GRANULES AND FAT OF STRIATED MUSCLE 13 



fiber from the same specimen. The droplets are fewer in number 

 and somewhat smaller than in the dark fibers. The arrangement 

 is similar in the two types. Droplets may be smaller than those 

 shown in the figures and placed nearer to the membrane Z, thus 

 making it difficult to see two distinct rows. Droplets in dark 

 fibers which show a large quantity of fat, may all be of small size, 

 not exceeding 1 ^ and arranged as in figure 3. Dark fibers may 

 show longitudinal rows of small droplets placed at frequent 

 intervals between less frequent rows of larger, elongated droplets. 

 In transverse sections the droplets are situated between the 

 muscle columns or Cohnheim's areas. The larger droplets are 

 at nodal points in the sarcoplasmic net work. No droplets are 

 found between the individual myofibrils within the muscle col- 

 umns. In muscle fibers of a type which have large true intersti- 

 tial granules (granules of segment Q, pectoral muscles of the 

 pigeon and bat and in cardiac muscle), the fat droplets when small 

 are placed in segment /, while larger droplets extend into segment 

 Q. In this type of muscle the droplets in segment J may be placed 

 on either side of the membrane of Krause, Z, but frequently drop- 

 lets are in a single row occupying the position of the membrane. 

 Fat droplets are also of almost constant occurrence in the sarco- 

 plasm beneath the sarcolemma and surrounding the muscle fiber 

 nuclei of skeletal muscle, and they are equally constant in the 

 central perinuclear sarcoplasmic accumulations of cardiac muscle. 



d. Morphology and position of true interstitial granules 



Figure 5 represents the true granules in one of the dark muscle 

 fibers of the pectoralis major of the pigeon, longitudinal section, 

 Weigert method. The granules are rod shaped, being approxi- 

 mately 1 M in diameter and 2 /x in length. In position they corre- 

 spond to segment Q and represent Holmgren's Q granules. Figure 

 6 represents a portion of a similar fiber, from the same muscle, 

 stained with Nile blue sulphate after formalin fixation. By 

 this method true interstitial granules are stained blue while fat 

 droplets are colored red. The fat droplets, for the most part, 

 are situated in segment / at the poles of the true interstitial gran- 



