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In cats anesthetized with chloralose the lengths and diameters of the different vascular segments in the tenuissimus muscle were measured in the vital microscope. The flow characteristics, including red cell velocity, were registered. Fixed and stained muscle specimens were analyzed histologically as well as Indian ink‐perfused muscles treated according to the method of Spalteholz. The average muscle fibre diameter was 44 μm. 62% of the muscle fibres were red (48% B‐fibres, diameter 41 μm and 14% C‐fibres, diameter 26 μm) and the remaining 38% were white (A‐fibres, diameter 55 μm). The average muscle fibre was surrounded by 3.6 capillaries. The capillaries were 1015 ± 16.3 (S.E.) μm long and 5.3 ± 0.0(3) (S.E.) μm in internal diameter. The calculated capillary surface area was 0.9 m2/100 cm3 of muscle tissue. Average red cell velocity in the capillaries was found to be 0.5 mm/s, but there was a wide variation with time and between different capillaries. Neither “spontaneous” changes in internal diameters of the microvessels nor a distinct precapillary sphincter activity could be observed. Arterio‐venous anastomoses within the muscle tissue were rare exceptions.

The gliding surfaces of the flexor tendons and the tendon sheath are delicate structures of the complex digital flexor system. Beside the fibrous parts, the tendon sheath also exhibits membranous synovial components, which represent a dialysing membrane producing a plasma ultrafiltrate--the synovial fluid. In this study, interest was focused on the vascularization of the synovial sheath. By a microangiographic method it was demonstrated that this membrane is richly vascularized and that the vascular plexus is in continuity on the outside of the fibrous pulleys. The friction surfaces of the system--the inside of the pulleys and the surface of the flexor tendons--are devoid of vessels, and here a differentiation into chondrocyte-like cells is observed. It is suggested that these tissue areas, in analogy to joint cartilage, are nourished by diffusion from the synovial fluid, and that the flexor system can be regarded as a specialized joint, sliding longitudinally and exhibiting an extremely large range of motion.

A venous microcannulation technique applied to the cat gastrocnemius muscle was developed which, based on morphological and functional demonstrations of anastomotic connections between two supplying segmental vascular circuits at the level of capillaries and/or post-capillary venules, seems to permit continuous recordings of hydrostatic pressure (denoted Pcvenule) transmitted from such anastomoses, that is, from a site close to the main fluid exchange vessels. For validity tests, such Pcvenule recordings were compared with simultaneous estimates of capillary pressure (Pc) with the isogravimetric technique (Pciso) and, further, with data for experimentally evoked changes of Pc derived from volumetric recordings of net transvascular fluid flux divided by the capillary filtration coefficient (delta Pcvol). Simultaneously obtained data for Pcvenule and Pciso showed close agreement, and the Pcvenule and Pcvol data showed a highly significant linear correlation over a wide range of Pc changes. These results indicate that reliable estimates of Pc can be obtained with the Pcvenule method. It allows for continuous Pc recordings without interfering with normal vascular reactivity and can be applied to non-isogravimetric conditions and combined with simultaneous observations of whole-organ transvascular fluid exchange. At normal arterial and venous pressures and vascular tone, Pcvenule averaged 16.2 +/- 0.2 mm Hg, at which a Starling fluid equilibrium prevailed, and increased with decreasing vascular tone, resulting in net transvascular fluid filtration.

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