FAT AND MITOCHOXDRIA IN CAKDIAC MUSCLE 9 



According to Knoll ('91) and Schaefer ('12) cardiac muscle 

 fibers correspond to the dark fibers of skeletal muscle. It has 

 been my experience, however, that in many eases the different 

 types of fibers, dark, light and intermediate are quite as well 

 marked in cardiac as in skeletal muscle. Figures 3, 4, and 5 

 show cardiac fibers of the rat, figure 10 those of a dog and figure 

 9 those of a fattened hog. The different types of fibers are clearly 

 shown in each of these figures. Dark fibers are designated D, 

 light fibers .L Figure 5 shows a longitudinal section from the 

 same specimen as the transverse section shown in figure 4. In 

 the longitudinal section it is seen that after a brief course fibers 

 of one type, dark or fatty, pass abruptlj^ into those of different 

 type, light or slightly fatty. This change of type occurs along the 

 transverse lines marked out by the intercalated disks. The 

 greater number of intercalated disks, however, mark no change 

 of type. A cardiac fiber of any given type includes, therefore, a 

 \'ariable number of so-called cardiac cells. In inanition, as we 

 shall see, fat gradually disappears from heart muscle. All the 

 fibers then appear light and it is frequently impossible to dis- 

 tinguish one type from the other, figure 1. Similarly when the 

 muscle fibers, as in certain fat fed animals, are loaded with fat 

 the light fibers may be so crowded with fat droplets as to present 

 the same appearance as dark fibers, figure 6. 



In the skeletal muscles of nearly all apparently normal mam- 

 mals including man, dark or fatty fibers are found, almost invari- 

 ably, side by side with others which are light or non-fatty and 

 as I have set forth the two types are also of frequent occurrence 

 in apparently normal cardiac muscle. This indicates that dark 

 or fatty fibers are to be considered normal, not pathological. 

 The occurrence of light and dark fibers is usually accounted for 

 on the theory that dark fibers have undergone pathological 

 'fatty degeneration' while light fibers have escaped the patho- 

 logical process. This explanation we cannot accept for reasons 

 given, as well as for others to be stated later. 



Distribution of fatty fibers in the heart. The hearts of the rats 

 here used were usually prepared for study by making frozen 

 sections extending transversely across both ventricles. Such 



