The blood supply of duck muscle spindles from the extensor pollicis and the extensor digitorum comminis wing muscles has been studied by light and electron microscopy. Capillaries usually accompany the nerve bundle that innervates the spindle, approaching at an oblique angle around the midequatorial region. Capillaries may run for some distance along the surface of the outer capsule, or penetrate partially into the outer capsule and lie between layers of the capsule cell processes. Some capillaries also penetrate the outer capsule, running into the periaxial space and continuing further towards the polar region. They have been shown to be in close contact with intrafusal muscle fibres, from the juxta-equatorial to the polar region, but have not been encountered among sensory terminals in the mid-equatorial region.
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