46 BULLETIN OF THE BUREAU OP FISHERIES. 



The epaxial division covers the deeper muscles presently to be described, but is not 

 strongly fused with their fascias. The hypaxial division is strongly attached into the 

 tips of the hemal spines of the last vertebrae of the caudal peduncle as well as into the 

 bases of the rays. Its superior margin covers the ventral inferior caudal flexor. 



The epaxial and hypaxial aponuroses are strongly united across the median line as 

 shown in the area between the terminal myomeres of the superficialis and the inter- 

 filamenti muscles. 



MUSCULI INTERFILAMENTI CAUDALIS, THE INTRINSIC MUSCLES OF MCMURRICH. 



This small superficial double fan-shaped muscle consists of dorsal and ventral 

 portions. The fibers originate in the fascia lying over, or covering, the termination of 

 the lateralis profundus muscle on the base of the tail. They are attached into the 

 admesial margins and the external surfaces of the bases of the seven caudal rays lying 

 on either side the median ray. This middle ray, so far as the muscular arrangements 

 indicate, is axial. The muscle fibers attached to its base above run diagonally upward 

 and caudalvvard, those below downward and caudalward. The superficial fibers form 

 a continuous layer, while the deep fibers run from ray to ray, as shown in figures 4,5, 

 and 6. When this muscle contracts it draws the caudal rays together, narrowing the 

 spread of the fin. 



The width of the caudal interfilamenti muscles, at the best, is about 10 mm. The 

 dorsal lobe is about 26 mm., the ventral lobe 24 mm. long. 



DEEP CAUDAL MUSCLES. 



The caudal fin is used by the salmon both as a steering rudder and as a propeller. 

 The deep ventral muscles move the parts of the fin to set its form for a rudder, but the 

 musculature which utilizes it as a propeller is limited to the great lateral muscles acting 

 on the fin as a whole. If, during the movements of the fin as a whole, it is set in some 

 special position or given a characteristic shape, that shape will be utilized for steering 

 the forward motion of the salmon. This activity is accomplished by means of the deep 

 caudal muscles, as can readily be seen by consideration of the effect of the contractions 

 of the muscles singly or in groups. There are six pairs of these deep muscles. They 

 vary considerably in detail of size and position but the usual type will now be described. 



FLEXOR CAUDALIS VENTRALIS SUPERFICIALIS. 



This is a delicate muscle slip which begins by a small flat tendon attached to the 

 bases of the hemal spines, the third and fourth from the end of the caudal peduncle. 

 A few fibers also arise from the fascia and tendons of the superficiahs in the median line. 

 It runs posteriorly to a slender tendon attached to the tip of the transverse spine on the 

 first caudal vertebra. The muscle is continued from this point to an insertion on the 

 base of the third caudal ray ventral to the axial ray. Its attachments lie over the inter- 

 filamenti. The ventral margin of the posterior division of this muscle is sometimes 

 fused with the dorsal margin of the next. 



