26 BULLETIN OF THE BUREAU OF FISHERIES. 



MUSCLES OF THE TRUNK. THE LONGITUDINAL MUSCLES. 



The longitudinal muscles have been developed out of the great lateral muscle and 

 form the major mass of muscle substance on each side of the body of. the king salmon 

 as in other fishes. The extreme anterior portion of the embryonic lateral muscle is 

 differentiated into numerous highly specialized small muscles in the head region, and 

 it is similarly, though less complexly, differentiated in the caudal region also. 



The great lateral muscle is subdivided both longitudinally and transversal . 

 Longitudinally the subdivisions are indicated superficially by more or less distinct 

 longitudinal connective tissue areas. The most developed and largest septum how- 

 ever, is indicated by the connective tissue band lying immediately under the lateral line, 

 where a thick septum extends from the under surface of the skin directly down to the 

 lateral-ventral surfaces of the centra of the vertebral column. This septum completely 

 divides the great lateral muscle into dorsal and ventral portions, the division extending 

 from the base of the skull to the middle of the base of the caudal fin. Kingsley, in his 

 Comparative Anatomy of Vertebrates, speaks of these great divisions as the epaxial 

 and hypaxial muscles. 



The extreme dorsal portion of the epaxial muscle on each side has become further 

 differentiated by the separation of a definite cylindrical bundle extending from the 

 occiput to the base of the tail, but interrupted at the dorsal fin, and modified at the 

 soft dorsal fin. This muscle is the homologue of Owen's supracarinalis. 



That portion of the lateral muscle l}ing below the lateral line, the hypaxial, has its 

 extreme ventral portion cut off into definite masses which for the most part are cylin- 

 drical in form. This portion is the homologue of what McMurrich " calls in the cat- 

 fish "the great ventral muscle," the infracarinaHs of Owen. It extends from the gular 

 plate to the base of the caudal fin, but is interrupted at the pelvic girdle and at the anal 

 fin respectively. 



The lateral muscles proper are further differentiated into a superficial and a deep 

 portion. These subdivisions are rather intimately bound together at their surfaces of 

 approximation. But in gross anatomical features, in minute histological structures, and 

 in physiological properties, they are so characteristically different that they were con- 

 sidered as distinct muscles. They have been described and given distinctive names by 

 the senior author.'' 



The entire list of longitudinal muscles, including the divisions of the lateral muscles 

 and special differentiations at the mid-dorsal and mid-ventral regions, is as follows: 

 L Divisions of the great lateral muscle proper. 



1. Musculus lateralis superficialis. 



a. The epaxial division. 



b. The hypaxial division. 



2. Musculus lateralis profundus. 



a. The epaxial division. 



b. The hypaxial division. 



o McMurrich, J. P.: Themyology of Amiurus. Proceedings of the Canadian Institute, vol. n, p. 330. 



6 Greene. Chas. W.: An undcscribed longitudinal differentiation of the great lateral muscle of the king salmon. Anatomi- 

 cal Record, 1913. vol. 7, p. 99-101. 



