8 H. HAYS BULLARD 



where the true interstitiul granules are large and numerous, the 

 types of fibers were distinguished by the Weigert and Altniann 

 methods as well as by the other methods just mentioned. 



Tli(^ light, dark and intermediate types of muscle fibers are 

 clearly marked in unstained frozen sections of fresh material 

 from normal cats, dogs and rats. If such sections be placed 

 in absolute alcohol for a few minutes and subsequently examined 

 under the microscope the dark and intermediate fibers assume 

 the appearance of light fibers. The fat droplets and also the alco- 

 hol-soluble portion of the true interstitial granules have been 

 removed by the alcohol, and the removal of these droplets and 

 granules causes the dark fibers to lose their opacity. In the dog, 

 cat and rat the true intersitial granules are small and the opacity 

 of the dark fibers is largely due to fat droplets. In the pectoral 

 muscles of the pigeon and the bat, not only fat droplets but also 

 true interstitial granules are an important factor in causing the 

 dark appearance of fibers. Sections of formalin fixed material 

 from the pectoral muscles of the pigeon after treatment with 

 absolute alcohol ten to twelve hours still show the dark fibers 

 much more opaciue than the light, while in similar preparations 

 from the dog, cat or rat, dark fibers have lost their opacity and 

 have the appearance of light fibers. The fat droplets have been 

 removed by the alcohol but the large true interstitial granules 

 of the pectoral muscles of the pigeon remain and cause the opaque 

 appearance of the dark fibers to be retained. The staining reac- 

 tions and solubility of fat droplets and true interstitial granules 

 will he dealt with later, but I may here mention that formalin 

 coagulates the ti'ue interstitial granules in such a manner as to 

 partially protect them from the action of fat solvents while fat 

 droplets are readily soluble both before and after formalin fixa- 

 tion. 



Figure 1 represents the types of fibers in the pectoralis major 

 of an (exceptionally wc^ll nourished white rat. The preparation 

 was stained by Herxheimer's method. Figure 2 shows a similar 

 specimen from a very emaciated rat. The dark fibers so apparent 

 in the well nourished animal hiivv largely disappeared in the 

 poorly nourished animal. The granules shown in the figures are 



