The main purpose of this paper and of others which follow is to bring proof of the existence in certain situations of a gradient of permeability along the capillaries which makes for the equalization of opportunity within the tissues served by them. Means are described whereby this equalization is achieved in different fashion elsewhere in the body, and evidence is brought forward of a significant permeability of the arterioles and venules. The Distribution of Vital Stains within Skeletal MuscleThe sheet muscles of guinea pigs and young rabbits are so transparent that the distribution of innocuous dyes can readily be followed during life.The smallest vessels of voluntary muscle are arranged almost diagrammatically (1), with arterioles and venules disposed alternately in the same plane, at approximate right angles to the muscle fibres (Figs. 1, 2, and 3; Schema I, A). The numerous, parallel capillaries which bridge the gaps between are, in the rabbit, often more than 1 ram. in length. Certain of them pass the nearest venule and course to that next beyond. All lie next the fibres which they nourish and adjoining ones are linked by occasional cross connections; but the existence of these does not significantly affect the rule that the blood leaving an arteriole finds its way back to the large vessels by way of the next venule.To witness the distribution of vital dyes a window can be inserted over the external oblique, a "white" muscle. The rabbit is etherized after a fast of 18 to 24 hours, laid--not stretched--on its back, an oval piece is snipped with scissors * Studies of Tissue Maintenance. IV. 807
The recognition that permeability increases progressively along the capillaries of voluntary muscle (1) has led us to study the state of affairs in other organs. An exceedingly pronounced gradient is demonstrable in the skin of the frog, and the small cutaneous venules are even more permeable than the adjoining portion of the capillaries (2). Because of the importance of mammalian skin in water storage and regulation, we have proceeded to study this organ. The skin of the albino mouse was chosen for the purpose because the spread of dyes from the blood can be followed with special ease in it. The Escape of Dyes from the Blood into the Skin of the BodyIn preliminary tests vital dyes of known diffusibility were injected into a tail vein of unanesthetized animals which were decapitated shortly after. The skin was found to color evenly with rapidly diffusible pigments, whereas with poorly diffusible ones, such as are especially suited to disclose local differences in vascular permeability, there was to be seen a deep staining of the tissue through which the small venules coursed, before any general coloration of the corium had taken place. The phenomenon was visible only when a skin flap was studied from its under side and even then ordinarily the subcutaneous fat prevented a clear view. To do away with it mice were kept on a reduced ration until they had become thin. They yielded pictures that were highly informative.
In previous papers (1, 2) experiments have been described which prove t h a t the permeability of the muscle capillaries of mammals and frogs increases progressively along these vessels, being greatest in the region where they join the venules. The present communication deals with the conditions in frog skin.The capillaries of the frog tongue, web, and mesentery have become classical objects of study while those of the skin of the body are employed but seldom. Yet in some species the skin offers opportunities beyond those of any of the stmc. tures mentioned. To watch tongue and mesentery they must of necessity be exposed and pinned out, subjected to more or less trauma, and kept wet with a drip, which differs from frog lymph (3). In the web two skin layers come together with result in complicated vascular pictures, and the circulation is far more frequently irregular than over the body. We have judged permeability by the escape of dyes from the blood. In the web and mesentery there is almost no interstitial tissue to stain, or to serve as a ground upon which the spread of dyes can be perceived. In the skin of the body on the other hand the capillaries ramify in a transparent tissue overlying a dense, grayish white substratum against which every least escape of dye is visible. The Vessds StudiedThe skin of Rana pipits and Rana damila~ presents the general features (Figs. 1, 2, 3, and 9) described for European frogs (4). The transparent epithelium, pierced here and there by gland outlets, covers a loose-textured stratum spongiosum of corium wherein lie pear shaped glands. In Rana damitans the spongiosum is so transparent, where it lacks granule cells, that the pitted surface of the underlying "sieve layer" of the stratum compactum can be closely inspected;
A mounting gradient of permeability exists along the capillaries of frog muscle. In chicken muscle on the other hand none has been demonstrated; but the close-knit vascularization is arranged in duplicate in such manner that the blood runs in opposite directions through the capillaries of nearly adjacent fibres. In a flight muscle of the pigeon there exists in addition to this artifice what appears to be a special collecting system of venous capillaries. In the mammalian diaphragm indications of such a system are also to be found, and a gradient of capillary permeability like that in the other skeletal muscles is probably present. These vascular conditions are briefly considered in terms of function.
The wall of the capillary has long been considered on good evidence as behaving like an indifferent semipermeable membrane separating the blood from the tissues. All along the course of the little vessel the permeability has been supposedly the same, differences in the happenings at various points being referable to the interplay of several factors, diffusion, hydrostatic pressure, and osmosis chief among them. It has been held that in the first portion of the capillary the force of hydrostatic pressure dominates over the osmotic pressure of the blood colloids, with the result that fluid is forced out into the tissue to be reabsorbed again as the vein is approached, because the drawing power of the colloids there prevails. The recognition that the permeability of the capillaries of some organs--muscle, skin--increases progressively in the direction of flow (1) has necessitated a reconsideration of the r61e of the factors just referred to. Recent experiments have shown that the gradient of capillary permeability exists independently of the hydrostatic pressure (2). The present work was undertaken to disclose the relation to it of osmotic conditions and especially of the influence of the osmotic pressure of the blood colloids. MethodsThe effect on the gradient of capillary permeability of greatly modifying the percentage of the circulating blood proteins has been directly followed. For this purpose several methods were employed.Adult rabbits of about 2000 gin., fasted 24 hours but with access to water, were used throughout. They were anesthetized with sodium iso-amyl-ethyl barbiturate (sodium amytal--Eli Lilly and Co.) given intravenously, 0.5 to 0.7 cc. of 10 per cent solution per kilo. Freshly prepared solutions were always employed. For
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