The alkalies, even the caustic, have but little action upon the arterial texture; immersed a long time in them, this texture remains almost untouched, loses but little by solution, cannot be broken as it can after being in the diluted acids, &c. _Membrane common to the system with Red Blood._ I call that the common membrane which lines the arteries, the left side of the heart and the pulmonary veins. It can be dissected with ease upon these two last organs. To separate it from the arteries, it is necessary to cut through by a very superficial circular section, the external fibrous layer, raise this layer by laminæ from below upwards; we come then to the internal membrane, which adheres but little to the preceding, and can be detached from it in the form of a canal, of very great extent. It is distinct from it, 1st. by its extreme tenuity, and the transparency that results from it; 2d. by its white colour; for it appears yellow only by being applied to the preceding; 3d. by the entire want of fibres. It is smooth and with a uniform texture like the serous membranes, which we may be convinced of by holding it up to the light. Besides, it differs essentially from these membranes by a kind of brittleness that characterizes it; it is broken and torn by the least effort. The whole resistance of the arteries resides in their fibrous coat. It appears that this membrane, though every where connected, has however some differences of structure in the different regions. 1st. It is evidently more delicate in the interior of the ventricle with red blood, than in the corresponding auricle and in the arteries. 2d. It yields in the heart and in the pulmonary veins, to dilatations much greater than those of which it is susceptible in the arteries, in which it would inevitably break, like the proper membrane, if the blood could produce as great differences of size in it, as it does in these organs. 3d. When we macerate the heart for some time, this internal membrane acquires in the auricle and upon the mitral valves, a very remarkable whiteness, and which is foreign to it in all the rest of its course. 4th. As to the action of the different agents, of the air, of water, of caloric, &c. it appears to me to be the same every where, and resembles precisely that upon the peculiar membrane. Only I have thought, that in the small arteries, the common membrane has the horny hardness more than this, which on this account wrinkles on the interior in different places, when a whole branch is immersed in boiling water; this does not take place in the great trunks. It is evident from this, that though the common membrane of red blood, is every where continuous, it is not uniform in its structure; we shall have occasion to make an analogous observation for the different portions of the two general mucous surfaces.