8 H. HAYS BULLARD 



similar to that occuiTing in the so-called light and dark fibers of 

 skeletal muscle. 



The existence of light, dark and intermediate fibers (by trans- 

 mitted light) has been known in skeletal muscle for a long time. 

 Knoll ('89-'91) and Schaffer ('93) described the dark fibers as 

 containing manj^ interstitial granules and fatty droplets while 

 the light fibers contained comparatively few granules and little 

 fat. Figure 14 shows a section of striated muscle fibers from the 

 diaphragm of a dog. At D is seen a fatty or dark fiber, and at 

 L a slightly fatty or light fiber. The work of Knoll and Schaffer 

 was done before Scharlach R. was extensively used as a fat stain 

 so that they did not obtain the exact picture shown in figure 14 

 but there can be little doubt that the types of voluntary striated 

 filbers here shown (fig. 14) correspond to the light and dark fibers 

 of Knoll. In skeletal muscle the fibers are of uniform type, 

 either light or dark throughout their entire length, and as was 

 pointed out by Knoll it is easy, in certain cases, to observe mor- 

 phological differences between the two types. A good example 

 of this is found in the breast muscle of the pigeon. Here the 

 light fibers are large, with nuclei placed within their substance and 

 contain little fat, while the dark fibers are small, with nuclei 

 peripherally situated, and contain a great deal of fat. It is cer- 

 tain that in the pigeon light and dark fibers are definitely fixed 

 types and not morphologically identical. In the skeletal muscles 

 of mammals a morphological difference is often observed in that 

 the dark fibers are of less diameter than the light. In a former 

 paper ('12 a) I reported having observed the two types of fibers 

 in the skeletal muscles of the human fetus and in the fetal calf. 

 I have not been able to differentiate the two types during the 

 first half of fetal life but in the human fetus they are well marked 

 as early as the sixth and seventh month. The relative number 

 and arrangement of the dark (fatty) fibers in the different muscles 

 of the human fetus is so similar to that found in the adult that it 

 seems certain that the dark fibers of the fetus remain true to 

 type in post-uterine life. In the mammalian fetal heart usually 

 the different types of fibers are not clearly marked although 

 they contain fat droplets. 



