54 



BULLETIN OF THE BUREAU OF FISHERIES. 



MUSCLES OF THE DORSAL FIN. 



Harrison " has briefly described the muscles of the dorsal fin of Salmo salar in con- 

 nection with his study of the development of the fins of teleosts. The muscles in the 

 king salmon are similar in character and arrangement. The number of dorsal fin rays 

 is greater in Oncorhynchus tschawylscha than in Salmo salar. The muscles of the fin 

 have a correspondingly greater number of divisions, one for each fin ray. A typical fin 

 ray is moved by three pairs of muscles, (i ) an inclinator, (2) an erector, and (3) a depres- 

 sor. Beside, the fin as a whole is moved forward by the pair of protractors and back- 

 ward by the pair of retractors described with the group of longitudinal muscles. The 

 specific fin muscles may be described more fully as follows: 



INCLIN.\TOR DORSALIS (THE SUPERFICIAL L.ATER.'^L MUSCLE OF MCMURRICH). 



This muscle in reality consists of a series of short muscles, i. e., independent slips, 

 corresponding in number with the dorsal fin rays. Each tiny slip has its origin in a 



fascia which is strongly attached 

 to the skin and which covers the 

 dorsal margin of the great lat- 

 eral muscle. The fibers of each 

 muscle slip converge as a conical 

 mass ending in a short tendon 

 inserted into the postero-lateral 

 margin of the base of each fin 

 ray. These muscle slips are 

 about 20 mm. long in the ante- 

 rior members of the series and 

 15 mm. in the posterior. The 

 extreme anterior three or four 

 slips are very rudimentary and 

 may readily be overlooked in the 

 dissection. There are 13 free 

 slips, 1 6 or 1 7 in all. In the sea 

 form the spaces between muscle slips are filled with subcutaneous fat. The ends of the 

 deep fin muscles are to be seen just between the ends or insertions of the inclinator slips. 

 Harrison was the first to describe these muscles carefully, and to him we owe the 

 name' 'inclinator." 



The contractions of the divisions of the inclinator muscles tend to bend or incline 

 the dorsal fin toward the side. 



ERECTOR DORSALIS. 



The erector muscle divisions lie between, hence alternate with, the depressor slips 

 of the double series. Each of these arises from the fascia between its interneural spine 

 and the one next in front of it, and from the posterior border of the latter. The mus- 



FlG. 9. — Superficial muscles of the dorsal fiu after removal of the skin and subcu- 

 taneous fat, left side. Inc. d., inclinator dorsalis, one slip for each fin ray; 

 p.d.^ protractor dorsalis; r.rf., retractor dorsalis; Lat., great lateral muscle, 

 epaxial portion. 



a Harrison. Ross G.. loc. cit. 



