53. One of these variations is that which the aspect of mucous membranes presents at their origin, when compared with their appearance in the more remote parts of the organs. Compare, for instance, the surface of the glans, the inner surface of the lips, the orifice of the urethra, &c., with any portion of the inner surfaces of the stomach, intestines, &c. In the first the corps papillaire will be seen slightly marked, and offering no villous character, the epidermis thick, very distinct, and easily separated, the chorion very evident, the vessels rather less superficial, the mucous glands numerous and very large, more especially in the mouth; in the other characters almost opposite will be observed; we should say, that the mucous membranes have at their origin a structure of a middle kind between the skin and their deeper portions. 54. Another variation of structure, not less striking, is that which is met with in that portion of mucous surface which lines the sinuses. Here it has more redness, and an extreme tenuity; the three layers cannot be distinguished; and although there is a considerable secretion of mucous fluids, there are no perceptible mucous glands. Such are the characters of those portions of the pituitary membrane, which are considered as adapted to augment the sensation of smell, but which do not perform that function in the manner generally understood. In fact, the instant when an odour enters the nose, having the air for its vehicle, it cannot at once pass into the sinuses, because the orifices by which these cavities communicate with the nose are very small; but it enters gradually, impregnates all the air which they contain, and not being able to escape readily, for the same reason that rendered its entrance difficult, the sensation is prolonged, which on the general pituitary membrane is soon dissipated by the action of the fresh air. Thus therefore the pituitary membrane is destined to receive the impressions of odours, and its extensions into the cavities of the sinuses to retain them.