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The Interstitial Granules of Striated Muscle etc. 301 



considerable detail. The dark fibers contain a relatively large amount 

 of sarcoplasm in wbicli the interstitial granules are embedded. The 

 light fibers have very much less sarcoplasm. In animals that have 

 muscles of different color, the red muscles are composed mainly of dark 

 fibers ; the pale muscles, of light fibers. In some birds (Gallus, Meleagris, 

 and Phasianus) the breast muscles are pale, and the muscles of the 

 lower extremities are red. In other birds the breast muscles are redder. 

 The leg muscles of wading birds are rich in dark fibers, as is also the 

 heart musculature of all birds. Among mammals there are more dark 

 fibers in the ox and sheep than in the calf and lamb. In the rabbit, as in 

 other mammals, the heart muscle, diaphragm, intercostals, and outer 

 eye muscles are red. In the guinea pig the superficial muscle layers of 

 the extremities are pale ; the deep layers are red. In the dog, cat, and bat 

 the muscles are generally red— there are no pronounced differences of 

 color. The degree of color in a muscle is not always in proportion to the 

 amount of droplets in its fibers. The following muscles are composed 

 mainly of dark fibers : the heart musculature of all animals ; the muscles 

 of mastication in mollusks, fishes, reptiles, and mammals; the respira- 

 tory muscles of amphibians, reptiles, and mammals; the outer eye 

 muscles. Judging from this distribution, Knoll expresses the view that 

 in general the most active muscles are the ones composed mainly of 

 dark fibers. He believes that age and domestication are factors that 

 have some influence in determining the proportion of the dark and 

 light fibers. Muscles that have come into disuse, as the breast muscles 

 of the domesticated fowls, are pale. Older animals seem to have more 

 dark fibers than younger ones. He finds no constant relation between the 

 rapidity of contraction and the amount of droplets in the muscle fiber. 

 As to the composition of the interstitial granules, Knoll states 

 that the strongly-refractive granules consist entirely of fat; the 

 weakly-refractive granules have a fatty marginal layer which stains 

 with osmic acid, and a central part which may be lecithin. The weakly- 

 refractive granules swell in water and stain intensely with gold chlorid. 

 Many transitions between the two types of granules occur. Knoll appar- 

 ently believed that the formation of fat in the fiber is not a con- 

 structive process. 



