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 nucleus — "StT" ****^ i 



Connective tissue 





nuclei i -'VS^m 



I 



T'ltl - \\ 





W" ■ 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 Jordan."^ 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 elements.-** 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 (e.g., Crustacea) they are branch- 

 ing; the fibrils (sarcostyles) are more separable than in vertebrates (Fig. 

 216, A). Insect muscles may have striations less than 5-6 /x apart and very 

 sharp. The fast muscles of flight in a variety of insects exhibit closer 

 striae (2yu,) than do their slower leg muscles (4 /x); in mandibular mus- 

 cles striae are still farther apart (6 /a).-^^ In other insects the ratio of the 



