If a fistula extends from the exterior of the abdomen into the intestines, its whole course is lined with callous bodies; these defend the cellular texture and the muscles through which the fistula passes. On the contrary, nothing similar appears upon the intestinal mucous surface, because its organization alone is sufficient to protect it. The urinary, salivary and lacrymal fluids never escape externally by artificial ducts formed in the neighbouring organs, without there are similar callous bodies in the course of these ducts; on the contrary, they pass with impunity over the mucous surfaces. Make in a limb a long and straight opening with a pointed instrument, and fix in it for a time a sound; a callous canal will be produced by it. Let a sound on the contrary remain in the urethra, no alteration of texture will result from it. Let us conclude from these different considerations, that the mucous system with the cutaneous alone, is organized so as to support the contact of all external bodies, and not to be affected by their presence, or at least experience only an increase of secretion, which is in no ways dangerous. Thus these two systems form two boundaries, the one internal and the other external, between which are placed the organs foreign, by their mode of sensibility and by that of their structure, to the external bodies. To these boundaries the excitement of these bodies is limited; their influence does not go beyond. So long as they do not pass these boundaries, the other organs do not feel them. We might say that the acute sensibility which each of them enjoys, is a kind of sentinel that nature has placed on the confines of the organic domain of the mind, to inform it of whatever can injure it. ARTICLE SECOND. ORGANIZATION OF THE MUCOUS SYSTEM. I. _Texture Peculiar to this Organization._ The mucous system presents two things to be considered in its peculiar texture, viz. 1st, a layer more or less thick which constitutes principally this texture, and which by analogy with the cutaneous corion, may be called the mucous corion; 2d, many small elongations which surmount it, and which are called villi or papillæ. As to the epidermis which covers it, I shall treat of it with the cutaneous epidermis. This texture has nothing similar to the substance that colours the skin, and which is between the papillæ and epidermis. We know in fact that negroes, as well as whites, have this texture of a bright red, which it derives from its vessels. _Mucous Corion._ This portion of the mucous texture, which is the most important, and which constitutes the thickness, form, and even the nature of it, has a soft and spongy appearance. We might say at first view that it was a consistent pulp, with which the extremely dense cellular texture that is subjacent to it had been covered. This softness is a character which distinguishes it from the cutaneous corion, which moreover has by its intimate nature but little resemblance to it.