SKELETAL MUSCULATURE OF THE KING SALMON. 29 



MYOMERES OF THE GREAT LATERAL MUSCLES. 



The entire lateral muscle mass, including the superficialis and profundus of both 

 the epaxial and the hypaxial divisions, is subdivided into vertically marked segments, 

 the myomeres (Wiedersheim). The myomeres are separated by connective tissue 

 septa, the myocommata. The septa, and hence the myomeres, are not simple vertical 

 sheaths but are very complexly folded "so as usually to form semiconical masses" 

 (Owen)." The surface markings of the septa, forming the borders between the 

 myomeres, present zig-zag lines across the sides of the fish. These septa are not so 

 simple as the surface lines would indicate, as shown in the figure presented (pi. 11). 

 From this figure of three myomeres taken from about the middle portion of the body 

 it is obvious that each myocomma as a whole forms a rather complex membrane. 

 Owen has described the form of the myocomma in Perca fluviatilis and illustrated the 

 same with a fair figure. Allis '' figures the surface markings of the anterior portion of 

 the body of Amia in his figure 33, the deep folds of the myomeres in figure 34, and the 

 septa after dissecting away the muscles in figure 35, all of the same region. Allis' 

 figures are splendid artistic reproductions of the anatomical facts. The region figured 

 by him is near the pectoral girdle where the myomeres and septal folds are relatively 

 simple. 



The form of the myomere and of the septum varies somewhat in different regions 

 of the body but is always complex and intricate. The variations are from one and 

 the same type. In that part of the body from which the figure is taken, in fact also 

 the myomeres of the entire side of the salmon, the surface markings have the general 

 outline of the letter "W" with the bottom of the letter turned toward the tail. The 

 middle limb of the curve coincides with the lateral line. (See pi. i.) For the entire 

 anterior half of the body the myocommata at the mid-line form sweeping curves. At 

 about the anterior border of the anal fin this curve gives way to a point of gradually 

 increasing sharpness. On the caudal peduncle at the lateral line each myocomma 

 makes a sharp pointed union as between the dorsal and ventral halves. 



The dorsal or epaxial half of the musculature has the bend in the myocommata 

 directed posteriorly. That portion of the myocomma on the surface between the lateral 

 line and the mid-dorsal bend runs in a sweeping curve, almost vertical at the anterior 

 portion of the body, set at an angle of about 60° under the dorsal fin, about 45° over the 

 middle portion of the anal fin, and about 30° on the caudal peduncle. From the middle 

 of the epaxial muscle to the dorsal margin the myocomma forms a sweeping curve toward 

 the head, at first at an angle of about 50°, then curving until just at the dorsal margin 

 the angle is about 10° to 15°, measured with reference to the lateral line. The line 

 marking the union between the dorsal and dorso-median curves of the myocommata 

 lies about three-fifths the distance from the lateral line to the base of the dorsal fin. 



The surface of the ventral half or hypaxial muscle shows similar curves of the 

 myocommata. The median portion very closely follows the angle formed by the ribs 

 along the sides of the abdominal wall. Posteriorly the inclination is ever increasing, 

 reaching its maximum of about 30° at the caudal peduncle. 



tt Owen, Richard: Comparative anatomy and physiology of vertebrates, vol. I, p. 203. 1866. 



& Allis, Edward Phelps: The cranial muscles and cranial and first spinal nerves in Amia calva. Journal of Morphology, 

 1897, vol. 13, p. 4S7-808. 



19371°— vol 33—15 3 



