491 



COMPARATIVE ANATOMY. 



muscles of the trunk, or lateral trunk-muscles. They consist 

 of two masses of muscle which occupy the sides of the body, and 

 extend from the head to the caudal end (m. laterales) ; they are 

 separated from one another along the middle line of the dorsal and 

 of the ventral surface. In the Myxinoidea, among the Cyclo- 

 stomata, the ventral portion of these masses of muscle is distin- 

 guished by the oblique course of its fibres. It is doubtful whether 

 this represents a new system or no. Each half is divided into a 

 dorsal and a ventral portion, which are separated from one another 

 along a horizontal plane drawn through the vertebral column ; so 

 that, altogether, there are four lateral muscles. 



Each of the four lateral trunk-muscles is divided, in Fishes, into 

 a number of segments by tendinous bands which correspond in 

 number to the vertebras (ligamenta intermuscularia) ; these may be 



easily made out on the surface of 

 the body, owing to their free edges 

 forming distinct tendinous bands. 

 The muscular fibres between any 

 two intermuscular ligaments take 

 a parallel course, and the ligaments, 

 therefore, afford insertion as well as 

 orio-in to one muscular segment. 

 The muscles, therefore, are only in- 

 directly connected with the skeleton. 

 The tendinous septa first keep to 

 one plane, but they then curve, 

 and curve in such a way that in 

 each dorsal muscle we can recog- 

 nise a lower layer, which is made 

 up of cones, one within the other, 

 and with their apices directed for- 

 wards (Fig. 276, A a), and an upper 

 one (6) which is made up of parts 

 of cones. The apices of these in- 

 complete cones look backwards. In the ventral muscles the rela- 

 tions are so far reversed that the cones (a 1 ) are placed above the 

 incomplete cones (&'). In a vertical section through the tail of a 

 Fish we see, therefore, two systems of concentric rings on either 

 side, which project into one another (these are the sections of hollow 

 cones) ; while above the upper ones, and below the lower ones, there 

 are curved lines of varying length (the sections of the incomplete 

 cones). 



In the Perennibranchiata, and in the larval stages of the other 

 Amphibia, we meet with essentially similar conditions ; we meet, 

 that is, with the same zigzag lines of the ligamenta intermuscularia, 

 only they are not so markedly sinuous. When these ligaments take 

 a more direct course the cones are not formed. In the adult Sala- 

 niandrina the ventral portion of the lateral muscles in the trunk 

 undergoes certain changes, and it is in the tail only that we can 



Fig. 276. A Transverse section through 

 the caudal muscles of Scomber 

 scomber, a Superior, a' Inferior 

 lateral muscles, b and b' Section of 

 imperfect upper and lower investing 

 cones. d Centrum of the vertebra. 

 B Zigzag lines of the superficial ends 

 of the Lig. intermuscularia in the tail 

 of Scomber (after J. Muller). 



