. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. Muscle and Electric Organs 581 and relative amount of connective tissue. Table 70 and Figures 216 and 217 suggest the nature of the histological series. The cross striations of muscle consist of alternating strongly birefringent and weakly birefringent regions, the anisotropic (A) and the isotropic (I) bands. The A band is normally seen as darker than the I band in fresh or fixed tissue. The light I bands are divided by so-called Z lines, which are transverse boundaries between the structural units, sarcomeres;
. Comparative animal physiology. Physiology, Comparative; Physiology, Comparative. Muscle and Electric Organs 581 and relative amount of connective tissue. Table 70 and Figures 216 and 217 suggest the nature of the histological series. The cross striations of muscle consist of alternating strongly birefringent and weakly birefringent regions, the anisotropic (A) and the isotropic (I) bands. The A band is normally seen as darker than the I band in fresh or fixed tissue. The light I bands are divided by so-called Z lines, which are transverse boundaries between the structural units, sarcomeres; thus one unit or sarcomere consists of two light areas on either side of a dark one; changes in the width of both bands and in the birefringence occur during contraction (Fig. 216, A). A muscle fiber consists of a bounding sarcolemma, fibrils which are aligned with respect to striations, and a ma- ::;, ; b ^fJi^M^ VA ⢠' ^^^ ' ' Muscle nucleusâ"StT" ****^ i Connective tissue nuclei i -'VS^m I. T'ltl - \\ W" â A Fig. 216. Views of muscle fibers: A, longitudinal, and B, cross views of striated fibers from sea-spider (Anoplodactylus). Sharp Q (or A) bands, J (or I) bands with 2 discs. Nuclei peripheral, fibrils nearly fill fibers. After ;^ C, longitudinal, and D, cross views of fibers from heart of Anodonta. Striations diagonal, fibrils peripheral and sparse, sarcoplasm abundant, nuclei central. Marceau.^*" trix or sarcoplasm. Outside it is a variable amount of connective tissue. The myofibrils are contractile, and the sarcoplasm may also contain some contractile ** Smooth muscle fibers of vertebrates contain fibrils which lack striations; optically they are weakly birefringent. In arthropod limb muscles the striations are sharp and fairly close to- gether, the fibers are long and, in many (, Crustacea) they are branch- ing; the fibrils (sarcostyles) are more separable than in vertebrates (Fig. 216, A). Insect muscles may have str
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