10 11. HAYS BULLARD 



fat droplets. Bell ('11) first clearly dciiionstrated that the fat 

 content of muscle is largely dependent upon the nutritive condi- 

 t ion of the animal. This subject will be referred to later. 



The relative n.umbcn- of light, dark and intermediate fibers 

 is known to be exceedingly variabl(\ For any given muscle of 

 a given species the percentage of each type is fairly constant under 

 normal nutritive conditions, although individual variation occurs. 

 Dark fibers have more interfibrillar substance or sarcoplasm and 

 are commonly of lesser diameter than light fibers. However, 

 the dark fibers in eye muscles (human) are as large or even larger 

 than the light fibers. In the pectoralis major of the pigeon light 

 fibers are exceptionally large with nuclei placed in the substance 

 of the fiber, while dark fibers are small and the nuclei are peripher- 

 ally situated. Mammalian skeletal muscle, in so far as I have 

 observed, has peripherally situated nuclei in both types of fibers. 

 The pectoral muscles of the bat are peculiar in that the fibers 

 are all small and correspond to dark fibers as found in the pigeon. 

 The fat content of the dark fibers of the bat as shown by Herx- 

 heimer's Scharlach R varies somewhat in the different fibers and 

 this may be considered as an indication of the two types. 



The two types of fibers, dark and light, are clearly marked in 

 the human fetus of seven months and of eight months. I have 

 also found the two types in the ox fetus from 45 to 65 cm. and 

 at full term. In so far as I know dark fibers in the fetus have not 

 been previously described. 



h. Relation of light and dark muscle fibers to ivhite and red muscle 



lied muscles, as the pectoralis major of the pigeon and bat, 

 connnonly show a high percentage of dark fibers, while the white 

 muscles of the ra})})it and certain other animals may be made up 

 largely or wholly of light fibers. By many text books and by even 

 a recent author, Ewald ('10), the terms red and white muscle 

 fibers are used synonymously with dark and light fibers. Such 

 a terminology is probably founded upon a niisq|nception and 

 should be discontinued. The white muscles of ijte frog during 

 the winter season show under the microscope a mrge percentage 

 of very dark fibers and frequently red nmscles, as cardiac muscle 



