When the skin leaves the back of the animal it mainly consists of four layers : the hair or wool; the epidermis, or outside skin; the true skin, or corium; and portions of the adipose of the slaughtered. The hair and outside skin and the fatty matter has to be removed before the operation of leather-making proper can be commenced. It is the true skin (cutis, dermis, or corium) that is converted into leather. This is composed of interlacing bundles of white fibres of the sort called "connective tissue." These white fibres are dense and resisting, and in the middle portion of the skin are closely interwoven, but near the flesh side they gradually become looser and more open, until it becomes exceedingly loose. Some of these very loose fibres are removed during "fleshing." The grain is exceedingly close and compact with these fibres. Here they are separated into their elementary fibrils, and are so interlaced that they can scarcely be recognized.

The true skin also contains a small proportion of fine yellow fibres called " elastic " fibres. They are very stable, and are very little changed in the tanning process. They are found where great elasticity is required for the functions of the animal. The fibres in the skins taken from young animals are more flexible than those found in the skins of older ones.