This work consists of 62 pages of electron micrographs of fair quality and 44 pages of text. The material examined consisted of thin sections of heart muscles from C3H and Webster strain mice. The photographs are claimed to prove that blood capillaries enter into the individual muscle fibers of the heart and that a higher concentration of sarcosomes is found between the myofibrils of the heart than in striated muscle. Numerous comments are made on the Z bands, sarcolemma, and fiber structures. A diagram of a heart muscle summarizes the histologic findings.