32 BULLETIN OF THE BUREAU OF FISHERIES. 



Furthermore, as a contraction progresses and the body of the salmon is sharply 

 curved, i. e., concave, to the side involved, the muscles pull even more directly on the 

 skeleton than at the beginning of the movement, as the figure shows. When the lateral 

 muscles on one side thus bend the ends of the body toward that side, the muscles will 

 pull along the line of oblique attachment of the anterior myocommata on the one hand 

 and the similar attachments of the posterior myocommata on the other, so that these 

 two sheaths serve as direct tendons for the muscle fibers. The arrangement is such that 

 this relation holds for almost every portion of the myomere. 



If the septa were simple vertical connective tissue sheaths the mechanical conditions 

 would be wholly changed. In such a case the power expended by the contraction of 

 each myomere would result in a pull on the adjacent myomeres only and from segment 

 to segment and not a direct pull on the skeleton. Only when the great lateral muscles 

 contracted for their full extent would the individual myomere exercise its greatest 

 mechanical possibility. Even then the fibers toward the surface of the myomere would 

 at the time of their maximal contraction soon reach their physiological limit of shortening. 

 The total effect would be to produce tension drawing the superficial part of the muscle 

 away from the skeleton in a relatively inefficient pull. The actual and natural arrange- 

 ment of the structures in the king salmon is far better and forms a wonderfully efficient 

 and economical mechanical-physiological device. 



SUPRACARINALES. THE DORSAL LONGITUDINAL MUSCLES. 



Lying along the extreme dorsal margins of the lateral muscles on either side of the 

 body are separate and well developed muscles, the supracarinales. These paired muscles 

 are imbedded in distinct and heavy connective tissue sheaths. In describing the supra- 

 carinales the muscles should be considered as made up of two divisions: (i) That portion 

 between the scapula and the anterior portion of the spinous dorsal, and (2) that portion 

 between the posterior margin of the spinous dorsal and the caudal fin. This latter is 

 sharply divided into an anterior and posterior division by the soft dorsal. These two 

 muscle divisions acting together tend to flex the body in the dorso-ventral plane, which 

 in the salmon would seem to be their chief function. Acting separately, each division 

 may be assumed to move the spinous dorsal fin, the first division forward, i. e., in pro- 

 traction, the second division backward, in retraction. From this latter function the 

 homologous muscles in other fishes have received their names and these names are used 

 here 



PROTRACTOR DORSALIS. 



This relatively strong muscle extends from the dorsal end of the scapula to the 

 anterior margin of the spinous dorsal and is about 25 cm. long in an 80 cm. salmon. 

 Anteriorly the fibers of the muscle are spread out into a relatively broad fan-shaped mass 

 about 2.5 cm. in width. The mass of the muscle is correspondingly thin in this region. 

 From the middle to the posterior end of the muscle the outline is almost circular, the 

 fibers forming a distinct strong cylindrical bundle even up to the point of insertion. 

 The diameter of this cylindrical mass is from 8 to 10 mm. in an 80 cm. standard fish. 

 Each muscle lies in a tendinous sheath (one on either side of the mid-line of the body). 

 The sheath is less strongly developed anteriorly. The different relations of the walls of 



