82 BULLETIN OF THE BUREAU OF FISHERIES. 



There is little or no intramuscular fat in the normal pink muscle. The loading of 

 the fat is intermuscular, in contrast with that in the dark muscle, where it is intramuscu- 

 lar. In the normal feeding, growing salmon there is no intramuscular fat, or at most 

 only a trace of fat in the pink muscle. This condition exists up to the time when the 

 salmon cease to feed. This statement is based on the examination of tissues of the 

 smaller salmon in the rivers and also on the examination of different sizes, including 

 the largest adults coming into the market at Monterey, Cal. In the latter there may 

 be an occasional trace of liposomes within the smallest fibers. Monterey fish that will 

 enter the Sacramento River basin can not be assumed to be wholly typical salmon that 

 have ceased feeding. Yet I think it safe to consider these as sufficiently mature adults 

 to serve for comparative purposes. 



In the quite young salmon, from 7 to i6 cm. long, there is no fat in the pink muscle, 

 either between the fibers or in the fibers along the lateral portion of the body. In the 

 ventral or "belly" muscle there is some intermuscular fat at this stage of development. 

 Salmon of this size are still feeding in fresh water. Of the sizes that one obtains at 

 Monterey, which of course are feeding in salt water, fat is beginning to be deposited in 

 the connective tissue between the fibers. This fat is relatively low in amount in the 

 smaller fish. There is great variation in its amount in different individual fishes at 

 Monterey, and while the number of fishes studied is very limited one can say that these 

 indicate that the fat increases in quantity with the size of the fish. 



An exception to the above description is found in a narrow zone of pink fibers lying 

 on the surface of the pink muscle. This zone is immediately covered by the deeper 

 layer of the dark muscle fibers. In these pink fibers there is always a slight amount of 

 intracellular fat. This is a special case, the significance of which will be discussed in 

 the chapter on the mechanism of fat transference in the salmon body. (Page 127.) 



Normal fat content of the trunk dark muscle. — The trunk dark muscle is described on 

 page 78 as characterized by an enormous loading of fat. 



The storage fat is both inter- and intramuscular. It is present between the fibers 

 in a relatively small number of medium-sized drops. These drops vary in size in the 

 adult salmon from 5 to 20 /x in diameter, and are sometimes larger. This fat is most 

 abundant in the immediate neighborhood of blood vessels. In longitudinal preparations 

 it is seen not to be uniformly distributed along the length of the fibers. 



The peculiar characteristic of the superficial lateral or dark muscle is its storage of 

 enormous quantities of intramuscular fat. The fat is distributed within the fiber in two 

 general relations. First, in the region between the sarcolemma and the substance of the 

 muscle fiber proper, especially in the young fish (fig. 7,pl. v). It often happens that there is 

 almost a complete ring of fat droplets surrounding the fiber and pushing the sarcolemma 

 out and away from the fiber wall. In a paraffin preparation there will be a series of vacu- 

 oles under the sarcolemma, where the fat is extracted. Sometimes these fat drops have 

 grown so large that they have fused or run together into larger masses of fat, but usually 

 the droplets are smaller and remain separated. In the maximum loading these droplets 

 are from 4 to 6 /( in diameter. This sarcolemmal fat is not uniformly present in all 

 regions of the muscle, and in regions where it is absent the sarcolemma is in close approxi- 

 mation to the outer wall of the muscle fiber as usual. 



Second, the intramuscular fat is present in large quantities buried within the substance 

 of the sarcoplasm. Especially favorable points for deposit are the angles formed by 



