The structure of the corium or true skin is quite different from that of the epidermis which has just been described, as it is principally composed of interlacing bundles of white fibres, of the kind known as “connective tissue” (see Fig. 14); these are composed of fibrils of extreme fineness, cemented together by a substance somewhat more soluble than the fibres themselves. The fibres are not themselves living cells, but are apparently produced by narrow spindle-shaped cells lying against them. The felted fibre-bundles are more loosely interwoven in the middle portion of the skin, but become compacter again near the flesh. In the case of sheep-skins this is especially marked, the middle part being full of fat-cells and very loose. Any ill treatment of the pelt during the wet-work is liable to still further loosen this middle layer so that grain and flesh may sometimes be torn apart. The flesh-splits of sheep-skins must have this loose fatty layer frized off before chamoising, and American “waxed fleshes” from ox-hides are levelled by splitting away this portion, and finished on the flesh. The outermost layer, just beneath the epidermis, is exceedingly close and compact, the fibre-bundles that run into it being separated into their elementary fibrils, which are so interlaced that they can scarcely be recognised. This is the pars papillaris, and forms the lighter-coloured layer, called (together with its very fine outer coating) the “grain” of leather. It is in this part that the fat-glands are embedded, while the hair-roots and sweat-glands pass through it into the looser tissue beneath. It receives its name from the small projections or papillæ, with which its outer surface is studded, and which form the characteristic grain of the various kinds of skin.[20] (See Fig. 9 and Plate I.)

The study of the structure of the grain, and especially of the arrangement of the hair-pores is very important, as it is usually the readiest means of identifying the kind of skin of which a leather is made, which in finished skins with artificially printed grain is often very difficult. (Plate II.) The examination is facilitated by wetting and stretching the skin, and by the use of a good lens, or a low power of the microscope.[21]

As stated above, the surface of skin which is next to the flesh is firmer than that in the centre, and as the fibres run nearly parallel with the surface it has a more or less membranous character. The skin is united to the body of the animal by a network of connective tissue (panniculus adiposus), which is frequently full of fat-cells and is then called adipose tissue. This constitutes the whitish layer which is removed, together with portions of actual flesh, in the operation of “fleshing.” If a minute portion of adipose tissue be examined microscopically, it will appear to consist of a mere mass of fat-globules entangled in connective tissue. If, however, it be stained with carmine or logwood it may be at once observed that each globule is contained in a cell, of which the nucleated protoplasm, by which the fat was secreted, is pressed closely against the wall (Fig. 15). Similar cells are contained in considerable quantities throughout the hide, and especially in the loose tissue of the central part; hence in leather manufacture it is impossible to expel or wash out the fat until the cells have been broken down by “liming” or in some other way.

Many animals (ox, horse, etc.) possess a thin layer of voluntary muscle (red flesh) spread over the inner side of the skin, and used for twitching to drive off flies. In rough fleshing this is sometimes left on and may be a cause of dark flesh in sole leather. Even in the finished leather its striped structure may be detected microscopically (Fig. 16).

Besides the connective-tissue fibres, the skin contains a small proportion of fine yellow “elastic” fibres. If a thin section of hide be soaked for a few minutes in a mixture of equal parts of water, glycerine, and strong acetic acid, and then examined under the microscope, the white connective-tissue fibres become swollen and transparent, and the yellow “elastic” fibres may be seen, as they are scarcely affected by the acid. The hair-bulbs and sweat- and fat-glands are also rendered distinctly visible by this treatment. On the other hand, the white gelatinous fibres are most easily seen by examining the section in a strong solution of common salt, or in one of ammonium sulphate; or by staining with some aniline dyes such as safranine. Sections are most readily cut for these purposes by the use of the freezing microtome, or after previous hardening in alcohol. For further details see L.I.L.B., p. 254.

