SKELETAL MUSCULATURE OF THE KING SALMON. 



55 



cular divisions of the erector dorsalis are separated from those of the depressor by very 

 thin connective tissue septa. But the whole group of muscle slips is encased in a much 

 thicker and tougher sheath. When the muscles are uncovered by removing the lateral 

 muscles the connective tissue sheath is more evident. This is seen to be intimately 

 attached along the line where the neural spines and interneurals are interlocked and 

 embedded in the median longitudinal septum. This sheath, the median septum, and 

 the partitions between muscle divisions serve to form a series of slender glove-finger- 

 like cavities enclosing the pairs of muscle slips on each side. 



Each muscle division of the erector is attached by a very short tendon into the 

 anterior margin of the base of the dorsal halfray. The largest erector divisions are 40 

 mm. in length. At the posterior and shorter margin of the muscle they are about 32 

 mm. in length. The anterior two or three muscle slips are rudimentary, very slender, 

 and more or less fused. 



The contractions of the erector muscle elevate the dorsal fin rays as the name 

 implies. The point of attach- 

 ment of the tendon of insertion 

 above or distal to the center of 

 movement of the joint favors 

 the erection. 



DEPRESSOR DORSALIS. 



The depressor muscle of the 

 dorsal fin is intimately associated 

 in position and attachments with 

 the erector dorsalis. The depres- 

 sor divisions are also segmental 

 in arrangement. They are very 

 slender slips of muscle which 

 arise each along the anterior bor- 

 der of the corresponding inter- 

 neural spine, and from the fascia 

 separating this muscle from the erector muscle in front of it. The fibers pass across the 

 end of the interneurals to insertions on the posterior border of the base of each dorsal ray. 

 The last muscle division of the posterior border of this series is very strongly developed. 

 It is somewhat broader than its mates and is attached into the bony plate previously de- 

 scribed for the retractor dorsalis muscle. 



Fig. 10. — Deep muscles of the dorsal fin after removal of the great lateral muscle; 

 e. d., erector dorsalis; d. d., depressor dorsalis p. d.. protractor dorsalis; r.d,, 

 retractor dorsalis. The skeleton is shown embedded in the median longitu- 

 dinal septum. 



MUSCLES OF THE ANAL FIN. 



The musculature of the anal fin is built on the same plan as that of the dorsal fin. 

 The modifications are slight and more or less unimportant. The fin has a protractor, the 

 retractor of the pelvis, and a retractor — both of which are divisions of the infracarinales 

 previously described as the most ventral longitudinal differentiation of the lateral muscu- 

 lature. The proper muscles of the fin are (i) the incHnator analis, (2) the erector analis, 

 (3) the depressor analis, and (4) the interfilamenti anahs. These muscles are in divisions 



