30 BULLETIN OP THE BUREAU OF FISHERIES. 



The ventral limb of the hypaxial portion of the myocomma, like the dorsal limb, 

 is very oblique, curving anteriorly. Directly under the pectoral fin this angle is about 

 70°, in the neighborhood of the ventral fins the angle is about 40°, and between the 

 ventral fin and the caudal fin it varies from 40° to 20° measured with reference to the 

 lateral line. The myocommata are placed most nearly horizontal just above the base 

 of the anal fin. 



The form of the septum, i. e., the myocomma, is more clearly shown from plate II 

 if one follows only the outlines of the most anterior of the four myocommata presented, 

 considering primarily the relations of the superficial margin to the deepest margin. The 

 deep margin is in contact with the skeleton and continuous with the median septum or 

 skeletal membrane. Considering the whole septum the superficial zigzag markings are 

 shallow while the zigzag outlines of the skeletal border are deep. In other words, the 

 skeletal boundaries of the septum in the mid-lateral line are attached several centimeters 

 in front of, i. e., cephalad to, the point at which the septum is attached to the skin on 

 the surface. In a similar manner, the skeletal borders of the mid-dorsal and of the 

 mid-ventral portions of the septum are attached back of, that is caudal to, the cor- 

 responding superficial borders. Posteriorly, i. e., over the anal fin (pi. i), this arrange- 

 ment of the myocommata and myomeres is much more extended in the longitudi- 

 nal axis of the salmon. When a given myocomma of the posterior surface of the epaxial 

 half of the body is exposed it is seen that the segment ends in a slender wedge directed 

 caudally, the surface in a particular case being 27 mm. farther back, i. e., caudally, 

 than the surface at the mid-line. The deep or skeletal attachment of the same sep- 

 tum was 55 mm. behind the mid-line surface point. Just at the lateral line the deep 

 portion of the septum dips far forward. The septa of the successive mj^omeres form 

 long slender conical sheaths extending from the under surface of the skin anteriorly 

 down to the skeleton. This distance amounts in the above case to 90 mm. 



The significance of this arrangement can be explained only when one keeps in mind 

 that the individual muscle fibers " of the myomeres run in lines closely paralleling the 

 axis of the fish. There are many variations from this rule; nevertheless, the general 

 effect is a relation between the muscle fiber and its septa which gives to the latter the 

 effect of tendons. This relation enormously strengthens the whole mechanism of 

 myomeres and septa as a power-producing machine. Figure i attempts to show this 

 advantageous arrangement in a diagrammatic way by a somewhat idealized section 

 through the anterior conelike fold just under the lateral line and of the posterior dorsal 

 fold above the lateral line. 



The alternate contractions of the great lateral muscles accomplish the propelling 

 of the body forward in the act of swimming. The skeleton is like a great flexible board. 

 The masses of the myomeres of either side are mechanically so knitted into this support 

 by the comple.K attachments of the myocommata to the skeleton that when a contrac- 

 tion occurs the force of the act is distributed over an unexpected distance along the 



fl Measurements of length of fibers in the myocommata at points on the surface: At the anterior margin of the dorsal fin 

 at the lateral line. 7,2 mm.; at the dorsal mid curve. 6 mm.; in front of the dorsal fin near the dorso-median line. 3.6 mm.; 

 ventrally 3 cm. below the lateral line, 7 mm.; 6 cm. below. 8.2 mm.; fibers running obliquely down and back just in front of the 

 pelvic fin, 5.S mm. 



Measurements just over the anal fin; Dorsal, 2.5 mm.; at the bend, 6.5 mm.; at the lateralline, 8 mm.; deep fibers directly 

 imder this region and i cm. dorsal to the lateral line the pink fibers measure 2.8 mm. On the ventral line of the muscle appar- 

 ently the same general variation in length of fibers occurs. At the point where the myocommata nm most obliquely just above 

 the base of the anal fin the fibers measure 3 mm. 



