578 Comparative Animal Physiology 



consist of separate fibers, and analysis of the ultimate contractile elements 

 of muscle is currently being pushed from microscopic to molecular dimen- 

 sions. Cross striations per se have no direct importance in contraction; 

 non-striated muscles can contract; extraction of myosin, the principal con- 

 tractile protein, leaves the striations intact.^^^ Yet cross-striated muscles 

 are in general faster than non-striated, and the material of the striations 

 may have important metabolic effects on the contractile proteins. An elec- 

 tron micrograph of a myofibril (Fig. 213) shows that it consists of myofila- 

 ments which pass continuously through the A and I bands and which do 

 not fold in contraction.^ ^^ These myofilaments sometimes show regular 

 spacings 400 a apart, but these spacings are much less regular and sharp 

 than are the 640 X spacings of collagen (Fig. 213). 



y 



3c 



Fig. 213. Electron micrograph of a myofibril of frog sartorius muscle— note filaments in 

 the fibril showing striations. Collagen fibrils cross the muscle fibril. Stained with phos- 

 photungstic acid. Scale 1 micron. From Hall, Jakus, and Schmitt."" 



Chemical extracts of the contractile protein of striated muscle can be 

 divided into two parts, actin and myosin, which combine to form the con- 

 tractile protein, actomyosin. Discussion of this important field of biophysics 

 is beyond the scope of this book, but molecular threads have been obtained 

 which may be the same as the contractile units of muscle. Figure 214, A 

 shows fibrous actin prepared by polymerization of globular actin and align- 

 ment of the threads gives the appearance of striae. Fibrous actin threads 

 (Fig. 214, B) strikingly resemble threads obtained by maceration of muscle 

 in a Waring blendor (Fig. 214, C). Molluscan smooth muscle yields, in 

 addition to "myosin," another protein, paramyosin, with much longer x-ray 

 spacings (1100 X) than those of "myosin." (Fig. 215.) Paramyosin is present 

 only in "holding" muscles and may not be contractile. 



Microscopic Histology of Muscle. The correlation between the time con- 

 stants of muscle contraction and muscle histology is striking. Muscles can 

 be arranged in a histological series according to: presence or absence of 

 striations, width and density of striations, fiber length in relation to muscle 

 length, distribution of fibrils and nuclei, proportion of sarcoplasm to fibrils, 