Ordinarily in the production of leather only the corium, or true skin is used, and in order to obtain it in a suitable condition for the various tanning processes, the hair or wool, together with the epithelium, must be completely removed without damaging the skin itself; and especial care must be taken that the grain, or portion next to the epidermis, does not suffer any injury during the treatment. All the methods employed depend upon the fact that the epidermis cells, especially the soft growing ones next to the corium, and those of the epidermis layer which surround the hair-roots, are more easily destroyed than the corium itself owing to their different chemical character. The “unhairing” process consists essentially in breaking down these cells by chemical or putrefactive agents, and removing the hair together with the rest of the epidermis by mechanical means. Of the various substances which may be used for this purpose, lime is one of the most convenient, as its solubility in water is so slight, that a solution of such a strength as to injure the hide cannot be easily made. Caustic alkalies, on the other hand, are much more soluble, and unless care be taken to use only the proper quantity, a dangerously strong solution may be made with consequent damage to the skin. The addition of small amounts of sulphides to the lime-solution accelerates the unhairing owing to their special solvent action on the epidermis-structures, and also in the case of alkaline sulphides, by the caustic alkali which is produced by their reaction with the lime. Even if used alone, strong solutions of alkaline sulphides rapidly destroy both hair and epidermis, converting them into a mass which may be swept off the skin like wet pulp, and yet they have practically no injurious action on the true skin.

In the “sweating” process the epidermic cells are broken down by putrefactive organisms and their products, so that the hair becomes loose and may then be either rubbed or scraped off. Ammonia, which is produced during the putrefaction, has also an important solvent action, and its presence doubtless tends to quicken the processes both of unhairing and of destruction.

To obtain useful knowledge of the structure of any particular skin, it is not necessary to have a very elaborate or expensive microscope, and it is quite possible to obtain useful information merely by the use of a good pocket lens, as for instance, in the examination of various forms of “grain,” and the embossing of one skin to imitate another.

For further details of the manipulation and selection of the microscope, the reader must consult L.I.L.B., pp. 234 et seq.

CHAPTER VIII.
THE CHEMICAL CONSTITUENTS OF SKIN.

The chemistry of the various constituents of skin is still very imperfectly understood, but Beilstein, in his great handbook of organic chemistry, places gelatin, albumens and keratins in the “aromatic” series, and implies therefore that they contain the “benzene” ring. It is at least certain that all are very complex.

The epidermis structures belong to the class of keratins, which are closely related to coagulated albumin; while the white fibres of the corium (or true skin) are either identical with gelatin, or only differ from it in their molecular condition or degree of hydration. This gelatinous tissue constitutes the bulk of the corium, but it also contains albumen as a constituent of the lymph and blood which supply its nourishment, keratins in the epithelial structures of the blood and lymph vessels, and “yellow fibres,” which are perhaps allied to the keratins, but which cannot well be isolated for analysis.

The white connective tissue of the corium is converted into gelatin (glutin) by boiling with water. Owing to the impossibility of obtaining unaltered hide-fibre free from the other constituents, and still more to that of deciding to what point it should be dried to remove uncombined water, it is impossible to prove by analysis whether its composition is identical with that of glutin; but as the white fibre constitutes by far the largest part of the corium, and the other constituents do not differ largely from it in their percentage composition, an analysis of carefully purified corium is practically identical with that of the actual fibre. The following analyses of hide and gelatin are therefore of interest.

The analyses of Von Schroeder and Paessler[22] are of special importance as being the average of a large number of separate determinations. Their nitrogen determinations are by Kjeldahl’s method. Small amounts of ash and traces of sulphur are neglected, and probably included in the O, which is obtained by difference.

Analyses of Purified Corium.

Analyses of Gelatin (free from Ash).

It will be noted that the above analyses of skin differ more widely among themselves than their average does from that of the gelatin analyses, though on the whole the nitrogen is somewhat higher in the latter. The molecular weight of gelatin must be very high,[24] and any empirical formula founded on ultimate analysis therefore quite hypothetical. Bleunard,[25] Schützenberger and Bourgois,[26] and Hofmeister agree on the formula C₇₆H₁₂₄N₂₄O₂₉, which leads to the following percentage composition:—

The addition of a molecule of water would make a difference in the percentage composition indicated by these formulæ which would be less than their probable experimental error, and the change may therefore be one of hydration.

Gelatin certainly contains both carboxyl and amido-groups, and is capable of combining with both acids and alkalies (see p. 84).

