204 



ANATOMY OF VERTEBRATES. 



muscle, with tendinous intersections directed downward and back- 

 ward — the lower portions, b, as a ventral longitudinal muscle, 

 with tendinous intersections directed downward and forward, 

 whilst the margins of the middle portions of the myocommas, c, 

 being curved, and usually bisected by the lateral mucous line, have 

 been taken as indications of two intermediate longitudinal muscles. 

 In the Sharks, instead of a curve the margins of the middle 

 portions of the myocommas form an angle with the apex turned 

 forward, fig. 132 ; and in the Rays the dorsal portions have 



132 



Muscles of fore part of Shark {Sqnalus glaucus). xi.iu. 



actually become insulated from the middle ones, and metamor- 

 phosed into a continuous longitudinal muscle, fig. 139, a, the 

 133 change being essentially the same with that which the 

 bony segments themselves undergo, when by anchy- 

 losis the sacral or cranial vertebras are blended into a 

 continuous longitudinal piece. In many bony fishes 

 the middle fibres of the caudal myocommas are dis- 

 posed in two cones ; a transverse section of the tail 

 10f as in fig. 133, shows the two concentric series of cut 

 Maekaroi. segments of the sheathed cones, on each side of the 

 spine. The portions of the myocommas above the lateral line 

 become grouped, in fish-like Batrachia and in Ophidia, into three 

 longitudinal muscles, comparable respectively to the ' spinalis 

 dorsi,' ' longissimus dorsi,' and ' sacrolumbalis,' the portions below 

 the line responding to certain intercostals and the ' rectus abdo- 

 minis,' of higher vertebrates. 



The myocommas of one side are separated from those of the 

 opposite side of the body by the vertebra?, by the interneural 

 and interha?mal aponeuroses, and by the abdominal cavity and its 

 proper walls, fig. 131, h, p. The ventral portions recede from each 

 other to give passage to the ventral fins, v, as in fig. 135, a : and 



