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In the gastrocnemius and biceps there were many swollen and homogeneous necrotic fibers such as that shown in Figure 2.

Such swollen fibers were deeply eosinophilic, contained a few pyknotic nuclei, and showed loss of cross-striations, obliteration of myofibrils, and prominent vacuolization.

The necrosis often involved only a portion of the length of a given fiber, and usually the immediately adjacent fibers were normal.

As shown in Figure 3, the protoplasm of other fibers was pale, granular, or flocculated and invaded by phagocytes.

Inflammatory cells were strikingly absent.

In association with these changes in the fibers, there were striking alterations in the muscle nuclei.

These were increased both in number and in size, contained prominent nucleoli, and were distributed throughout the fiber ( Figs. 2 - 5 ).

In contrast to the nuclear changes described above, another change in muscle nuclei was seen, usually occurring in fibers that were somewhat smaller than normal but that showed distinct cross-striations and myofibrillae.

The nuclei of these fibers, as is shown in Figures 3 and 4, showed remarkable proliferation and were closely approximated, forming a chainlike structure at either the center or the periphery of the fiber.

Individual nuclei were usually oval to round, though occasionally elongated, and frequently small and somewhat pyknotic.

At times, clumps of 10 to 15 closely-packed nuclei were also observed.

Occasionally there were small basophilic fibers that were devoid of myofibrillae and contained many vesicular nuclei with prominent nucleoli ( Fig. 5 ).

These were thought to represent regenerating fibers.

Trichrome stains failed to show fibrosis in the involved muscles.

In all of the sections examined, the arterioles and small arteries were essentially normal.