Reimer[27] obtained what he supposed to be pure unaltered fibre-substance by digestion of purified hide with ¹⁄₂ per cent. acetic acid for many days and subsequent neutralisation. His analysis showed C = 48·45 per cent., H = 6·66 per cent., N = 18·45 per cent., O = 26·44 per cent., thus deviating considerably from direct analysis of unaltered skin. It is obvious that little weight can be placed on this result, Reimer’s precipitate being probably a mere decomposition product.

Hofmeister[28] notes that on heating gelatin it loses water and forms an anhydride which he considers identical with collagen or hide-fibre. When gelatin is dried at a temperature of 130° C. it becomes incapable of solution in water, even at boiling temperature, and can only be dissolved by heating under pressure. It is certain that collagen (hide-fibre, ossein) is less easily soluble in hot water than ordinary gelatin.

So far as our present knowledge goes, we may regard hide-fibre as merely an organised and perhaps dehydrated gelatin.

Gelatin or glutin (not to be confounded with the gluten of cereals), when pure and dry is a colourless, transparent solid of horny toughness and of sp. gr. 1·3. It begins to melt about 140° C., at the same time undergoing decomposition. It is insoluble in hydrocarbons, in ether, or in strong alcohol. In cold water it swells to a transparent jelly, absorbing several times its weight of water, but does not dissolve. In hot water it is soluble, but a solution containing even 1 per cent. of good gelatin sets to a weak jelly on cooling. Gelatin jellies melt at temperatures which vary considerably with the quality or freedom from degradation products, but which within pretty wide limits (5-10 per cent.) are little affected by the concentration. A 10 per cent. solution of best hard gelatin melts about 38° C., while low glue may fail to set at 15° C. A useful technical test for the setting power of gelatin, based on this fact, consists in placing an angular fragment of the jelly in a small tube attached to a thermometer, and stirring in a beaker of water, which is slowly heated till the jelly melts, when the temperature is noted. The exact point is perhaps more easily seen if the tube is drawn to a conical point. The jelly may also be allowed to set in capillary tubes open at the bottom, and the moment noted when water rises into the tube. The temperature of fusion is raised by the addition of formaldehyde, salts of chromium, alumina and ferric salts, which produce a tanning effect, and in a less degree by sulphates, tartrates, acetates, some other salts, and diminished by iodides, bromides, chlorides and nitrates.[29] Solutions of gelatin too weak or too warm to gelatinise possess considerable viscosity. Gelatin may therefore be estimated, in the absence of other viscous matters, by the viscosimeter, an instrument which measures the time taken by a liquid in flowing through a capillary tube.[30] The firmness of a jelly, which is often important for commercial purposes, is frequently measured by Lipowitz’s method, in which a slightly convex disc, conveniently of exactly 1 cm. diameter, and cemented to the bottom of a thistle-head funnel tube, is loaded gradually with mercury till it sinks in the jelly. The jelly (5 or 10 per cent.) should be allowed to set some hours before the test is made.

Solutions of gelatin from skin and bone are powerfully lævorotatory to polarised light. At 30° C. (A)_D = -130°, but temperature and the reaction of the solution have much influence on the value found.

Gelatin is precipitated from aqueous solution by the addition of strong alcohol and concentrated solutions of ammonium sulphate and some other salts. Many other colloid bodies such as dextrin and gums behave similarly. In the absence of these substances, precipitation by alcohol may be utilised for the technical analysis of gelatins and glues, printers’ roller compositions and gelatin confectionery. 25 c.c. of the gelatinous solution, which is preferably of about 10 per cent., is placed in a small beaker tared together with a glass stirring rod, and thrice its volume of absolute alcohol added. On stirring, the gelatin sets firmly on the rod and sides of the beaker, and may be washed with dilute alcohol or even with cold water, dried and weighed. A very pure French gelatin gave 98·6 per cent., while a common bone-glue only yielded about 60 per cent. precipitate. Absolute alcohol withdraws water from gelatin-jelly, leaving a horny mass. Gelatin may also be precipitated completely by saturating its solution with sodium chloride, and then acidifying slightly with sulphuric or hydrochloric acid; and masses of jelly become hardened in acidified salt solution as in alcohol, though a neutral solution has little effect. The cause of this is difficult of explanation, but its bearing on the pickling of sheep-skins (p. 89) and the production of white leather (p. 186) is obvious.

