100 Investigations on the Life-History 



9. MICROSCOPICAL OBSERVATIONS ON MUSCLE 

 FAT IN THE SALMON. 



15Y S. 0. MAHALANOBIS, B.Sc., F.R.M.S. 



The chemical observations on the changes in the fats of the salmon 

 during its .sojourn in fresh water have .shown that the fish leaves its 

 marine feeding ground with the muscles loaded with fat, and that this 

 fat gradually diminishes in amount, being in part transmitted to the 

 ovary and in part used up as a source of energy. 



The object of the present enquy-y is to investigate more fully the 

 nature of this change. 



On this subject Miescher Ruesch says (p. 186) : * 



" That the lateral trunk muscles are the actual source of material 

 both for the nourishment of the animal and for the ripening of the 

 genitalia, is rendered evident by the microscope. Even the winter and 

 spring salmon show a sometimes more, sometimes less, well marked 

 series of fat droplets chiefly between the fine, cross-striped, elementary 

 fibrillse of the unequally thick muscle fibres, especially in the thinner 

 ones, such as we recognise as a sign of the so-called degeneration of muscle 

 fibres. The amount of these fat droplets increases to mid-summer, when 

 the ovary begins to grow more rapidly, and may lead to many fibres 

 becoming opaque. A separate thin muscle plite which lies along the 

 side of the body just under the skin degenerates most markedly. On the 

 other hand, all the remaining muscles of the breast, belly, back and anal 

 fins, of the jaw and hyoid bone, the upper and lower longitudinal muscles 

 (Langsmuskel), and the tail muscle, in the stricter sense, continue, so to 

 speak, fully intact and free of fat." 



Again on p. 208 he says : " . . . the trunk muscles, which already 

 in the salmon in March, nay even in the winter salmon (December), 

 show clear traces of commencing fatty degeneration (Fettentartung)." 



On p. 215, in describing the condition of the fish on the spawning 

 beds, he says : The flesh of the trunk is entirely opaque, whitish, and 

 entirely filled with fat droplets." As will Le seen presently, our obser- 

 vations do not agree with this last description. 



Miescher then goes 011 to develop a theory of the liquidation and 

 fatty degeneration of the muscles. After referring to the influence of 

 diminished vascular supply and diminished respiration upon the meta- 

 bolism, he concludes that the changes in the muscle arc caused by the 

 diminution in the blood supply. 



It is thus a matter of very considerable importance to re-investigate 

 the microscopic appearances of the muscle fibres and to determine how 

 far Miescher is correct in his conclusion that a fatty degeneration 

 occurs. 



It has already been shown (p. 93) that the salmon feeds during the 

 time it remains in the sea, and that a store of fat appears in the muscles, 

 which is evidently used up for the -nutrition of the animal, as well as the 

 ripening of the generative organs, during its sojourn in the river. 



The results of the chemical examination clearly point to two facts : 

 First, both at the mouth of the river and in upper water there is a fall 

 in the amount of muscle fat from May to November, the amount of fat 



* Statistische und biologische Beitrage, " Zur Kenntniss von Leben dea Rheinliirhsos 



