SKELETAL MUSCULATURE OK THE KING SALMON. 33 



this sheath are as follows: Superficially there is a relatively thin connective tissue 

 sheath separating the muscle from the skin covering it. This portion is heavily loaded 

 with fat. On the ventral surface of the muscle is a thick septum extending from the 

 skin to the median septum into which it is strongly knitted. The median wall is formed 

 by the superior portion of the median septum, in which are imbedded the interneurals of 

 the skeleton. The outlines of a cross section of the sheath are irregular though approxi- 

 mately circular, the outlines being slightly flattened where the septum is stronglj' 

 developed. 



The protractor dorsalis is segmental in the arrangement of its constituent fibers. 

 Connective tissue septa, the homologues of the myocommata of the lateralis superficialis 

 and profundus, extend through the muscle but in an irregular and complexly folded way. 

 In other words, the septa are not simple transverse membranes, but form cone-like 

 spirals. The fibers composing them are strongly interlaced producing in effect a ten- 

 dinous skeletal framework in which the muscle fibers are imbedded. 



The attachments of the muscles are as follows: The anterior end is attached into 

 the posterior margin of the dorsal end of the scapula and by a strong superficial apo- 

 neurosis into the skin over the scapula and occiput. This fascia extends forward to the 

 occipital and temporal bones. The tendinous fibers of the posterior end of the pro- 

 tractor are knitted into the anterior and superior margins of the two or three interneurals 

 lying under and supporting the most anterior rays of the dorsal fin. However, all along 

 the median border of the muscle tendinous slips are strongly inserted into the median 

 septum and the interhemals imbedded in this portion of the median septum. 



Contractions of the protractor fibers produce traction not only as between the dorsal 

 fin and the occiput, but all along the line of the dorsal margin of the median septum. 

 The whole mechanical effect of the attachments is more favorable for the production of 

 a strong dorsal flexion of the body of the fish than for a protraction of the dorsal fin. 



RETRACTOR DORSALIS. 



That portion of the supracarinalis lying between the posterior margin of the spinous 

 dorsal and the superior margin of the caudal fin receives the name of retractor dorsalis. 

 This muscle is a cylinder in form. The anterior attachment is by a short tendon inserted 

 into an irregular shaped vertical plate which forms a joint with the last interneural 

 spine, the spine lying under the most posterior dorsal ray. The plate is a modified 

 and enlarged free end of an interneural to receive the tendon of the retractor. The 

 posterior tendon of the retractor is rather broadly attached to the connective tissue 

 enclosing the dorsal ends of the interneural spines of the caudal peduncle which lie 

 under and support the dorsal rudimentary rays of the caudal fin. 



The retractor does not seem to be so intimately knitted into its division of the 

 median septum as in the case of the protractor; it is, indeed, free for most of its course. 

 The fact that the muscle is relatively short and smaller in its absolute size than the 

 protractor is probably associated with a development which has separated it from the 

 median septum. That portion of the retractor lying between the soft dorsal and the 

 caudal fin is very slender, 2 or 3 mm. only in diameter. Under the soft dorsal the muscle 

 is wholly tendinous and is closely attached to the base of the fin. Possibly it would 

 be better to consider the two divisions as distinct muscles separated by the soft dorsal. 