Decompositions.—When aqueous solutions of gelatin are heated under pressure, or in presence of glycerin and other bodies which raise the boiling-point, or more slowly at lower temperatures, they gradually lose the power of gelatinising on cooling, the gelatin being converted into modifications soluble in cold water, but still capable of being precipitated by tannin. Hofmeister[31] states that the gelatin takes up 3 molecules water and is split up into hemicollin, soluble in alcohol and not precipitated by platinic chloride solution; and semiglutin, insoluble in alcohol and precipitated by platinic chloride solution. Both are precipitated by mercuric chloride. Dry gelatin is soluble in glycerin at high temperatures, but probably suffers a similar change. Hence high temperatures and long-continued heating must be avoided in gelatin manufacture; and in making printers’ roller compositions, which are mixtures of gelatin and glycerin, the gelatin must be swollen with water and melted at a low temperature with the glycerin.

Gelatin is also converted into soluble forms (peptones), perhaps identical with the above, by the action of heat in presence of dilute acids and alkalies. These, like gelatin, are precipitated by tannin and by metaphosphoric acid.[32] Heated for longer periods or to higher temperatures with aqueous solutions of the caustic alkalies, baryta, or lime, gelatin is gradually broken down into simpler and simpler products, ending in nitrogen or ammonia, water and carbonic acid. Among the intermediate products may be mentioned various acids of the amido-acetic series, as amido-acetic (glycocine, glycocoll), amido-propionic (alanine), and amido-caproic (leucine); and of the amido-succinic series (amido-succinic = aspartic acid).[33]

Treatment with acids produces very similar effects. The first products are soluble peptones. Paal[34] on treating 100 parts of gelatin on the water-bath with 160 parts water and 40 parts concentrated HCl till the product was soluble in absolute alcohol, obtained, on purification, a white hygroscopic mass of peptone salts containing 10-12 per cent. of hydrochloric acid.[35]

The products of digestion of gelatin with gastric and pancreatic juice are peptones which do not differ materially from gelatin in ultimate composition, and the action is probably mainly hydrolytic.[36]

The earlier products of putrefaction are very similar. Many bacteria have the power of liquefying gelatin-jelly. This has been shown by Brunton and McFadyen[37] to be due not to the direct action of the bacteria, but to a soluble zymase secreted by them which peptonises the gelatin. Its action is favoured by an alkaline condition, and destroyed by a temperature of 100° C.[38] As putrefaction progresses, the solution becomes very acid from the formation of butyric acid, and later on ammonia and amido-acids are formed.

Fahrion,[39] starting with the idea that albuminoids and gelatin were condensation products of a lactone character (L.I.L.B. p. 185), and that they might, like lactones, be depolymerised by saponification, digested these bodies with alcoholic soda till they were dissolved, and on neutralising the solution with hydrochloric acid, of which the excess was driven off by repeated evaporation, and removing the sodium chloride by treatment with alcohol, obtained in each case bodies of acid reaction, which from their composition he supposed to be identical with Schützenberger’s proteic acid, C₈H₁₄N₂O₄, which is soluble in water and alcohol, insoluble in ether and petroleum, uncrystallisable, and forming uncrystallisable salts. Fahrion suggested that the nitrogenous character which Eitner attributed to his “dégras-former” (p. 370) was probably due to contamination by this body; and that its formation might be utilised in the analysis of leather and other proteid bodies. These products have since been further investigated by Prof. Paal and Dr. Schilling,[40] who show that they contain hydrochloric acid, to which their acid reaction is due, and that they are identical with the peptone salts previously obtained by Prof. Paal (v. s.) by digestion of proteids with hydrochloric acid. The free peptones are strongly basic.

By dry distillation of gelatin a mixture of pyrrol and pyridin bases are produced. This is commercially obtained by the distillation of bones, and is known as “bone oil,” or “Dippel’s animal oil.” Pyrrol, C₄H₅N, resembles phloroglucol in giving a purple-red colour to fir wood moistened with hydrochloric acid (p. 299).

