Distribution of fibre types in thirty-seven muscles of Didelphis virginiana.

The proportions of Type I and II fibers were determined in 37 muscles of Didelphis virginiana, representing 9 body regions. There was a distinct cranial/caudal gradation in the distribution of Type II fibers throughout the trunk musculature. Muscles of the fore- and hind-limbs showed a gradation of Type II fiber across the width of the muscle, being more prominent at the surface. The type II fibers in the tip of the tail showed peculiar staining properties in which the core of each fiber stained much more lightly than did the periphery.