Scanning electron microscopy of resin casts prepared by incomplete arterial injections showed that in the rat pancreas, the casting medium fills blood capillaries in the endocrine islets more promptly than those in the exocrine lobules and secretory ducts. Furthermore, the exocrine lobules containing endocrine islets allowed a more rapid resin flow through the insulo-acinar portal route than those lobules lacking an islet. The secretory ducts were the last portions to be filled with resin. Since the resin medium used in this study was as viscous as blood and injected under a physiological pressure, the microcirculatory modes demonstrated by the present method suggest the physiological flow of blood in the rat pancreas.

Thorough injection of low viscosity casting media into arteries (complete arterial injection) reproduces the whole extent of blood vascular beds, including thick venous portions (MURAKAMI, 1971; MURAKAMI et al., 1973). Small amounts of injection of the media into arteries (incomplete arterial injection) partially reproduce the vascular beds; the capillary-venular or venular system remains unfilled (MURAKAMI et al., 1983; 1987). Similar injection into veins (incomplete venous injection) can reproduce the venous or venocapillary system (MURAKAMI et al., 1983). A thorough injection into veins is not suited for casting a whole blood vascular bed, since it causes ruptures in capillaries (MURAKAMI et al., 1983). A combination of different, adequate casting methods, both complete and partial, thus allows a precise analysis of the fine vascular connections and arrangement of an organ (MURAKAMI, 1971; MURAKAMI et al., 1983, 1987). This paper demonstrates findings by scanning elec-