28 H. HAYS BULLARD 



SUMMARY 



The cardiac muscle fibers of mammals, both fetal and adult, 

 normally contain a variable nutritive reserve in the form of drop- 

 lets of neutral fat (figs. 2, 3, 4, 10, 13), and the existence in the 

 heart of any considerable quantity of 'invisible' neutral fat is 

 improbable. 



Fat droplets of normal cardiac muscle are arranged in longi- 

 tudinal and transverse rows in the sarcoplasm between the myo- 

 fibrillae or muscle columns (fig. 5). Large droplets are in the 

 Q band, smaller droplets in the J band. 



Fibers which contain very little fat (light by transmitted light) 

 may be found side by side with others which are crowded with 

 droplets (dark by transmitted light) (figs. 3, 4, 5, 9, 10, 13). The 

 occurrence of these two types of fibers in cardiac muscle is physio- 

 logical and they correspond to the so-called light (non-fatty) 

 and dark (fatty) fibers of skeletal muscle (fig. 14) which likewise 

 are of normal occurrence. 



In inanition the normal visible fat of cardiac muscle (fig. 2 or 

 3) is gradually decreased (fig. 1). When fatty foods are given a 

 pronounced increase is seen as soon as digestion is complete 



(fig- 4). 



The phospholipine (lecithine and related compounds) of car- 

 diac muscle is found in the true interstitial granules (mitochon- 

 dria) (figs. 15 and 16 at g.) and it is not markedly decreased in 

 inanition nor increased when fats are given in the food. Neutral 

 fat droplets in cardiac muscle do not arise from true interstitial 

 granules. 



Visible neutral fat is of normal occurrence in the muscle fibers 

 of the bundle of His (figs. 6, 7, 9, 10 at P) ; only a small amount 

 is found in the nodal tissue of the heart (figs. 11 and 12). 



