. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. J54 Biology of the Vertebrates ferentiated into alternate beadlike bands lying side by side across the bundles of fibers in such a fashion as to produce a striated effect. These beadlike parts of myofibrils are physically and chemically unlike the con- necting parts between the "beads," because they stain differentially with aniline dyes and refract light differently, the dark beads, or anisotropic bands, being doubly refractive in polarized ligh
. Biology of the vertebrates : a comparative study of man and his animal allies. Vertebrates; Vertebrates -- Anatomy; Anatomy, Comparative. J54 Biology of the Vertebrates ferentiated into alternate beadlike bands lying side by side across the bundles of fibers in such a fashion as to produce a striated effect. These beadlike parts of myofibrils are physically and chemically unlike the con- necting parts between the "beads," because they stain differentially with aniline dyes and refract light differently, the dark beads, or anisotropic bands, being doubly refractive in polarized light, while the isotropic bands, or parts between the beads, are singly refractive in polarized light. Moreover there is a physiological difference, as well as physical and chemical, in these parts of the contractile fibrils within a muscle fiber, since anisotropic bands shorten more than isotropic bands during contraction. In birds the "white meat" of the breast is characterized by an excess of myofibrils, while the "dark meat" has more sarcoplasm and less myofibrillar substance in its fibers. In general the striated muscles effect quick movements of comparatively short duration and are voluntary, that is, under the control of the will, while smooth muscle tissue is involuntary and much slower in action. There are certain notable exceptions to this generalization among invertebrates, for the body muscles of some mollusks are smooth and voluntary, while the visceral muscles of insects and crustaceans are typically striated and in- voluntary. 3. Cardiac Muscle The tissue of the muscular vertebrate heart is intermediate in char- acter between smooth and striated muscle, in that the component cells are comparatively short, branching, and in- voluntary in action, although striated in appearance and multinuclear (Fig. 109). The enormous dynamic force exer- cised by any kind of muscle tissue is seldom realized. The tireless heart of man, for example, knows no rest, a
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