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.) [20] It will be noted that the word “grain” is used by the tanner in at least three different senses, which are productive of much confusion. The extremely thin hyaline layer forms a natural glaze to the skin, and might well be spoken of as such; the form and arrangement of the _papillæ_ and hair-pores might be called the “pattern” of the grain, leaving the use of the word “grain” itself restricted to the _pars papillaris_. [Illustration: FIG. 15.--Fat-cells in connective tissue. _a_, fat-globule; _p_, protoplasm; _n_, nucleus; _m_, cell-wall. (Ranvier.)] 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]