Ringer's solution containing ferrocyanide ion was infused into the arterial system of lumbs. Some of these ions filtered across the blood capillary wall into the adjacent extracapillary fluids, from which diffusion caused these ions to enter the wall of the enclosing epimysial capsule. Ringer's solution containing ferric ion was then injected into the parenchyma (not intravascularly). In muscles, the intensely blue ferric ferrocyanide (Prussian blue) precipitate appeared on the walls of the

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... ous connective tissue capsules that enclosed each small cluster of muscle cells, and in the lymphatics of the extracapsular clefts. No Prussian blue appeared inside the capsules. These results indicate that "interstitial" fluids are divided into two discrete pools: (a) an intracapsular pool of capillary ultrafiltrates, and (b) an extracapsular pool in the trabecular clefts. Certain implications concerning the mixing of the tissue fluids, the estimate of capillary filtration rates and some of the functional differences between intravascular and intramuscular injections are discussed.