Reactions of Gelatin.—Gelatin is precipitated by mercuric chloride, in this respect resembling peptones, but not by potassium ferrocyanide, by which it is distinguished from albuminoids, and it differs from albumin in not being coagulated by heat. Solution of gelatin dissolves considerable quantities of calcium phosphate; hence this is always present in bone-glues. Gelatin and some of its decomposition products are precipitated by metaphosphoric acid.[41] The precipitate contains about 7 per cent. P₂O₅, but gradually loses it on washing. Various salts diminish the solubility of gelatin in hot water, and especially those of the alum type. Chrome alum and basic chrome salts are especially powerful, rendering it practically insoluble. The addition of about 3 per cent. ammonium or potassium dichromate causes glue or gelatin to become insoluble by the action of light with the formation of basic salts of chromium, and has been utilised in photography and as a waterproof cement. Other colloids besides gelatin are similarly affected.

Gelatin is precipitated by all tannins, even from very dilute solutions; one containing only 0·2 grm. per liter is rendered distinctly turbid by gallotannic acid or infusion of gall-nuts; but some other tannins give a less sensitive reaction. The precipitate is soluble to a considerable extent in excess of gelatin, so that in using the latter as a test for traces of tannin care must be taken to add a very small quantity only. The addition of a little alum renders the reaction more delicate. Whether the precipitate is a definite chemical compound has been disputed, as its composition varies according to whether gelatin or tannin is in excess. Böttinger[42] states that the precipitate produced by adding gelatin to excess of gallotannic acid contains 10·7 per cent. of nitrogen, indicating the presence of 66 per cent. of gelatin on the assumption that gelatin contains 16·5 per cent. N (see p. 57). Digested with water at 130° C., the precipitate is decomposed, yielding a solution which precipitates tannin, and probably indicating the formation of a more acid compound. Gelatin with excess of oak-bark tannin gives a precipitate containing 9·5 per cent. of nitrogen, corresponding to 57·5 per cent. of gelatin. Treated with water at 150° C., this precipitate yielded three products: one soluble in cold water, another in hot only and one insoluble. On addition of a solution of formaldehyde (formalin) to one of gelatin no visible action takes place in the cold, unless the solution of gelatin be very concentrated and alkaline, but on heating, the gelatin is rendered insoluble owing to the formation of a compound with the formaldehyde. From the very small amount of formalin which is required to produce formo-gelatin it is very doubtful if this is a definite compound.

Weiske[43] states that bone-gelatin, carefully freed from all mineral matter, is not precipitated by tannin till a trace of a salt (e. g. sodium chloride) is added. So far as is known, bone-gelatin is identical with that of skin.

Chondrin is the gelatinous body produced by the digestion of cartilage with water at 120° C. for three hours. In most of its physical properties it is identical with gelatin, but differs from the latter in being precipitated from its solution in water by acetic acid, lead acetate, alum, and the mineral acids when the latter are not present in excess. Chondrin also differs from gelatin in producing a substance capable of easily reducing cupric oxide when it is boiled for some time with dilute mineral acids. It is extremely probable that chondrin is merely an impure gelatine.[44]

