SKELETAL MUSCULATURE OP THE KING SALMON. 35 



The relation of the pair of muscles and their insertion into this triangle is shown in 

 figures 2 and 8. This bony triangle in its normal position in the body rests directly 

 under and indirectly supports the most anterior rays of the anal fin, with which it is 

 strongly connected by connective tissue fascias. It is a modified interhemal. 



The contractions of this division of the infracarinalis contribute to the ventral 

 flexion of the body. It does this by fixing both the anal fins and the pelvic fins. When 

 other muscles are relaxed so that these fins are movable the action of the muscle is to 

 produce retraction of the ventral fins, i. e., the pelvic girdle. If this latter arch is 

 fixed then protraction of the anal fin results, a movement by which the muscle may 

 well receive the alternate designation of protractor analis. 



RETRACTOR ANALIS. 



On either side of the mid-ventral line of the caudal peduncle lies a slender cylindri- 

 cal muscle, the retractor analis. The muscle is oval in cross section, about 4 mm. 

 broad by 2.5 mm. thick in an 80 cm. salmon. The fibers run up under the tendinous 

 end of the most posterior erector muscle of the anal fin, to be attached by a broad 

 tendon into the posterior margin of the modified cartilage which supports the most 

 posterior rays as previously described and indicated in figure 14. When the skin is 



removed and all the muscles are 

 in place this muscle has the ap- 

 pearance of running into the 



^ angle formed by the lateral mus- 



cles and the posterior margin of 



Fig. 2. — An antero-ventral view, somewhat diagrammatic, of the relation of the fin. 



the anterior rays of the anal fin, the supporting triangular cartilage, and "D *. • 1 4. t, 1 



the insertions of the pair of retractor ischii, i. e., protractor anahs muscles. x^OStenOriy tnC mUSClC rUnS 



The figure shows only indistinctly that the three] anal rays appear one under the baSCS of the Ventral 

 behind the other, the most anterior of course the shorter. . . , i ^ n 



aborted rays of the caudal fin to 

 be attached into the connective tissue and fascias and the ends of the hemal spines. 

 The muscle is slightly conical in shape, becoming more slender posteriorly. It is only 

 3 or 4 mm. in diameter at its thickest part. 



The function of the muscle is that of retraction of the anal fin, but the muscle is so 

 slightly developed that it can not produce extensive motion. 



MUSCLES OF THE HEAD REGION. 



The muscles of the head region may be grouped and discussed under the following 

 heads: 



A. Superficial dorsal head muscles. 



1. Adductor mandibulae, (a) cephalic portion; (b) mandibular portion. 



2. Levator arcus palatini. 



3. Dilatator operculi. 



4. Levator operculi. 



B. Deep dorsal head muscles. 



5. Adductor operculi. 



6. Adductor arcus palatini. 



