48 BULLETIN OP THE BUREAU OF FISHERIES. 



and the surface of the lowermost hypural bone, and from the connective tissue ovei 

 the bases of the corresponding caudal spines. The tendon is somewhat stronger at the 

 ends of this broad line of origin. The fibers of the muscle run dorsally and somewhat 

 caudally, converging as they go and ending in a broad and strong tendinous attachment 

 into the fourth, fifth, sixth, and sometimes seventh, long caudal rays dorsal to the axial 

 ray. 



The belly of this muscle is i6 mm. wide by 3 to 3.5 mm. thick and the muscular 

 portion is 22 mm. long, i. e., in the 80 cm. fish used as a standard. 



The contractions of the adductor lead to sharp flexion and adduction of the dorsal 

 caudal lobe. Since the tension on the lobe is almost directly from the point of attach- 

 ment of the tendon toward the median axial ray, this would naturally lead to an 

 approximation of the rays and a decrease in the spread of the fin. 



FIvEXOR CAUDALIS DORSALIS SUPERIOR. 



The dorsal flexor is the longest and strongest muscle of the deep caudal series. It 

 lies almost in the axial plane of the fish. It takes its origin from the median septum 

 over the fifth and fourth neural spines and from the third and fourth vertebrae, counting 

 from the posterior end of the caudal peduncle. Some fibers of origin are found along 

 the tips of the second and third neural spines. Fascias of the muscle are more or less 

 intimately attached to the median septum as far back as the first true caudal vertebra. 

 The insertion, in conjunction with the attachment of the adductor caudalis ventralis, 

 js by a thin flat but strong tendon ending on the lateral surface of the bases of the rays 

 of the most superior portion of the hypaxial division of the caudal fin. 



Occasionally the muscle is more strongly developed, in which case it has an origin 

 anterior to that described. 



Contractions of the superior flexor produce strong flexion of the dorsal caudal lobe 

 toward the side on which the contractions occur. Undoubtedly this muscle and the 

 one preceding it exert the most powerful influence in the control of the rudder-like 

 function of the caudal fin. 



FLEXOR CAUDALIS DORSALIS INFERIOR. 



The inferior dorsal flexor is a much more slender muscle than the preceding one. Its 

 origin is directly ventral to and lies parallel with the superior flexor. The fibers of 

 origin are from the connective tissue over the basal part of the bony plate formed by 

 the fusion of the neural spines of the last three vertebrae of the caudal peduncle. Some 

 fibers are also attached into the myocommata at its most anterior margin. The muscle 

 belly extends caudally in a line parallel with the general axis of the fish, running under 

 the adductor to a flat tendinous insertion into the bases of the median two or three 

 caudal rays next above the axial ray. 



Contractions of this muscle produce simple flexion of the middle portion of the 

 caudal fin. 



From the descriptions presented and the accompanying figures it is now more clear 

 that these muscles are the ones concerned in shaping the position and fonn of the caudal 

 fin during the active movements of forward swimming. The great lateral muscles 

 must be supposed to act on the caudal fin as a whole in the alternate propelling move- 