Coriin.—Rollet[45] has shown that when hide and other forms of connective tissue are soaked in lime- or baryta-water, the fibres become split up into finer fibrils, and as the action proceeds, these again separate into still finer ones, till the ultimate fibrils are so fine as to be only distinguished under a powerful microscope. At the same time, the alkaline solution dissolves the substance which cemented the fibres together, and this may be recovered by neutralising the solution with acetic acid, when the substance is thrown down as a flocculent precipitate. This was considered by Rollet to be an albuminoid substance; but Reimer[46] has shown that it is much more closely allied to the gelatinous fibres, and, indeed, is probably produced from them by the action of the alkaline solution. Reimer used limed calf-skin for his experiments, and subjected it to prolonged cleansing with distilled water, so that all soluble parts must have been pretty thoroughly removed beforehand. He then digested it in closed glasses with lime-water for 7-8 days, and precipitated the clear solution with dilute acetic acid. He found that the same portion of hide might be used again and again, without becoming exhausted, which strongly supports the supposition that the substance is merely a product of a partial decomposition of the hide-fibre, and indeed that there is no distinct “cementing substance,” but merely a difference in the hydration or physical condition of the fibre substance which causes it to split more readily in certain directions. The dissolved substance, which he called “coriin,” was purified by repeated solution in lime-water and reprecipitation by acetic acid. It was readily soluble in alkaline solutions but not in dilute acids, though in some cases it became so swollen and finely divided as to appear almost as if dissolved. It was, however, very soluble in common salt solution of about 10 per cent., from which it was precipitated both by the addition of much water and by saturating the solution with salt. Reimer found that a 10 per cent. salt solution was equally effective with lime-water in extracting coriin from the hide, and that it was partially precipitated on the addition of acid, and completely so on saturating the acidified solution with salt. Other salts of the alkalies and alkaline earths acted in a similar manner, so that Reimer was at first deceived when experimenting with baryta-water, because, being more concentrated than lime-water, the coriin remained dissolved in the barium salt formed on neutralising with acid, and it was necessary to dilute before a precipitate could be obtained. The slightly acid solution of coriin gave no precipitate either in the cold or on boiling with potassium ferrocyanide, being thus distinguished from albuminoids. The neutral or alkaline solution showed no precipitate with iron or mercuric chloride, copper sulphate, or with neutral lead acetate; but with basic lead acetate, basic iron sulphate, or an excess of tannin a precipitate was produced. Reimer’s analysis showed: Carbon, 45·91; hydrogen, 6·57; nitrogen, 17·82; oxygen, 29·60; and he gives a formula showing its relation to the original fibre, which does not seem supported by sufficient evidence. In all probability coriin is merely an impure degradation-product of hide-fibre or gelatin.

Hide Albumin.—The fresh hide contains a portion of actual albumin, viz. that of the blood-serum and of the lymph, which is not only contained in the abundant blood-vessels, but saturates the fibrous connective tissue, of which it forms the nourishment. This albumin is mostly removed from the skin by the liming and working on the beam, which is preparatory to tanning. Probably for sole-leather, the albumin itself would be rather advantageous if left in the hide, as it combines with tannin, and would assist in giving firmness and weight to the leather. It is, however, for reasons which will be seen hereafter, absolutely necessary to get rid of any lime which may be in combination with it. The blood must also be thoroughly cleansed from the hide before tanning, as its colouring matter contains iron, which, by combination with the tannin, produces a bad colour.

The albumins form a class of closely allied bodies of which white of egg may be taken as a type. They are also related to the casein of milk, to fibrin, and more distantly to gelatin. A good deal of information on the class may be found in Watt’s Dict. of Chem., 2nd ed., article ‘Proteids,’ and Beilstein’s article ‘Albuminaten,’ and in Allen’s ‘Commercial Organic Analysis,’ vol. iv.

The most characteristic property of albumins is that of coagulation by heat. The temperature at which this takes place differs somewhat in different members of the group, egg and serum albumin coagulating at 72-73° C. Dry albumins become insoluble if heated to 110° C. for some time. Traces of acid tend slightly to lower, and traces of alkali to raise the temperature of coagulation. Sodium chloride and some other neutral salts favour coagulation. Solutions of albumin become opalescent at a temperature slightly below that at which flakes form.

Albumins are also coagulated by alcohol and by strong mineral acids. Coagulated albumin is only soluble in strong acids and alkalies by aid of heat, and strongly resembles keratin (pp. 56, 68).

Solutions of albumin are lævorotatory to polarised light.

“Acid” and “Alkali” Albumins are formed by the action, in the cold, of dilute acids (such as acetic, hydrochloric) and alkalies on albumin solution. They are uncoagulable by heat, and are precipitated by careful neutralisation, but are soluble in excess of either acid or alkali, or alkaline carbonates. They are thrown out of solution by saturation with sodium chloride or magnesium sulphate. It is doubtful whether albumins combine with either acids or bases, and it is probable that the “acid” or “alkali” albumins are identical with the parapeptones formed in the first stage of peptic digestion.

On putrefaction, or on more severe treatment with acids and alkalies, albumins break down in a way similar to gelatin, and yield almost identical products (see p. 57); amido-acids of the acetic series, and tyrosin (para-oxy-α-amido-phenyl-propionic acid) and aspartic (amido-succinic) acid, being the most important.

Treatment with alcoholic soda (see p. 62) yields peptones similar to those of gelatin.[47]