solution with water; it gives a sticky precipitate with copper sulphate, and also with ferric chloride. It does not reduce alkaline solutions of copper salts. It gives no colour with iodine. It is precipitated by alcohol. Like vegetable gum, it yields oxalic acid after treatment with nitric acid, and lævulic acid after treatment with hydrochloric acid. Boiling a solution with 1 per cent. sulphuric acid gives it the power of reducing alkaline solutions of copper and bismuth salts: this is due to the conversion of the gum (CHO) into gummose (C,H,20), a reducing substance, which will not, however, undergo the alcoholic fermentation. This conversion is, however, slow and incomplete.

The physiological and pathological importance of animal gum seems to be much exaggerated by its discoverer: the following are, however, the chief points advanced by Landwehr in this relation: Animal gum is present in the fœtal tissues more abundantly than in the adult; in some animals, as in the frog, the gum is derived from the mucinoid envelope of the egg, in mammals from the mother; periods of activity of the generative organs are thus associated with an increased formation of animal gum; and pathological conditions of the female generative organs are often associated with excess of mucin and other compounds that contain gum, such as ovarian cysts and myxedema. The part that animal gum is supposed to play in chlorosis has been already dealt with (p. 300). Among other functions attributed to animal gum are: a part in the production of hydrochloric acid in the stomach; the aiding of the emulsification and absorp

1 Landwehr, Pflüger's Archiv, xxxix. 193; xl. 21.

2 Landwehr, Centralbl. med. Wissensch. 1885, p. 369. This observation has been confirmed by Wedenski, Zeit. physiol. Chem. xii. 122.

3 Wolfenden (Journal of Physiology, v. 91) shows that the envelope of the frog's egg consists of mucin.

4 No constant relation has been shown, however, to exist between diseases of the generative organs and myxedema (see further, next chapter).

tion of fats in the intestine; it is also regarded as the mother substance of milk sugar. These statements await verification before they can be received as more than interesting theories.

Further decomposition products of mucin.- Mucin yields leucine and tyrosine after prolonged boiling with strong sulphuric acid. Obolensky' obtained pyrocatechin (CHO) by boiling submaxillary mucin with caustic soda for fifteen to twenty minutes; but I have not succeeded in obtaining this substance from connective-tissue mucin in this way. Putrefaction produces the same decomposition products as it does from proteids.

Mucin is not digested by artificial gastric juice; if mucin, however, be dissolved in 2 per cent. hydrochloric acid, and the solution diluted till the percentage of HCI is 0.2, and then pepsin be added, albumoses and peptones will be formed at a suitable temperature (40° C.); but, as has been explained already, a solution of mucin made in this way is not really a solution of mucin at all, but a solution of the decomposition products of mucin, one of which is acid-albumin; and this it is which is converted into peptone by the action of pepsin. Pancreatic juice, in virtue of its alkalinity, readily dissolves connective-tissue mucin, and the results of artificial pancreatic digestion are albumoses, peptones, leucine, tyrosine, &c. from the proteid part of the molecule, and a reducing sugar from the animal gum.

CARTILAGE

In it are

In hyaline cartilage the matrix is free from fibres. embedded numerous cells, most commonly in groups of two or more; the cells are rounded except in the neighbourhood of adjoining fibrous tissue, where they are branched (transitional cartilage).

The matrix is much firmer than the ground substance of connective tissue proper, but, like it, is stained brown by nitrate of silver and subsequent exposure to light. It yields, on boiling, a substance called chondrin.

The cells lie in cavities in the matrix, which they apparently entirely fill in the natural condition. Each cavity is bounded and enclosed by a transparent capsule, which coheres intimately with the surrounding matrix, and it is only in young cartilage that it can be clearly distinguished from the matrix without the use of reagents. The cells in cartilage may be sometimes seen in a state of division, as indicated by the karyokinetic figures in their nuclei; the capsule divides with the cell, so that in a group of cells formed by subdivision a capsule may be traced surrounding the whole group, and secondary capsules enveloping each cell. It is doubtful how the capsule is produced, whether excreted by the cell which it subsequently encloses (Kölliker) or formed by a conversion of the superficial layer of the

1 Pflüger's Archiv, iv. 336.

I I

protoplasm of the cell-body (Max Schultze). There is at first no matrix but what is made up of the simple capsules.'

In fibro-cartilage the matrix, at first hyaline, is subsequently pervaded by elastic fibres (yellow or elastic fibro-cartilage), or by white fibres (white fibro-cartilage). The fibres here have the same properties and characteristics as those which occur in connective tissue proper.

Cartilage cells are rounded, oval, or bluntly angular masses of protoplasm embedded in which are fine curvilinear interlacing filaments and minute granules; by the use of osmic acid, fat-globules of varying size may often be demonstrated to exist; they are stained black by the osmic acid. Occasionally, and especially in young cartilage cells, glycogen appears to be present, for the cells are coloured brown with iodine (Neumann 2).

The following analyses by Hoppe-Seyler 3 exhibit the relative proportions of water, and of organic and inorganic matters in human hyaline cartilage :—

Articular
cartilage

676.7

735.9

323.3

264.1

301-3

248.7

22.0

15.4

The organic solids consist of those in the cells, which are mostly of proteid nature, and the organic basis of the matrix, which yields the albuminoid substance chondrin, and forms the greater part of the organic material of cartilage.

Chondrin

Preparation.-Chondrin may be obtained from hyaline cartilage by finely dividing the latter, and heating it with water in a digester or sealed glass tube to the temperature of 120° C. The solution so

For further particulars concerning the structure and development of cartilage see Quain's Anat. 9th edit. ii. 84.

Arch. f. mikr. Anat. vol. xiv. 1877.

3 Quoted by Gamgee, Physiol. Chem. p. 268.