A NEW TYPE OF FAT STORING MUSCLE 177 



There is nothing particularly characteristic of the sarcolemma 

 of the normal dark fiber except that it is strikingly widely sepa- 

 rated from the surface of the fiber. Where the sarcolemma is so 

 separated the space between it and the fiber is filled in with 

 definite spherical vacuoles from which fat has been dissolved in 

 the preparation. 



There is abundant evidence of the cleavage of these dark muscle 

 fibers in material obtained from the relatively young salmon. 

 This evidence consists in the arrangement of fibers in groups and 

 in the presence of various stages of the separation of fibers, a 

 point chiefly identified by the arrangement and the formation of 

 sarcolemmal partitions and by the disposal of fat in the fibers. 



The normal loading of fat 



The dark muscle is heavily loaded with fat. The fat is present 

 both between the muscle fibers and within the muscle fibers as 

 shown in fig. 1. 



The amount of fat between the fibers is relatively small. It 

 is present in drops from 5 to 20^ in diameter, chiefly in areas 

 which contain blood vessels. The fact peculiar to this muscle 

 and on which I wish to lay emphasis is the presence of enormous 

 quantities of intramuscular fat. I have followed this fat by the 

 special method of staining with scarlet red, Bell's modification of 

 the Herxheimer method, and have confirmed the observations 

 by paraffin sections of both young and adult tissues. 



The fat is distributed within the muscle fiber in two regions. 

 First, within the sarcoplasm throughout the substance of the 

 muscle, and second, under the sarcolemma but outside the sar- 

 coplasm. The sarcoplasmic fat is in droplets of extreme varia- 

 tion in size. In the normal mature muscle these fat drops run 

 from a fraction of a micron to as much as 6, or in rarer instances 

 even 10/x in diameter. In one typical young fish the average of 

 the large intramuscular droplets is from 4 to 6 /j. in diameter. The 

 fat droplets are located at the point corresponding with the angles 

 formed by Cohnheim's areas. Great variety exists as to the size 



