STORAGE OF FAT IN MUSCULAR TISSUE OF KING SALMON. 1 27 



TRANSFERENCE OF FAT IN THE DARK MUSCLE. 



It is Stated earlier in the paper that in the active feeding and gro\ving salmon large 

 quantities of fat are laid down in the dark muscle fibers. This deposit of fat begins in 

 the earliest stage observed in the young salmon. It reaches its maximum somewhere 

 near the time when the salmon begin their migration journey. At the Ilwaco station 

 the amount of fat deposited in this type of muscle is astoundingly large. (See fig. i , 

 pi. III.) The amount is especially significant when it is remembered that the deposit 

 has taken place as a storage process in a tissue that is supposed to be most active in the 

 giving off of mechanical energy. 



The variations noted in the dark muscle at the different stages in the migration 

 journey are variations in the amount and character of the distribution of fat. Extensive 

 discussion has been presented showing the facts as regards this picture at the different 

 migration stages. Attention is here called especially to two points, first, the striking 

 variation in the amount of fat of the dark muscle of different parts of one and the same 

 animal as given in such fish as no. 125 and no. 126. The second factor is the relatively 

 large amount of fat present as liposomes in the dark fibers at the death of the salmon. 



As regards the first point, it is obvious that the diminished quantity of fat along the 

 courses of certain of the smaller blood vessels, as shown in fish no. 120, also no. 126, 

 represents a process of fat erosion. It would seem that the fat in the process of being 

 removed is taken up first along the course of the blood vessels. Apparently we have to 

 do here with the simple process of lipolysis. If this be the correct view then it is evident 

 that the fat products of the dark muscle are handled in a way analogous to the fats in the 

 pink muscle in so far as the process of solution and utilization goes. Therefore there is 

 nothing peculiar about this tissue in this particular regard. 



In an animal in which the fats have reached a certain stage of consumption and in 

 which the processes of fat solution are going on rapidly we will have the greatest con- 

 trasts as between the highly vascular and the less vascular areas. The former favor in 

 every way the rapid solution and removal of the fats. In a comparatively large section 

 of dark muscle through, say, the trunk region of such an animal, one will notice a decided 

 mottled or marbled appearance of the section viewed under comparatively low magnifica- 

 tion. The less fat areas will be lighter, with less of the scarlet red stain, while the fatter 

 regions will be relatively deeper red in appearance. Often the light areas form distinct 

 patterns which conform to the smaller veins and arteries. 



In the salmon at this stage the contrasts as between the trunk muscle and the caudal 

 muscle are always sharp. The dark muscle, like the pink muscle, will contain relatively 

 less fat in the caudal region than in the trunk region. The microscope will show that 

 this caudal muscle fat is in smaller droplets which are fewer in number than in the trunk 

 region. In the less vascular areas of the caudal region there will often appear fibers that 

 have only traces, sometimes no fat. These factors are attributed to the more rapid 

 using of fat for the production of energy in the caudal muscles as the more active tissues. 



The utilization of the fats of the dark muscle does not present as acute a problem as 

 regards the numerous smaller liposomes which we found in the case of the pink muscle. 

 The salmon begins the fast with the dark muscle fully loaded with intracellular fat. 

 Therefore, the first change that will occur in this muscle will be a process of using up the 

 fat on hand in the cell. When at any time or for any reason this intracellular dark 

 muscle fat is wholly consumed, then the dark muscle will be in the same category as the 



