The number, distribution and density of muscle spindles in two wing muscles of the domestic duck.

A study by serial longitudinal sections of two dorsal wing muscles of the duck, the extensor pollicis and the extensor digitorum communis, has shown that: (1) The extensor pollicis muscle shows a spindle content range of 19--27, mean 23.2, giving a spindle density of 76.7/g; the extensor digitorum communis muscle shows a spindle content range of 37--59, mean 47.6, and a spindle density of 119.0/g. (2) Spindles in the extensor pollicis muscle are randomly distributed, while those in the extensor digitorum muscle are located primarily in the proximal two thirds of the muscle. (3) The present data have been discussed in relation to previous observations of muscle spindles occurring in some avian and mammalian skeletal muscles.