STORAGE OF FAT IN MUSCULAR TISSUE OF KING SALMON. 85 



region are two adjacent fibers, each containing an extra large fat drop in the center of the substance of 

 the fiber. One of these drops measures 15 /i in diameter, the other 20 /( in diameter. Here, also, the sur- 

 rounding muscle substance is loaded down with the usual tj-pe of small fat droplets. One can not assume 

 that the large drops of fat arise at the expense of formation of the smaller. Rather is it indicated that 

 these drops are the result of a most active fat storing in this fish at the time it was collected. 



The trunk pink muscle is free of fat in the main body of the muscle. There was neither fat between 

 the fibers nor within the fibers. This statement does not hold for a thin layer of pink muscle lying 

 just under the dark muscle. In this intermediate zone the pink muscles show a certain amount of 

 intracellular fat. These fat droplets are never as great in size as in the dark muscle, but are largest in 

 those fibers lying near the dark muscle layer. Passing from fiber to fiber in the direction away from the 

 dark muscle, the amount of intramuscular fat rapidly decreases. This zone is, on an average, only five 

 or six fibers thick. It tmderlies the whole extent of the thicker portion of the dark muscle. 



Microscopic examination of paraffin sections. — These transverse sections were especially fine as giving 

 a negative picture of the fat in the musculature. The sections show a thin membrane or sheath around 

 the dark fibers, the sarcolemma. The interest attaches to the location and relations of this sarcolemma 

 with reference to the substance of the fiber. The sarcolemma is in contact with the sarcoplasm for a 

 portion of its extent round the fibers, but is distinctly separated from it in most of its circumference. 

 The picture is as if the membrane were pushed out and away from the fiber. The space between the 

 sarcolemma and the proper substance of the muscle is subdivided by very delicate strands extending 

 across the intervening space and continuous with the interfibrillar substance. The form of the spaces, 

 their size, and arrangement, all strongly support the interpretation that these spaces are filled with 

 fat in the fresh condition. They are the cavities left when the fat drops are dissolved out, the fat that 

 in the frozen section is so much more difficult to determine as regards its exact relation to the sarcolemmal 

 sheath (fig. 7, pi. v). 



The central portion of the muscle fiber presents numerous clear areas around which the fibrillas 

 are arranged in irregular circles. Where such a group of fibrillae is unbroken, they usually stand with 

 their broad dimension radial to the center of the clear area. However, there is no particular 

 uniformity about the matter. This arrangement is best shown in figure 7, which is a camera-lucida 

 outline under an oil-immersion lens. The larger angles formed in these whirls of fibrilla are more 

 or less filled with irregularly arranged and smaller fibrillEe. Between the fibers and forming a slight 

 border along the rows of fibrillae is the sarcoplasm. In most instances this sarcoplasm is sufficient 

 in quantity to form a very thin sheet surrounding the clear areas already mentioned. The sarco- 

 plasm can usually be distinguished as an extremely thin sheet around the most superficial fibrillae. 

 It is connected by delicate strands here and there with the sarcolemma. 



The pink trunk muscle of these sections exhibits the great variation in size of fibers noted in the 

 frozen sections. The ends of the fibrillae are very distinct and clear. They are not broad and strap- 

 shaped, as in the dark muscle, but are more thread-like and smaller. In the deeper portion of the 

 pink muscle there is no evidence of interfibrillar spaces. In the intermediate zone of pink fibers, 

 located just tmder the dark muscle, the fibers are more or less marked by clear spaces. These areas 

 are relatively large and more numerous in the pink fibers lying nearest the dark and decrease in 

 number and size in those fibers further away. Some of the pink fibers show irregular groups of small 

 spaces just under the sarcolemma. In the smaller pink fibers the spaces are more numerous in the 

 center of the fiber. The arrangement of the transparent spaces witliin the fibers and between the 

 fibrillar portion of the muscle and tlie sarcolemma corresponds with the distribution of the fat drop- 

 lets in the fibers of the intermediate zone, as shown by the scarlet-red staining. 



Salmon [no. "5 and no. y6) collected at Monterey July 24. IQII, length 100 cm. (estimated). 



Microscopic observation of the trunk pink muscle transverse section, oil-immersion lens. — The material 

 was studie4 after three days' fixation in formalin. The striking picture is that of the intermuscular fat, 

 which is present in large quantity. The fat drops var>' in size from 3 /< up to 45 /( in diameter, the 

 smaller drops being very numerous. The fat is far greater in amount than in the young specimen 

 (no. 97) from Baird, on the McCloud River. 



This section is well fixed by its three days' immersion in formalin. It shows a splendid picture 

 of the fibrillar structure. The muscle fibers are without intracellular fat, or, at best, have only a 

 trace. The large and most of the medium-sized fibers are perfectly free of fat. There are a few of the 



