11. Whatever may be the nature of this intermediate membrane to the mucous and muscular coats, it evidently has a dense, close texture, which gives it a resistance very analogous to one of the fibrous membranes. It is from this that the organ receives its form; it is this which maintains and controls its shape, as may be proved by the following experiment. Take a portion of intestine: remove in any part of the bowel a part of this membrane, with the serous and muscular membranes: having applied a ligature to the inferior end, inflate it, the air will produce in the denuded part an hernia of the mucous coat. Take another portion of intestine, turn it, dissect off a small part of the mucous membrane and of this coat: inflation will produce upon the serous and muscular coats the same phenomenon as in the preceding case it did in the mucous membrane. It is therefore to this intermediate tunic that the mucous membrane owes its power of resistance to substances which distend it. This applies equally to the stomach, bladder, œsophagus, &c. 12. The free surface of mucous membranes, or that which is continually moistened by the fluid from which they borrow their name, presents two kinds of wrinkles or folds, the one inherent in their structure and which is constantly present, whatever may be their state of contraction or dilatation, such as the pylorus, the valvula conniventes, the valve of the colon, &c. These folds are formed, not merely by the mucous membranes, but also by the intermediate membrane mentioned above, and which in these parts takes a remarkable density and thickness. 13. The other folds may be called accidental, and are only observed during the contraction of the organ; such are those of the inner surface of the stomach, and of the large intestines, &c. In most of the human subjects brought to our amphitheatres, these folds in the stomach, of which so much has been said, are not perceptible, because generally the subject has died of a disease which has impaired the vital powers, without preventing all the action of this viscus; so that, although it is frequently found empty, its fibres are not in the least contracted. 14. In experiments on living animals, on the contrary, these folds are very apparent; and observe how they may be demonstrated. Let a dog eat or drink copiously; open it immediately, and make an incision into the stomach the whole length of its greater curvature, no fold will then appear, but it soon contracts, its edges are drawn in, and the whole of the mucous surface is covered with numerous prominent plicæ in the form of circumvolutions. The same result may be observed in the stomach of a recently killed animal by distending it with air, and then opening it; or, what is still better, by laying it open whilst empty, and stretching it, the folds will disappear, and when we cease to make the extension they immediately form again and are very apparent.