The blood supply of muscle spindles in some mammals and the pigeon.

The blood supply of muscle spindles was studied in serial cross sections in macaque, cat, rabbit, guinea pig, mouse and pigeon muscles which had been incubated in a medium containing 3,3' diaminobenzidine. Lumina of blood vessels were recognized by the reaction product that was localized within erythrocytes. The outer capsule was well vascularized, but few or no capillaries were seen in the periaxial space. The inner spindle capsule, which closely invests the axial bundle, was rarely contacted by periaxial capillaries at the equator and juxtequator. Capillaries occurred more frequently adjacent to intrafusal fibers at the polar region and beyond the end of the outer capsule. Shorter diffusion distances and, usually, higher capillary densities were found at the polar region than at the spindle midsection. This suggests that transcapillary exchange at the polar segment is nearer to conditions prevalent in extrafusal muscle than elsewhere in the spindle, provided the inner and outer capsules are not less permeable at the poles than at the midsection. Differences in blood supply among mammalian species appear to be related to receptor size.