55. With regard to the particular structure of that portion of mucous membrane which lines the sinuses I remark, that it is absolutely the same as of that which is spread over the surface of the internal ear, with the exception of a still more delicate tissue. All anatomists call this membrane the periosteum of the bony covering of the internal ear. The following considerations prove that it is not a fibrous membrane, analogous to that which covers the bones, but a mucous layer, like that of the sinuses. (1) It is evidently seen to be a continuation of the pituitary membrane by the medium of the Eustachian tube. (2) It is found to be habitually moist with a mucous fluid, which is discharged through that tube, a property foreign to fibrous membranes, both of whose surfaces are always attached to some parts of the animal structure. (3) No fibre can be distinguished in it. (4) Its spongy appearance, though whitish, its softness, the readiness with which it gives way to the least agent directed against it, with a view to tear it, form a character not to be found in any part of the periosteum. 56. I pass over the other variations of structure in mucous membranes in their different regions; in all they have real differences. I observe only, (1) That these variations distinguish them from serous membranes, whose aspect is everywhere the same, as may be seen by comparing the pericardium with the peritoneum, &c. (2) The sensibility of mucous membranes varies in a very peculiar manner in their different portions: thus an emetic irritates the stomach, but not the conjunctiva; the pituitary membrane perceives only odours; the mucous surface of the tongue flavours, &c. On the contrary, the contact of all kinds of bodies with the naked serous membranes produces phenomena exactly analogous. SECTION VII. OF THE VITAL POWERS OF THE MUCOUS MEMBRANES. 57. The sensibility of mucous membranes is one of the principal characteristics that distinguishes them from other analogous organs. This power, which belongs to organic bodies, is variable in every part, prompt to develop itself in some parts, under the influence of the least excitement, roused with difficulty in others, present in every part, liable to proceed by means of inflammation from the most obscure state to the last degree of intensity--this power is here remarkable for features very analogous to those which it presents in the cutaneous surface (to which, as we have stated, the mucous surface has great traits of resemblance) as respects its structure. It is to this analogy of sensibility that we must refer a crowd of phenomena, which are alternately exhibited in an inverse order upon both surfaces. I shall now point out some of these phenomena in succession.