42 H. HAYS BULLARD 



fed on fat meat showed an increased amount of fat in the cardiac 

 fibers but the increase was not so great as in skeletal muscle. 

 The muscle fibers of the pectoralis major of normal rats kept on a 

 diet of bread and lean meat show fat intermediate in amount to 

 that represented in figures 1 and 2. In several rats the superficial 

 fibers of the pectoralis major contained less fat than those some- 

 what removed from the surface. The fibers illustrated in the 

 figures were not superficially placed. After rats have been fed 

 on fat meat for a few days, the quantity of fat in the muscle 

 fibers appears to have been increased to the maximum. Further 

 feeding increases the amount of connective tissue fat but seems 

 to have no effect upon the fat in the muscle fibers. As already 

 mentioned, dark muscle fibers are more clearly marked in well 

 nourished animals than in emaciated animals of the same species. 

 The fat droplets of muscle fibers are clearly to be regarded 

 as reserve foodstuff. The work of Bell in this respect is so con- 

 vincing as scarcely to require confirmation. I have not made 

 observations on the effect of starch, sugar or protein diets, as 

 has Bell. 



VII. SUMMARY AND CONCLUSIONS 



The interstitial granules of striated muscle ma}'' be classed as 

 true interstitial granules and fat droplets as was done by Kolliker. 

 Both the granules and fat droplets are factors in bringing about 

 the dark or cloudy appearance of muscle fibers. 



The two types of fibers, dark and light, the occurrence of which 

 is well known in adult vertebrates, are also present in the muscles 

 of the fetus. 



The terms "red and white muscle" refer to macroscopical color 

 (lifTerences only. Applied to the microscopic appearance of 

 muscle fibers these terms become a misnomer if used as synony- 

 mous with "dark and light muscle fibers." 



The interstitial granules and fat droplets of muscle are some- 

 what uniformly arranged in longitudinal and in transverse rows 

 between the muscle columns. Small granules and fat droplets 

 form transverse rows in segment / on either side of the membrane 

 of Krause while those of larger size are in segment Q. The 



