The corium or true skin is principally composed of interlacing bundles of white fibres, of the kind known as "connective tissue "; these are themselves composed of fibrils of extreme fineness, cemented together by a substance of different composition from the fibres themselves. This may be demonstrated by steeping a small piece of hide for some days in a stoppered bottle in lime-, or baryta-water, in which the interfibrillar substance is soluble, and then teazing a small fragment of the fibre with needles on a glass microscope-slide, and examining with a power of at least 200-300 dia. In the middle portion of the skin, these bundles of fibre are closely interwoven ; but next the body, they gradually become looser and more open, forming the pars reticularis (or netted part) ; and the innermost layer is a mere network of loose membrane, generally loaded with masses of fat-cells. It is this which is removed in the " fleshing " process. On the other hand, 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 recognized. This is the pars papillaris, and forma the lighter-coloured layer, called 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 loose tissue beneath.
Besides the connective-tissue fibres, the skin contains a small proportion of fine yellow fibres, called " elastic " fibres. If a thin section of hide be soaked for a few minutes in strong acetic acid, and then examined under the microscope, the white connective-tissue fibres become swollen and transparent, and the yellow fibres may then be seen, as they are scarcely affected by the acid. The hair-bulbs and sweat- and fat-glands are also rendered distinctly visible.
The nerves of the skin are very numerous, each hair being supplied with fibres passing into both the papilla and sheath. They also pass into the skin papillae. They cannot readily be seen, without special preparation, and so far as is known, exercise no influence on the tanning process. " Breaking the nerve" is a technical term, which signifies a thorough stretch ing and softening of the skin, but has nothing to do with nerves properly so called. The blood- and lymph-vessels are, from the present point of view, somewhat more important. They may often be seen in sections, and are lined with nucleated cells, similar to those of the glands. These are surrounded by coatings unstriped muscular fibre, running both around and lengthways, and also by connective-tissue fibres. In the arteries, the muscular coating is much stronger than in the veins.
It may be thought that the apace above devoted to the anatomical structure of the skin is disproportionately large ; but there can be no doubt that, in order to make improvements, nothing is of more importance than a clear conception, even to the smallest details, of the materials and causes to be dealt with. The illustrations are from actual specimens, and enable the various parts of the hide to be identified under the microscope, which instrument is destined to play a most important part in the develop ment of tanning.
If it be required to see how far the cellular structure of the hide, such as hair-sheaths and fat glands, are affected nr destroyed in any stage of liming or bating, the following ready method may be employed. If a strip of bide be cut ; through from the grain side, as shown at a in Fig. 899, and the flap be turned down, and held between the finger and thumb, the fibrous tissue will be put on the stretch, and will then allow a moderately thin shaving (including the grain and parts immediately below it) to be cut by a sharp razor. The hide should be held in the position shown, and a steady drawing cut be made from flesh to grain, the razor being steadied on the tip of the forefinger, and its hollow surface flooded with water. If the thin section he now placed on a glass slide, moistened with a drop of water, and examined on the microscope under a strong light from above, with a 1-in. objective, the fat-glands will be seen as yellow masses, embedded in the white fibrous tissue. If a drop of a mixture of equal vols. of strong acetic acid, glycerine, and water be used to moisten the section, the fibrous tissue will become quite transparent, and whatever remains of the cellular tissue will be easily visible, and may even be studied under tolerably high powers if covered with a thin glass, and lighted by the mirror from below.
The same method is applicable for ascertaining the com pleteness of the tannage of leather, and to decide whether the bide fibre is really tanned, or only dyed. Actually tanned leather is unaffected by the acetic acid, but raw or only stained hide swells and becomes transparent.
To prepare the very thin sections necessary for detailed study of the hide, more complicated methods are required. Small slips of hide, not exceeding f in. wide, and cut directly across the lie of the hair, are placed first in weak alcohol (methylated spirit and water), and, after a few hours, are removed into strong methylated spirit. In 24 hours, the hide is hard enough to givo flue shavings, and may be cut either when held as above described, or when embedded in paraffin wax. The razor must be wet with alcohol, and the section be made exactly in the plane of the hair-roots, which may be seen with a hand-lena. The slices may now be stained by placing them in a watch-glue with water and a few drops of the logwood or picrocarmine staining-mixtures sold by opticians, and afterwards either examined in glycerine, or, after soaking some hours in absolute alcohol, may be transferred to clove-oil, and afterwards to a slide, and covered with a drop of dammar varnish and a cover-glass for permanent preservation. If picrocarmine be used, the connective-tissue fibres (gelatinous fibres) and the nuclei of the cells will be coloured red, and the cells themselves of both epidermis and glands, together with the muscles and elastic fibres, will be yellow.