The relative positions of the layers of the skin, the mode in which the hairs rise from them, and the form and situation of the sebaceous follicles, will be better understood by referring to the following cross section Fig 5. Pl. V., in which the line marked _a_ Represents the cuticle, _b_ The mucous layer, _c_ The true skin, _d_ Sebaceous follicles, _e_ Hairs rising from the true skin and inflections of the upper layers and piercing, _f_ The thin film of the yolk. (28.) _Periodical decidence of Wool._--It is affirmed that the only real difference between hair and wool is in the latter falling off periodically; but the same change takes place in animals totally covered with hair; in fact, almost every animal is subject to moulting, or a periodical decidence of its protecting covering. The fleece of the sheep has been proved in many instances not to be liable to _annual_ changes of this description;--Lord Western having, among others, shown that the wool of the merino may be retained at least three years without the slightest disposition to separate. The annual employment of shearing, to anticipate this falling off of the wool, is now common in most parts of the world; yet some, as the Icelanders and Kamtschadales, still retain the primeval custom of _rowing_, or pulling off the fleece in a mass, about the end of May, at which time it is nearly loosened. This loosening of the attachments of the fibre is supposed to be owing to a diminution, during winter, of the nutritive process at the root of the hair, so that the fibre is liable to give way at the tender part when the fleece becomes heavy during Spring. This attempt at an explanation meets, however, with the insurmountable objection, that long-woolled are less liable than short-woolled sheep to this occurrence, a circumstance which ought to be the reverse, if weight had any thing to do in the case.