34 BULLETIN OF THE BUREAU OF FISHERIES. 



INFRACARINALES. THE VENTRAL LONGITUDINAL MUSCLES. 



Longitudinal muscles lie along either side of the mid-ventral line. These muscles 

 are the homologues of Owen's infracarinales and of McMurrich's fifth portion of the 

 lateral longitudinal musculature. The muscle mass extends from the basibranchiostegal 

 plate to the base of the caudal fin. It is sharply separated from the surrounding 

 muscles for all of its extent except the anterior portion for about one-fourth of its extent. 



The infracarinales in the king salmon are divided into three portions, by the inter- 

 position of the pelvic arch and of the anal fin. These portions can be described under 

 the names of the protractor ischii, the retractor ischii (protractor analis), and the retractor 

 analis. 



PROTRACTOR ISCHII. 



This term has been given by Owen to the anterior portion of the infracarinalis. 

 In the king salmon this muscle division extends from the anterior margin of the pelvic 

 arch to the posterior margin of the basibranchiostegal plate, the paired muscles lying 

 on either side the mid-ventral plane. For the greater portion of its length the pro- 

 tractor ischii is inclosed in a cylindrical connective tissue sheath which contains a rela- 

 tively large amount of adipose tissue. In the mid-line between the two muscles the 

 adjacent portions of this connective tissue sheath form a pretty definite ventral median 

 septum. In the anterior third of the muscle this sheath is less definite and in most speci- 

 mens scarcely continuous for the full length. In this part of the muscle the form of 

 the muscle as a whole ceases to be cylindrical. The myomeres are not definitely separated 

 from those of the lateral muscle, and the septa are more or less continuous with those of 

 the neighboring lateral muscle. This portion of the protractor is spread out into a 

 slight spatula-shaped terminal mass in the region ventral and anterior to the pectoral 

 fin. The protractor ischii is composed of myomeres, relatively simple in arrangement 

 in the anterior third, and becoming more and more complexly folded into a sort of spiral 

 toward the posterior end of the muscle. In an 80 cm. salmon the diameter of the most 

 cylindrical portion of the muscle varies from 8 to 10 mm., i. e., just in front of the sym- 

 physis of the ischii. 



The protractor ends in a conical tip which is inserted into the fascias of the skin 

 and of the ventral fin muscles, the median septum, and the antero-ventral border of 

 the ischium itself. The tendons of insertion are formed by the ends of the whorls of 

 myocommatous connective tissue. These are best exposed by. a median incision through 

 the skin ventral to the protractor ischii itself. 



Contractions of this muscle accomplish two functions. If the axis of the body 

 is rigidly fixed by the action of other muscles then this muscle merely pulls the pelvic 

 girdle forward. It is from this action that it receives its name. However, it seems 

 that a more important function is found in a second action, namely, a strong ventral 

 flexion of the body. Then, too, in the spawning act, if one is to judge by external 

 appearances, the protractor ischii contributes sharply to the pressure that is brought 

 upon the abdominal cavity and which produces the extrusion of the eggs. 



RETRACTOR ISCHII (PROTRACTOR ANALIS). 



The retractor ischii consists of a cylindrical muscular slip which extends from 

 the posterior end of the pelvic arch directly caudalward and around the anal aperture 

 to be inserted with its fellow into a special bony triangle at the base of the anal fin. 



